fix: product creation and unit tests

src/FavoriteProducts.Domain/Resources/Errors/DomainErrors.Product.cs

@@ -19,5 +19,6 @@ public static class Product
         public static readonly DomainError<ProductEntity> DeleteProductFailed = new("delete_product_failed", "Failed to delete product.");
         public static readonly DomainError<ProductEntity> InvalidProductReviewScore = new("invalid_product_review_score", "Product can't have invalid review score");
         public static readonly DomainError<ProductEntity> CannotDecreaseReviewScore = new("cannot_decrease_review_score", "Product review score can't be decreased");
+        public static readonly DomainError<ProductEntity> AlreadyExists = new("product_already_exists", "Product already exists.");
     }
 }

src/FavoriteProducts.Domain/Resources/Products/Specifications/GetProductByTitleSpecification.cs

@@ -0,0 +1,11 @@
+using Ardalis.Specification;
+using FavoriteProducts.Domain.Resources.Products.ValueObjects;
+
+namespace FavoriteProducts.Domain.Resources.Products.Specifications;
+
+public sealed class GetProductByTitleSpecification : SingleResultSpecification<Product>
+{
+    public GetProductByTitleSpecification(ProductTitle title) => 
+        Query
+        .Where(product => product.Title == title);
+}

src/FavoriteProducts.Infrastructure/Data/Relational/EntityMappings/ProductMapping.cs

@@ -10,7 +10,7 @@ public sealed class ProductMapping : AuditableEntityMapping<Product>
     public override void Configure(EntityTypeBuilder<Product> builder)
     {
         base.Configure(builder);
-        
+
         builder.ToTable("products");
 
         builder.Property(x => x.Title)
@@ -23,7 +23,10 @@ public override void Configure(EntityTypeBuilder<Product> builder)
                 )
                 .HasAnnotation("MaxLength", ProductTitle.MaxLength)
                 .HasAnnotation("MinLength", ProductTitle.MinLength);
-
+
+        builder.HasIndex(x => x.Title)
+                .IsUnique();
+
         builder.Property(p => p.ReviewScore)
                 .HasColumnName("review_score")
                 .IsRequired()

src/FavoriteProducts.Infrastructure/Migrations/20240804210216_add-product-title-unique-index.cs

@@ -0,0 +1,28 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace FavoriteProducts.Infrastructure.Migrations
+{
+    /// <inheritdoc />
+    public partial class addproducttitleuniqueindex : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateIndex(
+                name: "IX_products_title",
+                table: "products",
+                column: "title",
+                unique: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "IX_products_title",
+                table: "products");
+        }
+    }
+}

src/FavoriteProducts.Infrastructure/Migrations/FavoriteProductsContextModelSnapshot.cs

@@ -166,6 +166,9 @@ protected override void BuildModel(ModelBuilder modelBuilder)
 
                     b.HasKey("Id");
 
+                    b.HasIndex("Title")
+                        .IsUnique();
+
                     b.ToTable("products", (string)null);
                 });
 

src/FavoriteProducts.UseCases/Features/Products/Commands/Create/CreateProductCommandHandler.cs

@@ -1,7 +1,7 @@
 using FavoriteProducts.Domain.Core.Abstractions;
 using FavoriteProducts.Domain.Core.Results;
-using FavoriteProducts.Domain.Resources.Errors;
 using FavoriteProducts.Domain.Resources.Products;
+using FavoriteProducts.Domain.Resources.Products.Specifications;
 using FavoriteProducts.Domain.Resources.Products.ValueObjects;
 using FavoriteProducts.UseCases.Abstractions;
 using FavoriteProducts.UseCases.Features.Products.Dtos;
@@ -12,27 +12,34 @@ public sealed class CreateProductCommandHandler(
     IRepository<Product> productRepository) : ICommandHandler<CreateProductCommand, ProductDto>
 {
     public async Task<Result<ProductDto>> Handle(CreateProductCommand request, CancellationToken cancellationToken)
-    {
-        var maybeFields = TryBuildFields(request);
-        if (maybeFields.IsFailure)
+    {   
+        var maybeTitle = ProductTitle.Create(request.Title);
+        var maybeDescription = ProductDescription.Create(request.Description);
+        var maybePrice = ProductPrice.Create(request.Price);
+        var maybeBrand = ProductBrand.Create(request.Brand);
+        var maybeReviewScore = ProductReviewScore.Create(request.ReviewScore);
+        var composed = new List<Result>
         {
-            return maybeFields.Error;
-        }
-
-        var (
             maybeTitle,
-            maybeBrand,
             maybeDescription,
             maybePrice,
+            maybeBrand,
             maybeReviewScore
-            ) = maybeFields.Value;
+        };
+        if (composed.Any(result => result.IsFailure))
+        {
+            return Error.MultipleErrors(composed.Select(result => result.Error));
+        }
 
+        var existingProduct = await productRepository.SingleOrDefaultAsync(
+            new GetProductByTitleSpecification(maybeTitle.Value), cancellationToken);
+
         var maybeProduct = Product.Create(
-            maybeTitle,
-            maybeDescription,
-            maybePrice,
-            maybeBrand,
-            maybeReviewScore,
+            maybeTitle.Value,
+            maybeDescription.Value,
+            maybePrice.Value,
+            maybeBrand.Value,
+            maybeReviewScore.Value,
             request.ImageUrl);
         if (maybeProduct.IsFailure)
         {
@@ -42,47 +49,4 @@ public async Task<Result<ProductDto>> Handle(CreateProductCommand request, Cance
         var product = await productRepository.AddAsync(maybeProduct.Value, cancellationToken);
         return ProductDto.FromEntity(product);
     }
-
-    private static Result<(ProductTitle, ProductBrand, ProductDescription, ProductPrice, ProductReviewScore)>
-        TryBuildFields(
-            CreateProductCommand request)
-    {
-        var maybeTitle = ProductTitle.Create(request.Title);
-        if (maybeTitle.IsFailure)
-        {
-            return maybeTitle.Error;
-        }
-
-        var maybeBrand = ProductBrand.Create(request.Brand);
-        if (maybeBrand.IsFailure)
-        {
-            return maybeBrand.Error;
-        }
-
-        var maybeDescription = ProductDescription.Create(request.Description);
-        if (maybeDescription.IsFailure)
-        {
-            return maybeDescription.Error;
-        }
-
-        var maybePrice = ProductPrice.Create(request.Price);
-        if (maybePrice.IsFailure)
-        {
-            return maybePrice.Error;
-        }
-
-        var maybeReviewScore = ProductReviewScore.Create(request.ReviewScore);
-        if (maybeReviewScore.IsFailure)
-        {
-            return maybeReviewScore.Error;
-        }
-
-        return (
-            maybeTitle.Value,
-            maybeBrand.Value,
-            maybeDescription.Value,
-            maybePrice.Value,
-            maybeReviewScore.Value
-        );
-    }
 }