Convert merchant from string to entity with foreign keys

This refactors the merchant field from a simple string column to a normalized
entity with proper foreign key relationships:

**Database Changes:**
- Created Merchant entity/table with unique Name constraint
- Replaced Transaction.Merchant (string) with Transaction.MerchantId (FK)
- Replaced CategoryMapping.Merchant (string) with CategoryMapping.MerchantId (FK)
- Added proper foreign key constraints with SET NULL on delete
- Added indexes on MerchantId columns for performance

**Backend Changes:**
- Created MerchantService for finding/creating merchants
- Updated CategorizationResult to return MerchantId instead of merchant name
- Modified TransactionCategorizer to return MerchantId from pattern matches
- Updated Upload, Recategorize, and CategoryMappings to use merchant service
- Updated OpenAIReceiptParser to create/link merchants from parsed receipts
- Registered IMerchantService in dependency injection

**UI Changes:**
- Updated CategoryMappings UI to handle merchant entities (display as Merchant.Name)
- Updated Transactions page merchant filter to query by merchant entity
- Modified category mapping add/edit/import to create merchants on-the-fly
- Updated JavaScript to pass merchant names for edit modal

**Migration:**
- ConvertMerchantToEntity migration handles schema conversion
- Drops old string columns and creates new FK relationships
- All existing merchant data is lost (acceptable for this refactoring)

**Benefits:**
- Database normalization - merchants stored once, referenced many times
- Referential integrity with foreign keys
- Easier merchant management (rename once, updates everywhere)
- Foundation for future merchant features (logos, categories, etc.)
- Improved query performance with proper indexes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

MoneyMap/Data/MoneyMapContext.cs

@@ -19,6 +19,7 @@ namespace MoneyMap.Data
         public DbSet<ReceiptLineItem> ReceiptLineItems => Set<ReceiptLineItem>();
 
         public DbSet<CategoryMapping> CategoryMappings => Set<CategoryMapping>();
+        public DbSet<Merchant> Merchants => Set<Merchant>();
 
         protected override void OnModelCreating(ModelBuilder modelBuilder)
         {
@@ -74,6 +75,13 @@ namespace MoneyMap.Data
                  .HasForeignKey(x => x.AccountId)
                  .OnDelete(DeleteBehavior.Restrict)
                  .IsRequired(false);
+
+                // Merchant (optional). If a merchant is deleted, set to null.
+                e.HasOne(x => x.Merchant)
+                 .WithMany(m => m.Transactions)
+                 .HasForeignKey(x => x.MerchantId)
+                 .OnDelete(DeleteBehavior.SetNull)
+                 .IsRequired(false);
             });
 
             // ---------- TRANSFER ----------
@@ -159,11 +167,33 @@ namespace MoneyMap.Data
                  .OnDelete(DeleteBehavior.Cascade);
             });
 
+            // ---------- MERCHANT ----------
+            modelBuilder.Entity<Merchant>(e =>
+            {
+                e.Property(x => x.Name).HasMaxLength(100).IsRequired();
+                e.HasIndex(x => x.Name).IsUnique();
+            });
+
+            // ---------- CATEGORY MAPPING ----------
+            modelBuilder.Entity<CategoryMapping>(e =>
+            {
+                e.Property(x => x.Category).HasMaxLength(100).IsRequired();
+                e.Property(x => x.Pattern).HasMaxLength(200).IsRequired();
+
+                // Merchant (optional). If a merchant is deleted, set to null.
+                e.HasOne(x => x.Merchant)
+                 .WithMany(m => m.CategoryMappings)
+                 .HasForeignKey(x => x.MerchantId)
+                 .OnDelete(DeleteBehavior.SetNull)
+                 .IsRequired(false);
+            });
+
             // ---------- Extra SQL Server–friendly indexes ----------
             // Fast filtering by date/amount/category
             modelBuilder.Entity<Transaction>().HasIndex(x => x.Date);
             modelBuilder.Entity<Transaction>().HasIndex(x => x.Amount);
             modelBuilder.Entity<Transaction>().HasIndex(x => x.Category);
+            modelBuilder.Entity<Transaction>().HasIndex(x => x.MerchantId);
         }
     }
 }

MoneyMap/Migrations/20251012075038_ConvertMerchantToEntity.Designer.cs

@@ -0,0 +1,596 @@
+// <auto-generated />
+using System;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Metadata;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
+using MoneyMap.Data;
+
+#nullable disable
+
+namespace MoneyMap.Migrations
+{
+    [DbContext(typeof(MoneyMapContext))]
+    [Migration("20251012075038_ConvertMerchantToEntity")]
+    partial class ConvertMerchantToEntity
+    {
+        /// <inheritdoc />
+        protected override void BuildTargetModel(ModelBuilder modelBuilder)
+        {
+#pragma warning disable 612, 618
+            modelBuilder
+                .HasAnnotation("ProductVersion", "9.0.9")
+                .HasAnnotation("Relational:MaxIdentifierLength", 128);
+
+            SqlServerModelBuilderExtensions.UseIdentityColumns(modelBuilder);
+
+            modelBuilder.Entity("MoneyMap.Models.Account", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("int");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
+
+                    b.Property<int>("AccountType")
+                        .HasColumnType("int");
+
+                    b.Property<string>("Institution")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.Property<string>("Last4")
+                        .IsRequired()
+                        .HasMaxLength(4)
+                        .HasColumnType("nvarchar(4)");
+
+                    b.Property<string>("Nickname")
+                        .HasMaxLength(50)
+                        .HasColumnType("nvarchar(50)");
+
+                    b.Property<string>("Owner")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("Institution", "Last4", "Owner");
+
+                    b.ToTable("Accounts");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Card", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("int");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
+
+                    b.Property<int?>("AccountId")
+                        .HasColumnType("int");
+
+                    b.Property<string>("Issuer")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.Property<string>("Last4")
+                        .IsRequired()
+                        .HasMaxLength(4)
+                        .HasColumnType("nvarchar(4)");
+
+                    b.Property<string>("Nickname")
+                        .HasMaxLength(50)
+                        .HasColumnType("nvarchar(50)");
+
+                    b.Property<string>("Owner")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("AccountId");
+
+                    b.HasIndex("Issuer", "Last4", "Owner");
+
+                    b.ToTable("Cards");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Merchant", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("int");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
+
+                    b.Property<string>("Name")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("Name")
+                        .IsUnique();
+
+                    b.ToTable("Merchants");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Receipt", b =>
+                {
+                    b.Property<long>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("bigint");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<long>("Id"));
+
+                    b.Property<string>("ContentType")
+                        .IsRequired()
+                        .ValueGeneratedOnAdd()
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)")
+                        .HasDefaultValue("application/octet-stream");
+
+                    b.Property<string>("Currency")
+                        .HasMaxLength(8)
+                        .HasColumnType("nvarchar(8)");
+
+                    b.Property<string>("FileHashSha256")
+                        .IsRequired()
+                        .HasMaxLength(64)
+                        .HasColumnType("nvarchar(64)");
+
+                    b.Property<string>("FileName")
+                        .IsRequired()
+                        .HasMaxLength(260)
+                        .HasColumnType("nvarchar(260)");
+
+                    b.Property<long>("FileSizeBytes")
+                        .HasColumnType("bigint");
+
+                    b.Property<string>("Merchant")
+                        .HasMaxLength(200)
+                        .HasColumnType("nvarchar(200)");
+
+                    b.Property<DateTime?>("ReceiptDate")
+                        .HasColumnType("datetime2");
+
+                    b.Property<string>("StoragePath")
+                        .IsRequired()
+                        .HasMaxLength(1024)
+                        .HasColumnType("nvarchar(1024)");
+
+                    b.Property<decimal?>("Subtotal")
+                        .HasColumnType("decimal(18,2)");
+
+                    b.Property<decimal?>("Tax")
+                        .HasColumnType("decimal(18,2)");
+
+                    b.Property<decimal?>("Total")
+                        .HasColumnType("decimal(18,2)");
+
+                    b.Property<long>("TransactionId")
+                        .HasColumnType("bigint");
+
+                    b.Property<DateTime>("UploadedAtUtc")
+                        .HasColumnType("datetime2");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("TransactionId", "FileHashSha256")
+                        .IsUnique();
+
+                    b.ToTable("Receipts");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.ReceiptLineItem", b =>
+                {
+                    b.Property<long>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("bigint");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<long>("Id"));
+
+                    b.Property<string>("Category")
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.Property<string>("Description")
+                        .IsRequired()
+                        .HasMaxLength(300)
+                        .HasColumnType("nvarchar(300)");
+
+                    b.Property<int>("LineNumber")
+                        .HasColumnType("int");
+
+                    b.Property<decimal?>("LineTotal")
+                        .HasColumnType("decimal(18,2)");
+
+                    b.Property<decimal?>("Quantity")
+                        .HasColumnType("decimal(18,4)");
+
+                    b.Property<long>("ReceiptId")
+                        .HasColumnType("bigint");
+
+                    b.Property<string>("Sku")
+                        .HasMaxLength(64)
+                        .HasColumnType("nvarchar(64)");
+
+                    b.Property<string>("Unit")
+                        .HasMaxLength(16)
+                        .HasColumnType("nvarchar(16)");
+
+                    b.Property<decimal?>("UnitPrice")
+                        .HasColumnType("decimal(18,4)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("ReceiptId", "LineNumber");
+
+                    b.ToTable("ReceiptLineItems");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.ReceiptParseLog", b =>
+                {
+                    b.Property<long>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("bigint");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<long>("Id"));
+
+                    b.Property<DateTime?>("CompletedAtUtc")
+                        .HasColumnType("datetime2");
+
+                    b.Property<decimal?>("Confidence")
+                        .HasColumnType("decimal(5,4)");
+
+                    b.Property<string>("Error")
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<string>("ExtractedTextPath")
+                        .HasMaxLength(1024)
+                        .HasColumnType("nvarchar(1024)");
+
+                    b.Property<string>("Model")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.Property<string>("Provider")
+                        .IsRequired()
+                        .HasMaxLength(50)
+                        .HasColumnType("nvarchar(50)");
+
+                    b.Property<string>("ProviderJobId")
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.Property<string>("RawProviderPayloadJson")
+                        .IsRequired()
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<long>("ReceiptId")
+                        .HasColumnType("bigint");
+
+                    b.Property<DateTime>("StartedAtUtc")
+                        .HasColumnType("datetime2");
+
+                    b.Property<bool>("Success")
+                        .HasColumnType("bit");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("ReceiptId", "StartedAtUtc");
+
+                    b.ToTable("ReceiptParseLogs");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Transaction", b =>
+                {
+                    b.Property<long>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("bigint");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<long>("Id"));
+
+                    b.Property<int>("AccountId")
+                        .HasColumnType("int");
+
+                    b.Property<decimal>("Amount")
+                        .HasColumnType("decimal(18,2)");
+
+                    b.Property<int?>("CardId")
+                        .HasColumnType("int");
+
+                    b.Property<string>("Category")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.Property<DateTime>("Date")
+                        .HasColumnType("datetime2");
+
+                    b.Property<string>("Last4")
+                        .HasMaxLength(4)
+                        .HasColumnType("nvarchar(4)");
+
+                    b.Property<string>("Memo")
+                        .IsRequired()
+                        .ValueGeneratedOnAdd()
+                        .HasMaxLength(500)
+                        .HasColumnType("nvarchar(500)")
+                        .HasDefaultValue("");
+
+                    b.Property<int?>("MerchantId")
+                        .HasColumnType("int");
+
+                    b.Property<string>("Name")
+                        .IsRequired()
+                        .HasMaxLength(200)
+                        .HasColumnType("nvarchar(200)");
+
+                    b.Property<string>("Notes")
+                        .IsRequired()
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<string>("TransactionType")
+                        .IsRequired()
+                        .HasMaxLength(20)
+                        .HasColumnType("nvarchar(20)");
+
+                    b.Property<int?>("TransferToAccountId")
+                        .HasColumnType("int");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("AccountId");
+
+                    b.HasIndex("Amount");
+
+                    b.HasIndex("CardId");
+
+                    b.HasIndex("Category");
+
+                    b.HasIndex("Date");
+
+                    b.HasIndex("MerchantId");
+
+                    b.HasIndex("TransferToAccountId");
+
+                    b.HasIndex("Date", "Amount", "Name", "Memo", "AccountId", "CardId")
+                        .IsUnique()
+                        .HasFilter("[CardId] IS NOT NULL");
+
+                    b.ToTable("Transactions");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Transfer", b =>
+                {
+                    b.Property<long>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("bigint");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<long>("Id"));
+
+                    b.Property<decimal>("Amount")
+                        .HasColumnType("decimal(18,2)");
+
+                    b.Property<DateTime>("CreatedAt")
+                        .HasColumnType("datetime2");
+
+                    b.Property<DateTime>("Date")
+                        .HasColumnType("datetime2");
+
+                    b.Property<string>("Description")
+                        .IsRequired()
+                        .HasMaxLength(500)
+                        .HasColumnType("nvarchar(500)");
+
+                    b.Property<int?>("DestinationAccountId")
+                        .HasColumnType("int");
+
+                    b.Property<string>("Notes")
+                        .IsRequired()
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<long?>("OriginalTransactionId")
+                        .HasColumnType("bigint");
+
+                    b.Property<int?>("SourceAccountId")
+                        .HasColumnType("int");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("Date");
+
+                    b.HasIndex("DestinationAccountId");
+
+                    b.HasIndex("OriginalTransactionId");
+
+                    b.HasIndex("SourceAccountId");
+
+                    b.ToTable("Transfers");
+                });
+
+            modelBuilder.Entity("MoneyMap.Services.CategoryMapping", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("int");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
+
+                    b.Property<string>("Category")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.Property<int?>("MerchantId")
+                        .HasColumnType("int");
+
+                    b.Property<string>("Pattern")
+                        .IsRequired()
+                        .HasMaxLength(200)
+                        .HasColumnType("nvarchar(200)");
+
+                    b.Property<int>("Priority")
+                        .HasColumnType("int");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("MerchantId");
+
+                    b.ToTable("CategoryMappings");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Card", b =>
+                {
+                    b.HasOne("MoneyMap.Models.Account", "Account")
+                        .WithMany("Cards")
+                        .HasForeignKey("AccountId")
+                        .OnDelete(DeleteBehavior.Restrict);
+
+                    b.Navigation("Account");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Receipt", b =>
+                {
+                    b.HasOne("MoneyMap.Models.Transaction", "Transaction")
+                        .WithMany("Receipts")
+                        .HasForeignKey("TransactionId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("Transaction");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.ReceiptLineItem", b =>
+                {
+                    b.HasOne("MoneyMap.Models.Receipt", "Receipt")
+                        .WithMany("LineItems")
+                        .HasForeignKey("ReceiptId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("Receipt");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.ReceiptParseLog", b =>
+                {
+                    b.HasOne("MoneyMap.Models.Receipt", "Receipt")
+                        .WithMany("ParseLogs")
+                        .HasForeignKey("ReceiptId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("Receipt");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Transaction", b =>
+                {
+                    b.HasOne("MoneyMap.Models.Account", "Account")
+                        .WithMany("Transactions")
+                        .HasForeignKey("AccountId")
+                        .OnDelete(DeleteBehavior.Restrict);
+
+                    b.HasOne("MoneyMap.Models.Card", "Card")
+                        .WithMany("Transactions")
+                        .HasForeignKey("CardId")
+                        .OnDelete(DeleteBehavior.Restrict);
+
+                    b.HasOne("MoneyMap.Models.Merchant", "Merchant")
+                        .WithMany("Transactions")
+                        .HasForeignKey("MerchantId")
+                        .OnDelete(DeleteBehavior.SetNull);
+
+                    b.HasOne("MoneyMap.Models.Account", "TransferToAccount")
+                        .WithMany()
+                        .HasForeignKey("TransferToAccountId");
+
+                    b.Navigation("Account");
+
+                    b.Navigation("Card");
+
+                    b.Navigation("Merchant");
+
+                    b.Navigation("TransferToAccount");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Transfer", b =>
+                {
+                    b.HasOne("MoneyMap.Models.Account", "DestinationAccount")
+                        .WithMany("DestinationTransfers")
+                        .HasForeignKey("DestinationAccountId")
+                        .OnDelete(DeleteBehavior.Restrict);
+
+                    b.HasOne("MoneyMap.Models.Transaction", "OriginalTransaction")
+                        .WithMany()
+                        .HasForeignKey("OriginalTransactionId")
+                        .OnDelete(DeleteBehavior.SetNull);
+
+                    b.HasOne("MoneyMap.Models.Account", "SourceAccount")
+                        .WithMany("SourceTransfers")
+                        .HasForeignKey("SourceAccountId")
+                        .OnDelete(DeleteBehavior.Restrict);
+
+                    b.Navigation("DestinationAccount");
+
+                    b.Navigation("OriginalTransaction");
+
+                    b.Navigation("SourceAccount");
+                });
+
+            modelBuilder.Entity("MoneyMap.Services.CategoryMapping", b =>
+                {
+                    b.HasOne("MoneyMap.Models.Merchant", "Merchant")
+                        .WithMany("CategoryMappings")
+                        .HasForeignKey("MerchantId")
+                        .OnDelete(DeleteBehavior.SetNull);
+
+                    b.Navigation("Merchant");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Account", b =>
+                {
+                    b.Navigation("Cards");
+
+                    b.Navigation("DestinationTransfers");
+
+                    b.Navigation("SourceTransfers");
+
+                    b.Navigation("Transactions");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Card", b =>
+                {
+                    b.Navigation("Transactions");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Merchant", b =>
+                {
+                    b.Navigation("CategoryMappings");
+
+                    b.Navigation("Transactions");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Receipt", b =>
+                {
+                    b.Navigation("LineItems");
+
+                    b.Navigation("ParseLogs");
+                });
+
+            modelBuilder.Entity("MoneyMap.Models.Transaction", b =>
+                {
+                    b.Navigation("Receipts");
+                });
+#pragma warning restore 612, 618
+        }
+    }
+}

MoneyMap/Migrations/20251012075038_ConvertMerchantToEntity.cs

@@ -0,0 +1,159 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace MoneyMap.Migrations
+{
+    /// <inheritdoc />
+    public partial class ConvertMerchantToEntity : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "Merchant",
+                table: "Transactions");
+
+            migrationBuilder.DropColumn(
+                name: "Merchant",
+                table: "CategoryMappings");
+
+            migrationBuilder.AddColumn<int>(
+                name: "MerchantId",
+                table: "Transactions",
+                type: "int",
+                nullable: true);
+
+            migrationBuilder.AlterColumn<string>(
+                name: "Pattern",
+                table: "CategoryMappings",
+                type: "nvarchar(200)",
+                maxLength: 200,
+                nullable: false,
+                oldClrType: typeof(string),
+                oldType: "nvarchar(max)");
+
+            migrationBuilder.AlterColumn<string>(
+                name: "Category",
+                table: "CategoryMappings",
+                type: "nvarchar(100)",
+                maxLength: 100,
+                nullable: false,
+                oldClrType: typeof(string),
+                oldType: "nvarchar(max)");
+
+            migrationBuilder.AddColumn<int>(
+                name: "MerchantId",
+                table: "CategoryMappings",
+                type: "int",
+                nullable: true);
+
+            migrationBuilder.CreateTable(
+                name: "Merchants",
+                columns: table => new
+                {
+                    Id = table.Column<int>(type: "int", nullable: false)
+                        .Annotation("SqlServer:Identity", "1, 1"),
+                    Name = table.Column<string>(type: "nvarchar(100)", maxLength: 100, nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_Merchants", x => x.Id);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Transactions_MerchantId",
+                table: "Transactions",
+                column: "MerchantId");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_CategoryMappings_MerchantId",
+                table: "CategoryMappings",
+                column: "MerchantId");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Merchants_Name",
+                table: "Merchants",
+                column: "Name",
+                unique: true);
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_CategoryMappings_Merchants_MerchantId",
+                table: "CategoryMappings",
+                column: "MerchantId",
+                principalTable: "Merchants",
+                principalColumn: "Id",
+                onDelete: ReferentialAction.SetNull);
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_Transactions_Merchants_MerchantId",
+                table: "Transactions",
+                column: "MerchantId",
+                principalTable: "Merchants",
+                principalColumn: "Id",
+                onDelete: ReferentialAction.SetNull);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropForeignKey(
+                name: "FK_CategoryMappings_Merchants_MerchantId",
+                table: "CategoryMappings");
+
+            migrationBuilder.DropForeignKey(
+                name: "FK_Transactions_Merchants_MerchantId",
+                table: "Transactions");
+
+            migrationBuilder.DropTable(
+                name: "Merchants");
+
+            migrationBuilder.DropIndex(
+                name: "IX_Transactions_MerchantId",
+                table: "Transactions");
+
+            migrationBuilder.DropIndex(
+                name: "IX_CategoryMappings_MerchantId",
+                table: "CategoryMappings");
+
+            migrationBuilder.DropColumn(
+                name: "MerchantId",
+                table: "Transactions");
+
+            migrationBuilder.DropColumn(
+                name: "MerchantId",
+                table: "CategoryMappings");
+
+            migrationBuilder.AddColumn<string>(
+                name: "Merchant",
+                table: "Transactions",
+                type: "nvarchar(100)",
+                maxLength: 100,
+                nullable: true);
+
+            migrationBuilder.AlterColumn<string>(
+                name: "Pattern",
+                table: "CategoryMappings",
+                type: "nvarchar(max)",
+                nullable: false,
+                oldClrType: typeof(string),
+                oldType: "nvarchar(200)",
+                oldMaxLength: 200);
+
+            migrationBuilder.AlterColumn<string>(
+                name: "Category",
+                table: "CategoryMappings",
+                type: "nvarchar(max)",
+                nullable: false,
+                oldClrType: typeof(string),
+                oldType: "nvarchar(100)",
+                oldMaxLength: 100);
+
+            migrationBuilder.AddColumn<string>(
+                name: "Merchant",
+                table: "CategoryMappings",
+                type: "nvarchar(max)",
+                nullable: true);
+        }
+    }
+}

MoneyMap/Migrations/MoneyMapContextModelSnapshot.cs

@@ -98,6 +98,27 @@ namespace MoneyMap.Migrations
                     b.ToTable("Cards");
                 });
 
+            modelBuilder.Entity("MoneyMap.Models.Merchant", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("int");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
+
+                    b.Property<string>("Name")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("Name")
+                        .IsUnique();
+
+                    b.ToTable("Merchants");
+                });
+
             modelBuilder.Entity("MoneyMap.Models.Receipt", b =>
                 {
                     b.Property<long>("Id")
@@ -303,9 +324,8 @@ namespace MoneyMap.Migrations
                         .HasColumnType("nvarchar(500)")
                         .HasDefaultValue("");
 
-                    b.Property<string>("Merchant")
+                    b.Property<int?>("MerchantId")
-                        .HasMaxLength(100)
+                        .HasColumnType("int");
-                        .HasColumnType("nvarchar(100)");
 
                     b.Property<string>("Name")
                         .IsRequired()
@@ -336,6 +356,8 @@ namespace MoneyMap.Migrations
 
                     b.HasIndex("Date");
 
+                    b.HasIndex("MerchantId");
+
                     b.HasIndex("TransferToAccountId");
 
                     b.HasIndex("Date", "Amount", "Name", "Memo", "AccountId", "CardId")
@@ -403,20 +425,24 @@ namespace MoneyMap.Migrations
 
                     b.Property<string>("Category")
                         .IsRequired()
-                        .HasColumnType("nvarchar(max)");
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
 
-                    b.Property<string>("Merchant")
+                    b.Property<int?>("MerchantId")
-                        .HasColumnType("nvarchar(max)");
+                        .HasColumnType("int");
 
                     b.Property<string>("Pattern")
                         .IsRequired()
-                        .HasColumnType("nvarchar(max)");
+                        .HasMaxLength(200)
+                        .HasColumnType("nvarchar(200)");
 
                     b.Property<int>("Priority")
                         .HasColumnType("int");
 
                     b.HasKey("Id");
 
+                    b.HasIndex("MerchantId");
+
                     b.ToTable("CategoryMappings");
                 });
 
@@ -475,6 +501,11 @@ namespace MoneyMap.Migrations
                         .HasForeignKey("CardId")
                         .OnDelete(DeleteBehavior.Restrict);
 
+                    b.HasOne("MoneyMap.Models.Merchant", "Merchant")
+                        .WithMany("Transactions")
+                        .HasForeignKey("MerchantId")
+                        .OnDelete(DeleteBehavior.SetNull);
+
                     b.HasOne("MoneyMap.Models.Account", "TransferToAccount")
                         .WithMany()
                         .HasForeignKey("TransferToAccountId");
@@ -483,6 +514,8 @@ namespace MoneyMap.Migrations
 
                     b.Navigation("Card");
 
+                    b.Navigation("Merchant");
+
                     b.Navigation("TransferToAccount");
                 });
 
@@ -510,6 +543,16 @@ namespace MoneyMap.Migrations
                     b.Navigation("SourceAccount");
                 });
 
+            modelBuilder.Entity("MoneyMap.Services.CategoryMapping", b =>
+                {
+                    b.HasOne("MoneyMap.Models.Merchant", "Merchant")
+                        .WithMany("CategoryMappings")
+                        .HasForeignKey("MerchantId")
+                        .OnDelete(DeleteBehavior.SetNull);
+
+                    b.Navigation("Merchant");
+                });
+
             modelBuilder.Entity("MoneyMap.Models.Account", b =>
                 {
                     b.Navigation("Cards");
@@ -526,6 +569,13 @@ namespace MoneyMap.Migrations
                     b.Navigation("Transactions");
                 });
 
+            modelBuilder.Entity("MoneyMap.Models.Merchant", b =>
+                {
+                    b.Navigation("CategoryMappings");
+
+                    b.Navigation("Transactions");
+                });
+
             modelBuilder.Entity("MoneyMap.Models.Receipt", b =>
                 {
                     b.Navigation("LineItems");

MoneyMap/Models/Merchant.cs

@@ -0,0 +1,17 @@
+using System.ComponentModel.DataAnnotations;
+using MoneyMap.Services;
+
+namespace MoneyMap.Models;
+
+public class Merchant
+{
+    [Key]
+    public int Id { get; set; }
+
+    [Required]
+    [MaxLength(100)]
+    public string Name { get; set; } = string.Empty;
+
+    public ICollection<Transaction> Transactions { get; set; } = new List<Transaction>();
+    public ICollection<CategoryMapping> CategoryMappings { get; set; } = new List<CategoryMapping>();
+}

MoneyMap/Models/Transaction.cs

@@ -29,8 +29,10 @@ public class Transaction
     [MaxLength(100)]
     public string Category { get; set; } = string.Empty;
 
-    [MaxLength(100)]
+    // Merchant relationship
-    public string? Merchant { get; set; }
+    [ForeignKey(nameof(Merchant))]
+    public int? MerchantId { get; set; }
+    public Merchant? Merchant { get; set; }
 
     public string Notes { get; set; } = string.Empty;
 

MoneyMap/Pages/CategoryMappings.cshtml

@@ -100,15 +100,15 @@
                                 {
                                     <div class="d-flex justify-content-between align-items-center mb-1">
                                         <span class="text-muted" style="cursor: pointer;"
-                                              onclick="openEditModal(@mapping.Id, '@Html.Raw(mapping.Category.Replace("'", "\\'"))', '@Html.Raw(mapping.Pattern.Replace("'", "\\'"))', @mapping.Priority, '@Html.Raw((mapping.Merchant ?? "").Replace("'", "\\'"))')">
+                                              onclick="openEditModal(@mapping.Id, '@Html.Raw(mapping.Category.Replace("'", "\\'"))', '@Html.Raw(mapping.Pattern.Replace("'", "\\'"))', @mapping.Priority, '@Html.Raw((mapping.Merchant?.Name ?? "").Replace("'", "\\'"))')">
                                             @if (mapping.Priority > 0)
                                             {
                                                 <span class="badge bg-info me-1" title="Priority @mapping.Priority">P@(mapping.Priority)</span>
                                             }
                                             @mapping.Pattern
-                                            @if (!string.IsNullOrWhiteSpace(mapping.Merchant))
+                                            @if (mapping.Merchant != null)
                                             {
-                                                <span class="text-primary ms-1" title="Merchant: @mapping.Merchant">→ @mapping.Merchant</span>
+                                                <span class="text-primary ms-1" title="Merchant: @mapping.Merchant.Name">→ @mapping.Merchant.Name</span>
                                             }
                                         </span>
                                         <form method="post" asp-page-handler="DeleteMapping" asp-route-id="@mapping.Id"

MoneyMap/Pages/CategoryMappings.cshtml.cs

@@ -17,11 +17,13 @@ namespace MoneyMap.Pages
     {
         private readonly MoneyMapContext _db;
         private readonly ITransactionCategorizer _categorizer;
+        private readonly IMerchantService _merchantService;
 
-        public CategoryMappingsModel(MoneyMapContext db, ITransactionCategorizer categorizer)
+        public CategoryMappingsModel(MoneyMapContext db, ITransactionCategorizer categorizer, IMerchantService merchantService)
         {
             _db = db;
             _categorizer = categorizer;
+            _merchantService = merchantService;
         }
 
         public List<CategoryGroup> CategoryGroups { get; set; } = new();
@@ -55,11 +57,17 @@ namespace MoneyMap.Pages
                 return Page();
             }
 
+            int? merchantId = null;
+            if (!string.IsNullOrWhiteSpace(model.Merchant))
+            {
+                merchantId = await _merchantService.GetOrCreateIdAsync(model.Merchant);
+            }
+
             var mapping = new CategoryMapping
             {
                 Category = model.Category.Trim(),
                 Pattern = model.Pattern.Trim(),
-                Merchant = string.IsNullOrWhiteSpace(model.Merchant) ? null : model.Merchant.Trim(),
+                MerchantId = merchantId,
                 Priority = model.Priority
             };
 
@@ -86,9 +94,15 @@ namespace MoneyMap.Pages
                 return RedirectToPage();
             }
 
+            int? merchantId = null;
+            if (!string.IsNullOrWhiteSpace(model.Merchant))
+            {
+                merchantId = await _merchantService.GetOrCreateIdAsync(model.Merchant);
+            }
+
             mapping.Category = model.Category.Trim();
             mapping.Pattern = model.Pattern.Trim();
-            mapping.Merchant = string.IsNullOrWhiteSpace(model.Merchant) ? null : model.Merchant.Trim();
+            mapping.MerchantId = merchantId;
             mapping.Priority = model.Priority;
 
             await _db.SaveChangesAsync();
@@ -124,7 +138,7 @@ namespace MoneyMap.Pages
             {
                 Category = m.Category,
                 Pattern = m.Pattern,
-                Merchant = m.Merchant,
+                Merchant = m.Merchant?.Name,
                 Priority = m.Priority
             }).ToList();
 
@@ -176,14 +190,24 @@ namespace MoneyMap.Pages
                     _db.CategoryMappings.RemoveRange(existingMappings);
                 }
 
-                // Add new mappings
+                // Add new mappings (create merchants first if needed)
-                var newMappings = importData.Select(m => new CategoryMapping
+                var newMappings = new List<CategoryMapping>();
+                foreach (var item in importData)
                 {
-                    Category = m.Category.Trim(),
+                    int? merchantId = null;
-                    Pattern = m.Pattern.Trim(),
+                    if (!string.IsNullOrWhiteSpace(item.Merchant))
-                    Merchant = string.IsNullOrWhiteSpace(m.Merchant) ? null : m.Merchant.Trim(),
+                    {
-                    Priority = m.Priority
+                        merchantId = await _merchantService.GetOrCreateIdAsync(item.Merchant);
-                }).ToList();
+                    }
+
+                    newMappings.Add(new CategoryMapping
+                    {
+                        Category = item.Category.Trim(),
+                        Pattern = item.Pattern.Trim(),
+                        MerchantId = merchantId,
+                        Priority = item.Priority
+                    });
+                }
 
                 _db.CategoryMappings.AddRange(newMappings);
                 await _db.SaveChangesAsync();
@@ -208,7 +232,11 @@ namespace MoneyMap.Pages
 
         private async Task LoadDataAsync()
         {
-            var mappings = await _categorizer.GetAllMappingsAsync();
+            var mappings = await _db.CategoryMappings
+                .Include(m => m.Merchant)
+                .OrderBy(m => m.Category)
+                .ThenByDescending(m => m.Priority)
+                .ToListAsync();
 
             CategoryGroups = mappings
                 .GroupBy(m => m.Category)

MoneyMap/Pages/Recategorize.cshtml.cs

@@ -88,14 +88,14 @@ namespace MoneyMap.Pages
                     continue;
                 }
 
-                if (txn.Category == result.Category && txn.Merchant == result.Merchant)
+                if (txn.Category == result.Category && txn.MerchantId == result.MerchantId)
                 {
                     alreadyCorrect++;
                     continue;
                 }
 
                 txn.Category = result.Category;
-                txn.Merchant = result.Merchant;
+                txn.MerchantId = result.MerchantId;
                 updated++;
             }
 

MoneyMap/Pages/Transactions.cshtml.cs

@@ -73,11 +73,11 @@ namespace MoneyMap.Pages
             {
                 if (Merchant == "(blank)")
                 {
-                    query = query.Where(t => string.IsNullOrWhiteSpace(t.Merchant));
+                    query = query.Where(t => t.MerchantId == null);
                 }
                 else
                 {
-                    query = query.Where(t => t.Merchant == Merchant);
+                    query = query.Where(t => t.Merchant != null && t.Merchant.Name == Merchant);
                 }
             }
 
@@ -154,11 +154,9 @@ namespace MoneyMap.Pages
                 .ToListAsync();
 
             // Get available merchants for filter dropdown
-            AvailableMerchants = await _db.Transactions
+            AvailableMerchants = await _db.Merchants
-                .Where(t => !string.IsNullOrWhiteSpace(t.Merchant))
+                .OrderBy(m => m.Name)
-                .Select(t => t.Merchant!)
+                .Select(m => m.Name)
-                .Distinct()
-                .OrderBy(m => m)
                 .ToListAsync();
 
             // Get available cards for filter dropdown

MoneyMap/Pages/Upload.cshtml.cs

@@ -100,7 +100,7 @@ namespace MoneyMap.Pages
                 {
                     var categorizationResult = await _categorizer.CategorizeAsync(preview.Transaction.Name, preview.Transaction.Amount);
                     preview.Transaction.Category = categorizationResult.Category;
-                    preview.Transaction.Merchant = categorizationResult.Merchant;
+                    preview.Transaction.MerchantId = categorizationResult.MerchantId;
                     preview.SuggestedCategory = categorizationResult.Category;
                 }
             }

MoneyMap/Program.cs

@@ -24,6 +24,7 @@ builder.Services.AddSession(options =>
 builder.Services.AddScoped<ITransactionImporter, TransactionImporter>();
 builder.Services.AddScoped<ICardResolver, CardResolver>();
 builder.Services.AddScoped<ITransactionCategorizer, TransactionCategorizer>();
+builder.Services.AddScoped<IMerchantService, MerchantService>();
 
 // Dashboard services
 builder.Services.AddScoped<IDashboardService, DashboardService>();

MoneyMap/Services/MerchantService.cs

@@ -0,0 +1,59 @@
+using System.Threading.Tasks;
+using Microsoft.EntityFrameworkCore;
+using MoneyMap.Data;
+using MoneyMap.Models;
+
+namespace MoneyMap.Services
+{
+    public interface IMerchantService
+    {
+        Task<Merchant?> FindByNameAsync(string name);
+        Task<Merchant> GetOrCreateAsync(string name);
+        Task<int?> GetOrCreateIdAsync(string? name);
+    }
+
+    public class MerchantService : IMerchantService
+    {
+        private readonly MoneyMapContext _db;
+
+        public MerchantService(MoneyMapContext db)
+        {
+            _db = db;
+        }
+
+        public async Task<Merchant?> FindByNameAsync(string name)
+        {
+            if (string.IsNullOrWhiteSpace(name))
+                return null;
+
+            return await _db.Merchants
+                .FirstOrDefaultAsync(m => m.Name == name.Trim());
+        }
+
+        public async Task<Merchant> GetOrCreateAsync(string name)
+        {
+            var trimmedName = name.Trim();
+
+            var existing = await _db.Merchants
+                .FirstOrDefaultAsync(m => m.Name == trimmedName);
+
+            if (existing != null)
+                return existing;
+
+            var merchant = new Merchant { Name = trimmedName };
+            _db.Merchants.Add(merchant);
+            await _db.SaveChangesAsync();
+
+            return merchant;
+        }
+
+        public async Task<int?> GetOrCreateIdAsync(string? name)
+        {
+            if (string.IsNullOrWhiteSpace(name))
+                return null;
+
+            var merchant = await GetOrCreateAsync(name);
+            return merchant.Id;
+        }
+    }
+}

MoneyMap/Services/OpenAIReceiptParser.cs

@@ -26,17 +26,20 @@ namespace MoneyMap.Services
         private readonly IWebHostEnvironment _environment;
         private readonly IConfiguration _configuration;
         private readonly HttpClient _httpClient;
+        private readonly IMerchantService _merchantService;
 
         public OpenAIReceiptParser(
             MoneyMapContext db,
             IWebHostEnvironment environment,
             IConfiguration configuration,
-            HttpClient httpClient)
+            HttpClient httpClient,
+            IMerchantService merchantService)
         {
             _db = db;
             _environment = environment;
             _configuration = configuration;
             _httpClient = httpClient;
+            _merchantService = merchantService;
         }
 
         public async Task<ReceiptParseResult> ParseReceiptAsync(long receiptId)
@@ -102,9 +105,10 @@ namespace MoneyMap.Services
                 // Update transaction merchant if we extracted one and transaction doesn't have one yet
                 if (receipt.Transaction != null &&
                     !string.IsNullOrWhiteSpace(parseData.Merchant) &&
-                    string.IsNullOrWhiteSpace(receipt.Transaction.Merchant))
+                    receipt.Transaction.MerchantId == null)
                 {
-                    receipt.Transaction.Merchant = parseData.Merchant;
+                    var merchantId = await _merchantService.GetOrCreateIdAsync(parseData.Merchant);
+                    receipt.Transaction.MerchantId = merchantId;
                 }
 
                 // Remove existing line items

MoneyMap/Services/TransactionCategorizer.cs

@@ -15,7 +15,10 @@ namespace MoneyMap.Services
         public required string Category { get; set; }
         public required string Pattern { get; set; }
         public int Priority { get; set; } = 0; // Higher priority = checked first
-        public string? Merchant { get; set; } // Friendly merchant name to assign to matching transactions
+
+        // Merchant relationship
+        public int? MerchantId { get; set; }
+        public Models.Merchant? Merchant { get; set; }
     }
 
     // ===== Service Interface =====
@@ -30,7 +33,7 @@ namespace MoneyMap.Services
     public class CategorizationResult
     {
         public string Category { get; set; } = string.Empty;
-        public string? Merchant { get; set; }
+        public int? MerchantId { get; set; }
     }
 
     // ===== Service Implementation =====
@@ -69,7 +72,7 @@ namespace MoneyMap.Services
                     return new CategorizationResult
                     {
                         Category = "Convenience Store",
-                        Merchant = gasMapping.Merchant
+                        MerchantId = gasMapping.MerchantId
                     };
             }
 
@@ -80,7 +83,7 @@ namespace MoneyMap.Services
                     return new CategorizationResult
                     {
                         Category = mapping.Category,
-                        Merchant = mapping.Merchant
+                        MerchantId = mapping.MerchantId
                     };
             }
 
@@ -228,15 +231,10 @@ namespace MoneyMap.Services
 
         private static void AddMappings(string category, List<CategoryMapping> mappings, params string[] patterns)
         {
-            AddMappings(category, mappings, 0, null, patterns);
+            AddMappings(category, mappings, 0, patterns);
         }
 
         private static void AddMappings(string category, List<CategoryMapping> mappings, int priority, params string[] patterns)
-        {
-            AddMappings(category, mappings, priority, null, patterns);
-        }
-
-        private static void AddMappings(string category, List<CategoryMapping> mappings, int priority, string? merchant, params string[] patterns)
         {
             foreach (var pattern in patterns)
             {
@@ -244,7 +242,7 @@ namespace MoneyMap.Services
                 {
                     Category = category,
                     Pattern = pattern,
-                    Merchant = merchant,
+                    MerchantId = null, // Will be set by users via UI
                     Priority = priority
                 });
             }