refactor(infra,api.tests): fix my null lies

src/FiscalOS.API/Accounts/AccountsExtensions.cs

@@ -6,8 +6,10 @@ internal static class AccountsExtensions
 
   public static RouteGroupBuilder MapAccountsEndpoints(this WebApplication app)
   {
-    var accountsGroup = app.MapGroup(RouteGroupPrefix).RequireAuthorization();
+    var accountsGroup = app.MapGroup(RouteGroupPrefix)
+      .RequireAuthorization();
 
+    accountsGroup.MapAddEndpoint();
     accountsGroup.MapConnectEndpoint();
 
     return accountsGroup;

src/FiscalOS.API/Accounts/Add/Endpoint.cs

@@ -0,0 +1,55 @@
+using FiscalOS.Core.Accounts;
+
+namespace FiscalOS.API.Accounts.Add;
+
+internal static class Endpoint
+{
+  private const string Route = "/";
+
+  public static RouteHandlerBuilder MapAddEndpoint(this RouteGroupBuilder groupBuilder)
+  {
+    return groupBuilder.MapPost(Route, HandleAsync);
+  }
+
+  private static async Task<IResult> HandleAsync(
+    HttpContext httpContext,
+    [FromBody] Request request,
+    [FromServices] AppDbContext appDbContext,
+    CancellationToken ct
+  )
+  {
+    var userId = httpContext.GetUserId();
+
+    var user = await appDbContext.Users
+      .Include(u => u.Institutions.Where(i => i.Metadata is PlaidMetadata && ((PlaidMetadata)i.Metadata).PlaidId == request.PlaidInstitutionId))
+      .Include(u => u.Accounts.Where(a => a.Metadata is PlaidAccountMetadata && ((PlaidAccountMetadata)a.Metadata).PlaidId == request.PlaidAccountId))
+      .ThenInclude(a => a.Metadata)
+      .SingleOrDefaultAsync(u => u.Id == userId, ct);
+
+    if (user is null)
+    {
+      return Results.Unauthorized();
+    }
+
+    if (user.Institutions.Any() is false)
+    {
+      return Results.ValidationProblem(new Dictionary<string, string[]>
+      {
+        ["PlaidInstitutionId"] = ["The PlaidInstitutionId field is invalid. No institution connected with the given PlaidInstitutionId was found for the user."],
+      });
+    }
+
+    if (user.Accounts.Any())
+    {
+      return Results.Conflict();
+    }
+
+    var accountMetadata = PlaidAccountMetadata.From(request.PlaidAccountId, request.PlaidAccountName);
+    var account = Account.From(user.Institutions.First().Id, request.PlaidAccountName, accountMetadata);
+    user.AddAccount(account);
+
+    await appDbContext.SaveChangesAsync(ct);
+
+    return Results.Ok();
+  }
+}

src/FiscalOS.API/Accounts/Add/Request.cs

@@ -0,0 +1,30 @@
+
+namespace FiscalOS.API.Accounts.Add;
+
+public record Request : IValidatableObject
+{
+  public string PlaidInstitutionId { get; init; } = string.Empty;
+  public string PlaidAccountId { get; init; } = string.Empty;
+  public string PlaidAccountName { get; init; } = string.Empty;
+
+  public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
+  {
+    if (string.IsNullOrWhiteSpace(PlaidInstitutionId))
+    {
+      var fieldName = nameof(PlaidInstitutionId);
+      yield return new($"The {fieldName} field is required.", [fieldName]);
+    }
+
+    if (string.IsNullOrWhiteSpace(PlaidAccountId))
+    {
+      var fieldName = nameof(PlaidAccountId);
+      yield return new($"The {fieldName} field is required.", [fieldName]);
+    }
+
+    if (string.IsNullOrWhiteSpace(PlaidAccountName))
+    {
+      var fieldName = nameof(PlaidAccountName);
+      yield return new($"The {fieldName} field is required.", [fieldName]);
+    }
+  }
+}

src/FiscalOS.API/Accounts/Connect/Endpoint.cs

@@ -34,20 +34,12 @@ internal static class Endpoint
 
     if (user is null)
     {
-      return Results.Problem(
+      return Results.Unauthorized();
-        statusCode: StatusCodes.Status401Unauthorized,
-        title: "Unauthorized",
-        detail: "You are not authorized to connect an institution. Please log in and try again."
-      );
     }
 
     if (user.Institutions.Any())
     {
-      return Results.Problem(
+      return Results.Conflict();
-        statusCode: StatusCodes.Status409Conflict,
-        title: "Institution already connected",
-        detail: "The user has already connected an institution with the provided Plaid Institution Id."
-      );
     }
 
     var (itemId, accessToken) = await plaidService.ExchangeTokenAsync(request.PublicToken);

src/FiscalOS.API/Usings.cs

@@ -3,6 +3,7 @@ global using System.Security.Claims;
 global using System.Text.Json.Serialization;
 
 global using FiscalOS.API.Accounts;
+global using FiscalOS.API.Accounts.Add;
 global using FiscalOS.API.Accounts.Connect;
 global using FiscalOS.API.Http;
 global using FiscalOS.API.Login;

src/FiscalOS.Core/Accounts/Account.cs

@@ -0,0 +1,20 @@
+namespace FiscalOS.Core.Accounts;
+
+public sealed class Account : Entity
+{
+  public Guid UserId { get; init; }
+  public Guid InstitutionId { get; init; }
+  public Institution? Institution { get; init; }
+  public string Name { get; init; } = string.Empty;
+  public AccountMetadata? Metadata { get; init; }
+
+  public static Account From(Guid institutionId, string name, AccountMetadata accountMetadata)
+  {
+    return new()
+    {
+      InstitutionId = institutionId,
+      Name = name,
+      Metadata = accountMetadata,
+    };
+  }
+}

src/FiscalOS.Core/Accounts/AccountMetadata.cs

@@ -0,0 +1,15 @@
+namespace FiscalOS.Core.Accounts;
+
+public abstract class AccountMetadata : Entity
+{
+  private readonly string _type;
+
+  public string Type => _type;
+
+  public Guid AccountId { get; init; }
+
+  protected AccountMetadata(string type)
+  {
+    _type = type;
+  }
+}

src/FiscalOS.Core/Accounts/Institution.cs

@@ -4,7 +4,7 @@ public sealed class Institution : Entity
 {
   public Guid UserId { get; init; }
   public string Name { get; init; } = string.Empty;
-  public InstitutionMetadata Metadata { get; init; } = null!;
+  public InstitutionMetadata? Metadata { get; init; }
 
   private Institution()
   {

src/FiscalOS.Core/Identity/User.cs

@@ -13,6 +13,9 @@ public sealed class User : Entity
   private readonly List<Institution> _institutions = [];
   public IEnumerable<Institution> Institutions => _institutions;
 
+  private readonly List<Account> _accounts = [];
+  public IEnumerable<Account> Accounts => _accounts;
+
   private User()
   {
   }
@@ -50,4 +53,11 @@ public sealed class User : Entity
 
     _institutions.Add(institution);
   }
+
+  public void AddAccount(Account account)
+  {
+    ArgumentNullException.ThrowIfNull(account, nameof(account));
+
+    _accounts.Add(account);
+  }
 }

src/FiscalOS.Infra/Accounts/Plaid/PlaidAccountMetadata.cs

@@ -0,0 +1,21 @@
+namespace FiscalOS.Infra.Accounts.Plaid;
+
+public sealed class PlaidAccountMetadata : AccountMetadata
+{
+  public const string TypeValue = Providers.Plaid;
+  public string PlaidId { get; init; } = string.Empty;
+  public string PlaidName { get; init; } = string.Empty;
+
+  private PlaidAccountMetadata() : base(TypeValue)
+  {
+  }
+
+  public static PlaidAccountMetadata From(string plaidId, string plaidName)
+  {
+    return new()
+    {
+      PlaidId = plaidId,
+      PlaidName = plaidName,
+    };
+  }
+}

src/FiscalOS.Infra/Accounts/Plaid/PlaidMetadata.cs

@@ -2,7 +2,7 @@ namespace FiscalOS.Infra.Accounts.Plaid;
 
 public sealed class PlaidMetadata : InstitutionMetadata
 {
-  public const string TypeValue = "Plaid";
+  public const string TypeValue = Providers.Plaid;
   public string PlaidId { get; init; } = string.Empty;
   public string PlaidName { get; init; } = string.Empty;
   public string EncryptedAccessToken { get; init; } = string.Empty;

src/FiscalOS.Infra/Accounts/Providers.cs

@@ -0,0 +1,6 @@
+namespace FiscalOS.Infra.Accounts;
+
+public static class Providers
+{
+  public const string Plaid = nameof(Plaid);
+}

src/FiscalOS.Infra/Data/AppDbContext.cs

@@ -1,3 +1,4 @@
+using Account = FiscalOS.Core.Accounts.Account;
 using Institution = FiscalOS.Core.Accounts.Institution;
 
 namespace FiscalOS.Infra.Data;
@@ -65,6 +66,11 @@ public sealed class AppDbContext(
         .WithOne()
         .HasForeignKey(static i => i.UserId)
         .OnDelete(DeleteBehavior.Cascade);
+
+      eb.HasMany(static u => u.Accounts)
+        .WithOne()
+        .HasForeignKey(static a => a.UserId)
+        .OnDelete(DeleteBehavior.Cascade);
     });
 
     modelBuilder.Entity<RefreshToken>(static eb =>
@@ -104,5 +110,37 @@ public sealed class AppDbContext(
       eb.Property(static m => m.PlaidName);
       eb.Property(static m => m.EncryptedAccessToken);
     });
+
+    modelBuilder.Entity<Account>(static eb =>
+    {
+      eb.Property(static a => a.Name);
+
+      eb.HasOne(static a => a.Institution)
+        .WithMany()
+        .HasForeignKey(static a => a.InstitutionId)
+        .OnDelete(DeleteBehavior.Cascade);
+
+      eb.HasOne(static a => a.Metadata)
+        .WithOne()
+        .HasForeignKey<AccountMetadata>(static m => m.AccountId)
+        .OnDelete(DeleteBehavior.Cascade);
+    });
+
+    modelBuilder.Entity<AccountMetadata>(static eb =>
+    {
+      eb.HasDiscriminator(static m => m.Type)
+        .HasValue<PlaidAccountMetadata>(PlaidAccountMetadata.TypeValue);
+
+      eb.Property(static m => m.AccountId);
+      eb.Property(static m => m.Type);
+    });
+
+    modelBuilder.Entity<PlaidAccountMetadata>(static eb =>
+    {
+      eb.HasBaseType<AccountMetadata>();
+
+      eb.Property(static m => m.PlaidId);
+      eb.Property(static m => m.PlaidName);
+    });
   }
 }

src/FiscalOS.Infra/Migrations/20260213130218_AccountAndMetadata.Designer.cs

@@ -0,0 +1,327 @@
+// <auto-generated />
+using System;
+using FiscalOS.Infra.Data;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
+
+#nullable disable
+
+namespace FiscalOS.Infra.Migrations
+{
+    [DbContext(typeof(AppDbContext))]
+    [Migration("20260213130218_AccountAndMetadata")]
+    partial class AccountAndMetadata
+    {
+        /// <inheritdoc />
+        protected override void BuildTargetModel(ModelBuilder modelBuilder)
+        {
+#pragma warning disable 612, 618
+            modelBuilder.HasAnnotation("ProductVersion", "10.0.2");
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Account", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("InstituionId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Name")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("UserId")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("InstituionId");
+
+                    b.HasIndex("UserId");
+
+                    b.ToTable("Account");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.AccountMetadata", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("AccountId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Type")
+                        .IsRequired()
+                        .HasMaxLength(21)
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("AccountId")
+                        .IsUnique();
+
+                    b.ToTable("AccountMetadata");
+
+                    b.HasDiscriminator<string>("Type").HasValue("AccountMetadata");
+
+                    b.UseTphMappingStrategy();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Institution", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Name")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("UserId")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("UserId");
+
+                    b.ToTable("Institution");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.InstitutionMetadata", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("InstitutionId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Type")
+                        .IsRequired()
+                        .HasMaxLength(21)
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("InstitutionId")
+                        .IsUnique();
+
+                    b.ToTable("InstitutionMetadata");
+
+                    b.HasDiscriminator<string>("Type").HasValue("InstitutionMetadata");
+
+                    b.UseTphMappingStrategy();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Identity.RefreshToken", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("ExpiresAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<bool>("Revoked")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("Token")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("UserId")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("Token")
+                        .IsUnique();
+
+                    b.HasIndex("UserId");
+
+                    b.ToTable("RefreshTokens");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Identity.User", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("EncryptedDataKey")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("EncryptionKeyId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("HashedPassword")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Username")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("Username")
+                        .IsUnique();
+
+                    b.ToTable("Users");
+                });
+
+            modelBuilder.Entity("FiscalOS.Infra.Accounts.Plaid.PlaidAccountMetadata", b =>
+                {
+                    b.HasBaseType("FiscalOS.Core.Accounts.AccountMetadata");
+
+                    b.Property<string>("PlaidId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("PlaidName")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.HasDiscriminator().HasValue("Plaid");
+                });
+
+            modelBuilder.Entity("FiscalOS.Infra.Accounts.Plaid.PlaidMetadata", b =>
+                {
+                    b.HasBaseType("FiscalOS.Core.Accounts.InstitutionMetadata");
+
+                    b.Property<string>("EncryptedAccessToken")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("PlaidId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("PlaidName")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.HasDiscriminator().HasValue("Plaid");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Account", b =>
+                {
+                    b.HasOne("FiscalOS.Core.Accounts.Institution", "Institution")
+                        .WithMany()
+                        .HasForeignKey("InstituionId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.HasOne("FiscalOS.Core.Identity.User", null)
+                        .WithMany("Accounts")
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("Institution");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.AccountMetadata", b =>
+                {
+                    b.HasOne("FiscalOS.Core.Accounts.Account", null)
+                        .WithOne("Metadata")
+                        .HasForeignKey("FiscalOS.Core.Accounts.AccountMetadata", "AccountId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Institution", b =>
+                {
+                    b.HasOne("FiscalOS.Core.Identity.User", null)
+                        .WithMany("Institutions")
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.InstitutionMetadata", b =>
+                {
+                    b.HasOne("FiscalOS.Core.Accounts.Institution", null)
+                        .WithOne("Metadata")
+                        .HasForeignKey("FiscalOS.Core.Accounts.InstitutionMetadata", "InstitutionId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Identity.RefreshToken", b =>
+                {
+                    b.HasOne("FiscalOS.Core.Identity.User", "User")
+                        .WithMany("RefreshTokens")
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("User");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Account", b =>
+                {
+                    b.Navigation("Metadata")
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Institution", b =>
+                {
+                    b.Navigation("Metadata")
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Identity.User", b =>
+                {
+                    b.Navigation("Accounts");
+
+                    b.Navigation("Institutions");
+
+                    b.Navigation("RefreshTokens");
+                });
+#pragma warning restore 612, 618
+        }
+    }
+}

src/FiscalOS.Infra/Migrations/20260213130218_AccountAndMetadata.cs

@@ -0,0 +1,92 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace FiscalOS.Infra.Migrations
+{
+    /// <inheritdoc />
+    public partial class AccountAndMetadata : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "Account",
+                columns: table => new
+                {
+                    Id = table.Column<Guid>(type: "TEXT", nullable: false),
+                    UserId = table.Column<Guid>(type: "TEXT", nullable: false),
+                    InstituionId = table.Column<Guid>(type: "TEXT", nullable: false),
+                    Name = table.Column<string>(type: "TEXT", nullable: false),
+                    CreatedAt = table.Column<DateTimeOffset>(type: "TEXT", nullable: false),
+                    UpdatedAt = table.Column<DateTimeOffset>(type: "TEXT", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_Account", x => x.Id);
+                    table.ForeignKey(
+                        name: "FK_Account_Institution_InstituionId",
+                        column: x => x.InstituionId,
+                        principalTable: "Institution",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                    table.ForeignKey(
+                        name: "FK_Account_Users_UserId",
+                        column: x => x.UserId,
+                        principalTable: "Users",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                });
+
+            migrationBuilder.CreateTable(
+                name: "AccountMetadata",
+                columns: table => new
+                {
+                    Id = table.Column<Guid>(type: "TEXT", nullable: false),
+                    Type = table.Column<string>(type: "TEXT", maxLength: 21, nullable: false),
+                    AccountId = table.Column<Guid>(type: "TEXT", nullable: false),
+                    PlaidId = table.Column<string>(type: "TEXT", nullable: true),
+                    PlaidName = table.Column<string>(type: "TEXT", nullable: true),
+                    CreatedAt = table.Column<DateTimeOffset>(type: "TEXT", nullable: false),
+                    UpdatedAt = table.Column<DateTimeOffset>(type: "TEXT", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_AccountMetadata", x => x.Id);
+                    table.ForeignKey(
+                        name: "FK_AccountMetadata_Account_AccountId",
+                        column: x => x.AccountId,
+                        principalTable: "Account",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Account_InstituionId",
+                table: "Account",
+                column: "InstituionId");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Account_UserId",
+                table: "Account",
+                column: "UserId");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_AccountMetadata_AccountId",
+                table: "AccountMetadata",
+                column: "AccountId",
+                unique: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "AccountMetadata");
+
+            migrationBuilder.DropTable(
+                name: "Account");
+        }
+    }
+}

src/FiscalOS.Infra/Migrations/20260213180002_FixAccountInstitutionColumnName.Designer.cs

@@ -0,0 +1,327 @@
+// <auto-generated />
+using System;
+using FiscalOS.Infra.Data;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
+
+#nullable disable
+
+namespace FiscalOS.Infra.Migrations
+{
+    [DbContext(typeof(AppDbContext))]
+    [Migration("20260213180002_FixAccountInstitutionColumnName")]
+    partial class FixAccountInstitutionColumnName
+    {
+        /// <inheritdoc />
+        protected override void BuildTargetModel(ModelBuilder modelBuilder)
+        {
+#pragma warning disable 612, 618
+            modelBuilder.HasAnnotation("ProductVersion", "10.0.2");
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Account", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("InstitutionId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Name")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("UserId")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("InstitutionId");
+
+                    b.HasIndex("UserId");
+
+                    b.ToTable("Account");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.AccountMetadata", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("AccountId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Type")
+                        .IsRequired()
+                        .HasMaxLength(21)
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("AccountId")
+                        .IsUnique();
+
+                    b.ToTable("AccountMetadata");
+
+                    b.HasDiscriminator<string>("Type").HasValue("AccountMetadata");
+
+                    b.UseTphMappingStrategy();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Institution", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Name")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("UserId")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("UserId");
+
+                    b.ToTable("Institution");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.InstitutionMetadata", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("InstitutionId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Type")
+                        .IsRequired()
+                        .HasMaxLength(21)
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("InstitutionId")
+                        .IsUnique();
+
+                    b.ToTable("InstitutionMetadata");
+
+                    b.HasDiscriminator<string>("Type").HasValue("InstitutionMetadata");
+
+                    b.UseTphMappingStrategy();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Identity.RefreshToken", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("ExpiresAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<bool>("Revoked")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("Token")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("UserId")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("Token")
+                        .IsUnique();
+
+                    b.HasIndex("UserId");
+
+                    b.ToTable("RefreshTokens");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Identity.User", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("EncryptedDataKey")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("EncryptionKeyId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("HashedPassword")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Username")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("Username")
+                        .IsUnique();
+
+                    b.ToTable("Users");
+                });
+
+            modelBuilder.Entity("FiscalOS.Infra.Accounts.Plaid.PlaidAccountMetadata", b =>
+                {
+                    b.HasBaseType("FiscalOS.Core.Accounts.AccountMetadata");
+
+                    b.Property<string>("PlaidId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("PlaidName")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.HasDiscriminator().HasValue("Plaid");
+                });
+
+            modelBuilder.Entity("FiscalOS.Infra.Accounts.Plaid.PlaidMetadata", b =>
+                {
+                    b.HasBaseType("FiscalOS.Core.Accounts.InstitutionMetadata");
+
+                    b.Property<string>("EncryptedAccessToken")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("PlaidId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("PlaidName")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.HasDiscriminator().HasValue("Plaid");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Account", b =>
+                {
+                    b.HasOne("FiscalOS.Core.Accounts.Institution", "Institution")
+                        .WithMany()
+                        .HasForeignKey("InstitutionId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.HasOne("FiscalOS.Core.Identity.User", null)
+                        .WithMany("Accounts")
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("Institution");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.AccountMetadata", b =>
+                {
+                    b.HasOne("FiscalOS.Core.Accounts.Account", null)
+                        .WithOne("Metadata")
+                        .HasForeignKey("FiscalOS.Core.Accounts.AccountMetadata", "AccountId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Institution", b =>
+                {
+                    b.HasOne("FiscalOS.Core.Identity.User", null)
+                        .WithMany("Institutions")
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.InstitutionMetadata", b =>
+                {
+                    b.HasOne("FiscalOS.Core.Accounts.Institution", null)
+                        .WithOne("Metadata")
+                        .HasForeignKey("FiscalOS.Core.Accounts.InstitutionMetadata", "InstitutionId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Identity.RefreshToken", b =>
+                {
+                    b.HasOne("FiscalOS.Core.Identity.User", "User")
+                        .WithMany("RefreshTokens")
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("User");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Account", b =>
+                {
+                    b.Navigation("Metadata")
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Institution", b =>
+                {
+                    b.Navigation("Metadata")
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Identity.User", b =>
+                {
+                    b.Navigation("Accounts");
+
+                    b.Navigation("Institutions");
+
+                    b.Navigation("RefreshTokens");
+                });
+#pragma warning restore 612, 618
+        }
+    }
+}

src/FiscalOS.Infra/Migrations/20260213180002_FixAccountInstitutionColumnName.cs

@@ -0,0 +1,62 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace FiscalOS.Infra.Migrations
+{
+    /// <inheritdoc />
+    public partial class FixAccountInstitutionColumnName : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropForeignKey(
+                name: "FK_Account_Institution_InstituionId",
+                table: "Account");
+
+            migrationBuilder.RenameColumn(
+                name: "InstituionId",
+                table: "Account",
+                newName: "InstitutionId");
+
+            migrationBuilder.RenameIndex(
+                name: "IX_Account_InstituionId",
+                table: "Account",
+                newName: "IX_Account_InstitutionId");
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_Account_Institution_InstitutionId",
+                table: "Account",
+                column: "InstitutionId",
+                principalTable: "Institution",
+                principalColumn: "Id",
+                onDelete: ReferentialAction.Cascade);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropForeignKey(
+                name: "FK_Account_Institution_InstitutionId",
+                table: "Account");
+
+            migrationBuilder.RenameColumn(
+                name: "InstitutionId",
+                table: "Account",
+                newName: "InstituionId");
+
+            migrationBuilder.RenameIndex(
+                name: "IX_Account_InstitutionId",
+                table: "Account",
+                newName: "IX_Account_InstituionId");
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_Account_Institution_InstituionId",
+                table: "Account",
+                column: "InstituionId",
+                principalTable: "Institution",
+                principalColumn: "Id",
+                onDelete: ReferentialAction.Cascade);
+        }
+    }
+}

src/FiscalOS.Infra/Migrations/AppDbContextModelSnapshot.cs

@@ -17,6 +17,69 @@ namespace FiscalOS.Infra.Migrations
 #pragma warning disable 612, 618
             modelBuilder.HasAnnotation("ProductVersion", "10.0.2");
 
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Account", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("InstitutionId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Name")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("UserId")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("InstitutionId");
+
+                    b.HasIndex("UserId");
+
+                    b.ToTable("Account");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.AccountMetadata", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("AccountId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Type")
+                        .IsRequired()
+                        .HasMaxLength(21)
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("AccountId")
+                        .IsUnique();
+
+                    b.ToTable("AccountMetadata");
+
+                    b.HasDiscriminator<string>("Type").HasValue("AccountMetadata");
+
+                    b.UseTphMappingStrategy();
+                });
+
             modelBuilder.Entity("FiscalOS.Core.Accounts.Institution", b =>
                 {
                     b.Property<Guid>("Id")
@@ -146,6 +209,21 @@ namespace FiscalOS.Infra.Migrations
                     b.ToTable("Users");
                 });
 
+            modelBuilder.Entity("FiscalOS.Infra.Accounts.Plaid.PlaidAccountMetadata", b =>
+                {
+                    b.HasBaseType("FiscalOS.Core.Accounts.AccountMetadata");
+
+                    b.Property<string>("PlaidId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("PlaidName")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.HasDiscriminator().HasValue("Plaid");
+                });
+
             modelBuilder.Entity("FiscalOS.Infra.Accounts.Plaid.PlaidMetadata", b =>
                 {
                     b.HasBaseType("FiscalOS.Core.Accounts.InstitutionMetadata");
@@ -165,6 +243,32 @@ namespace FiscalOS.Infra.Migrations
                     b.HasDiscriminator().HasValue("Plaid");
                 });
 
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Account", b =>
+                {
+                    b.HasOne("FiscalOS.Core.Accounts.Institution", "Institution")
+                        .WithMany()
+                        .HasForeignKey("InstitutionId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.HasOne("FiscalOS.Core.Identity.User", null)
+                        .WithMany("Accounts")
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("Institution");
+                });
+
+            modelBuilder.Entity("FiscalOS.Core.Accounts.AccountMetadata", b =>
+                {
+                    b.HasOne("FiscalOS.Core.Accounts.Account", null)
+                        .WithOne("Metadata")
+                        .HasForeignKey("FiscalOS.Core.Accounts.AccountMetadata", "AccountId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
             modelBuilder.Entity("FiscalOS.Core.Accounts.Institution", b =>
                 {
                     b.HasOne("FiscalOS.Core.Identity.User", null)
@@ -194,6 +298,12 @@ namespace FiscalOS.Infra.Migrations
                     b.Navigation("User");
                 });
 
+            modelBuilder.Entity("FiscalOS.Core.Accounts.Account", b =>
+                {
+                    b.Navigation("Metadata")
+                        .IsRequired();
+                });
+
             modelBuilder.Entity("FiscalOS.Core.Accounts.Institution", b =>
                 {
                     b.Navigation("Metadata")
@@ -202,6 +312,8 @@ namespace FiscalOS.Infra.Migrations
 
             modelBuilder.Entity("FiscalOS.Core.Identity.User", b =>
                 {
+                    b.Navigation("Accounts");
+
                     b.Navigation("Institutions");
 
                     b.Navigation("RefreshTokens");

tests/FiscalOS.API.Tests/Integration/AddTests.cs

@@ -0,0 +1,304 @@
+using Account = FiscalOS.Core.Accounts.Account;
+using Institution = FiscalOS.Core.Accounts.Institution;
+
+namespace FiscalOS.API.Tests.Integration;
+
+public class AddTests(TestApi testApi) : IntegrationTest(testApi)
+{
+  private static readonly Uri AddUri = new("/accounts", UriKind.Relative);
+
+  [Fact]
+  public async Task Add_WhenCalledWhenNotLoggedIn_ItShouldReturn401WithProblemDetails()
+  {
+    var response = await Client.PostAsync(AddUri, null, TestContext.Current.CancellationToken);
+
+    await response.Should().BeProblemDetails(HttpStatusCode.Unauthorized);
+  }
+
+  [Fact]
+  public async Task Add_WhenCalledWithoutInstitutionIdOrAccountIdOrAccountName_ItShouldReturn400WithProblemDetails()
+  {
+    var jwt = JwtTokenBuilder.New()
+      .WithClaim(JwtRegisteredClaimNames.Sub, Guid.NewGuid().ToString())
+      .Build();
+
+    using var content = new StringContent(JsonSerializer.Serialize(new { }), Encoding.UTF8, "application/json");
+    using var request = new HttpRequestMessage(HttpMethod.Post, AddUri)
+    {
+      Content = content
+    };
+    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", jwt);
+
+    var response = await Client.SendAsync(request, TestContext.Current.CancellationToken);
+
+    await response.Should().BeValidationProblemDetails(new Dictionary<string, string[]>()
+    {
+      ["PlaidInstitutionId"] = ["The PlaidInstitutionId field is required."],
+      ["PlaidAccountId"] = ["The PlaidAccountId field is required."],
+      ["PlaidAccountName"] = ["The PlaidAccountName field is required."],
+    });
+  }
+
+  [Fact]
+  public async Task Add_WhenCalledWithoutInstitutionId_ItShouldReturn400WithProblemDetails()
+  {
+    var jwt = JwtTokenBuilder.New()
+      .WithClaim(JwtRegisteredClaimNames.Sub, Guid.NewGuid().ToString())
+      .Build();
+
+    var json = JsonSerializer.Serialize(new
+    {
+      plaidAccountId = "accountId",
+      plaidAccountName = "Some Account",
+    });
+    using var content = new StringContent(json, Encoding.UTF8, "application/json");
+    using var request = new HttpRequestMessage(HttpMethod.Post, AddUri)
+    {
+      Content = content
+    };
+    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", jwt);
+
+    var response = await Client.SendAsync(request, TestContext.Current.CancellationToken);
+
+    await response.Should().BeValidationProblemDetails(new Dictionary<string, string[]>()
+    {
+      ["PlaidInstitutionId"] = ["The PlaidInstitutionId field is required."],
+    });
+  }
+
+  [Fact]
+  public async Task Add_WhenCalledWithoutAccountId_ItShouldReturn400WithProblemDetails()
+  {
+    var jwt = JwtTokenBuilder.New()
+      .WithClaim(JwtRegisteredClaimNames.Sub, Guid.NewGuid().ToString())
+      .Build();
+
+    var json = JsonSerializer.Serialize(new
+    {
+      plaidInstitutionId = "institutionId",
+      plaidAccountName = "Some Account",
+    });
+    using var content = new StringContent(json, Encoding.UTF8, "application/json");
+    using var request = new HttpRequestMessage(HttpMethod.Post, AddUri)
+    {
+      Content = content
+    };
+    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", jwt);
+
+    var response = await Client.SendAsync(request, TestContext.Current.CancellationToken);
+
+    await response.Should().BeValidationProblemDetails(new Dictionary<string, string[]>()
+    {
+      ["PlaidAccountId"] = ["The PlaidAccountId field is required."],
+    });
+  }
+
+  [Fact]
+  public async Task Add_WhenCalledWithoutAccountName_ItShouldReturn400WithProblemDetails()
+  {
+    var jwt = JwtTokenBuilder.New()
+      .WithClaim(JwtRegisteredClaimNames.Sub, Guid.NewGuid().ToString())
+      .Build();
+
+    var json = JsonSerializer.Serialize(new
+    {
+      plaidInstitutionId = "institutionId",
+      plaidAccountId = "accountId",
+    });
+    using var content = new StringContent(json, Encoding.UTF8, "application/json");
+    using var request = new HttpRequestMessage(HttpMethod.Post, AddUri)
+    {
+      Content = content
+    };
+    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", jwt);
+
+    var response = await Client.SendAsync(request, TestContext.Current.CancellationToken);
+
+    await response.Should().BeValidationProblemDetails(new Dictionary<string, string[]>()
+    {
+      ["PlaidAccountName"] = ["The PlaidAccountName field is required."],
+    });
+  }
+
+  [Fact]
+  public async Task Add_WhenCalledWithNonExistentUser_ItShouldReturn401WithProblemDetails()
+  {
+    var jwt = JwtTokenBuilder.New()
+      .WithClaim(JwtRegisteredClaimNames.Sub, Guid.NewGuid().ToString())
+      .Build();
+
+    var json = JsonSerializer.Serialize(new
+    {
+      plaidInstitutionId = "id",
+      plaidAccountId = "id",
+      plaidAccountName = "Some Account",
+    });
+    using var content = new StringContent(json, Encoding.UTF8, "application/json");
+    using var request = new HttpRequestMessage(HttpMethod.Post, AddUri)
+    {
+      Content = content,
+    };
+    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", jwt);
+
+    var response = await Client.SendAsync(request, TestContext.Current.CancellationToken);
+
+    await response.Should().BeProblemDetails(HttpStatusCode.Unauthorized);
+  }
+
+  [Fact]
+  public async Task Add_WhenCalledWithPlaidInstitutionIdThatHasNotBeenAdded_ItShouldReturn400WithProblemDetails()
+  {
+    var user = await ExecuteAsync(static async (context, ct, sp) =>
+    {
+      var passwordHasher = sp.GetRequiredService<IPasswordHasher>();
+      var encryptor = sp.GetRequiredService<IEncryptor>();
+
+      var userEncryptionKey = await encryptor.GenerateEncryptedKeyAsync(ct);
+      var user = User.From("User1", passwordHasher.Hash("@Password1"), userEncryptionKey);
+
+
+      await context.AddAsync(user, ct);
+      await context.SaveChangesAsync(ct);
+      return user;
+    }, TestContext.Current.CancellationToken);
+
+    var jwt = JwtTokenBuilder.New()
+      .WithClaim(JwtRegisteredClaimNames.Sub, user.Id.ToString())
+      .Build();
+
+    var json = JsonSerializer.Serialize(new
+    {
+      plaidInstitutionId = "id",
+      plaidAccountId = "id",
+      plaidAccountName = "Some Account",
+    });
+    using var content = new StringContent(json, Encoding.UTF8, "application/json");
+    using var request = new HttpRequestMessage(HttpMethod.Post, AddUri)
+    {
+      Content = content,
+    };
+    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", jwt);
+
+    var response = await Client.SendAsync(request, TestContext.Current.CancellationToken);
+
+    await response.Should().BeValidationProblemDetails(new Dictionary<string, string[]>()
+    {
+      ["PlaidInstitutionId"] = ["The PlaidInstitutionId field is invalid. No institution connected with the given PlaidInstitutionId was found for the user."],
+    });
+  }
+
+  [Fact]
+  public async Task Add_WhenCalledWithPlaidAccountIdThatHasAlreadyBeenAdded_ItShouldReturn409WithProblemDetails()
+  {
+    var (user, institution, account) = await ExecuteAsync(static async (context, ct, sp) =>
+    {
+      var passwordHasher = sp.GetRequiredService<IPasswordHasher>();
+      var encryptor = sp.GetRequiredService<IEncryptor>();
+
+      var userEncryptionKey = await encryptor.GenerateEncryptedKeyAsync(ct);
+      var user = User.From("User1", passwordHasher.Hash("@Password1"), userEncryptionKey);
+
+      await context.AddAsync(user, ct);
+
+      var encryptedAccessToken = await encryptor.EncryptAsyncFor(user, "accessToken", ct);
+      var plaidMetadata = PlaidMetadata.From("alreadyExists", "Some Bank", encryptedAccessToken);
+      var institution = Institution.From("Some Bank", plaidMetadata);
+
+      await context.AddAsync(institution, ct);
+
+      var plaidAccountMetadata = PlaidAccountMetadata.From("accountId", "Some Account");
+      var account = Account.From(institution.Id, "Some Account", plaidAccountMetadata);
+
+      user.AddInstitution(institution);
+      user.AddAccount(account);
+
+      await context.SaveChangesAsync(ct);
+      return (user, institution, account);
+    }, TestContext.Current.CancellationToken);
+
+    var jwt = JwtTokenBuilder.New()
+      .WithClaim(JwtRegisteredClaimNames.Sub, user.Id.ToString())
+      .Build();
+
+    var json = JsonSerializer.Serialize(new
+    {
+      plaidInstitutionId = ((PlaidMetadata)institution.Metadata!).PlaidId,
+      plaidAccountId = ((PlaidAccountMetadata)account.Metadata!).PlaidId,
+      plaidAccountName = ((PlaidAccountMetadata)account.Metadata).PlaidName,
+    });
+    using var content = new StringContent(json, Encoding.UTF8, "application/json");
+    using var request = new HttpRequestMessage(HttpMethod.Post, AddUri)
+    {
+      Content = content,
+    };
+    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", jwt);
+
+    var response = await Client.SendAsync(request, TestContext.Current.CancellationToken);
+
+    await response.Should().BeProblemDetails(HttpStatusCode.Conflict);
+  }
+
+  [Fact]
+  public async Task Add_WhenCalledWithNewAccount_ItShouldReturn200()
+  {
+    var (user, institution) = await ExecuteAsync(async (context, ct, sp) =>
+    {
+      var passwordHasher = sp.GetRequiredService<IPasswordHasher>();
+      var encryptor = sp.GetRequiredService<IEncryptor>();
+      var plaidClient = sp.GetRequiredService<PlaidClient>();
+
+      var userEncryptionKey = await encryptor.GenerateEncryptedKeyAsync(ct);
+      var user = User.From("User1", passwordHasher.Hash("@Password1"), userEncryptionKey);
+
+      var encryptedAccessToken = await encryptor.EncryptAsyncFor(user, "accessToken", ct);
+      var plaidMetadata = PlaidMetadata.From("id", "Some Bank", encryptedAccessToken);
+      var institution = Institution.From("Some Bank", plaidMetadata);
+
+      user.AddInstitution(institution);
+
+      await context.AddAsync(user, ct);
+      await context.SaveChangesAsync(ct);
+      return (user, institution);
+    }, TestContext.Current.CancellationToken);
+
+    var jwt = JwtTokenBuilder.New()
+      .WithClaim(JwtRegisteredClaimNames.Sub, user.Id.ToString())
+      .Build();
+
+    var newAccountId = "newAccountId";
+    var newAccountName = "New Account";
+
+    var json = JsonSerializer.Serialize(new
+    {
+      plaidInstitutionId = ((PlaidMetadata)institution.Metadata!).PlaidId,
+      plaidAccountId = newAccountId,
+      plaidAccountName = newAccountName,
+    });
+    using var content = new StringContent(json, Encoding.UTF8, "application/json");
+    using var request = new HttpRequestMessage(HttpMethod.Post, AddUri)
+    {
+      Content = content,
+    };
+    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", jwt);
+
+    var response = await Client.SendAsync(request, TestContext.Current.CancellationToken);
+
+    response.StatusCode.Should().Be(HttpStatusCode.OK);
+
+    var updatedUser = await ExecuteAsync(
+      async (context, ct) => await context.Set<User>()
+        .Include(u => u.Accounts)
+        .ThenInclude(a => a.Metadata)
+        .FirstAsync(u => u.Id == user.Id, ct),
+      TestContext.Current.CancellationToken
+    );
+
+    updatedUser.Accounts.Should().ContainSingle(
+      a => a.Name == newAccountName &&
+        a.InstitutionId == institution.Id &&
+        a.Metadata is PlaidAccountMetadata &&
+        ((PlaidAccountMetadata)a.Metadata).PlaidId == newAccountId &&
+        ((PlaidAccountMetadata)a.Metadata).PlaidName == newAccountName
+    );
+  }
+}

tests/FiscalOS.API.Tests/Integration/ConnectTests.cs

@@ -136,7 +136,7 @@ public class ConnectTests(TestApi testApi) : IntegrationTest(testApi)
     var json = JsonSerializer.Serialize(new
     {
       publicToken = "token",
-      plaidInstitutionId = ((PlaidMetadata)institution.Metadata).PlaidId,
+      plaidInstitutionId = ((PlaidMetadata)institution.Metadata!).PlaidId,
     });
     using var content = new StringContent(json, Encoding.UTF8, "application/json");
     using var request = new HttpRequestMessage(HttpMethod.Post, ConnectUri)
@@ -167,13 +167,13 @@ public class ConnectTests(TestApi testApi) : IntegrationTest(testApi)
       await context.AddAsync(user, ct);
       await context.SaveChangesAsync(ct);
 
-      var publicToken = await plaidClient.SandboxPublicTokenCreateAsync(new()
+      var publicTokenResponse = await plaidClient.SandboxPublicTokenCreateAsync(new()
       {
         InstitutionId = plaidInstitutionId,
         InitialProducts = [Products.Transactions],
       });
 
-      return (user, publicToken.PublicToken);
+      return (user, publicTokenResponse.PublicToken);
     }, TestContext.Current.CancellationToken);
 
     var jwt = JwtTokenBuilder.New()