Fix: Manually load ProductVariants with separate query instead of Include

**Root Cause**: EF Core Include() was not properly materializing the Variants navigation
property despite correct SQL JOIN generation.

**Solution**: Load variants separately and manually group by ProductId for DTO mapping.
This bypasses EF Core's navigation property fixup issues.

🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>

LittleShop/Services/ProductService.cs

@@ -22,11 +22,19 @@ public class ProductService : IProductService
             .Include(p => p.Category)
             .Include(p => p.Photos)
             .Include(p => p.MultiBuys)
-            .Include(p => p.Variants)
             .Where(p => p.IsActive)
-            .AsSplitQuery()
             .ToListAsync();
 
+        // Manually load variants for all products
+        var productIds = products.Select(p => p.Id).ToList();
+        var allVariants = await _context.ProductVariants
+            .Where(v => productIds.Contains(v.ProductId))
+            .ToListAsync();
+
+        // Group variants by ProductId for quick lookup
+        var variantsByProduct = allVariants.GroupBy(v => v.ProductId)
+            .ToDictionary(g => g.Key, g => g.ToList());
+
         return products.Select(p => new ProductDto
             {
                 Id = p.Id,
@@ -63,7 +71,8 @@ public class ProductService : IProductService
                     CreatedAt = mb.CreatedAt,
                     UpdatedAt = mb.UpdatedAt
                 }).ToList(),
-                Variants = p.Variants.Select(v => new ProductVariantDto
+                Variants = variantsByProduct.ContainsKey(p.Id)
+                    ? variantsByProduct[p.Id].Select(v => new ProductVariantDto
                     {
                         Id = v.Id,
                         ProductId = v.ProductId,
@@ -76,6 +85,7 @@ public class ProductService : IProductService
                         CreatedAt = v.CreatedAt,
                         UpdatedAt = v.UpdatedAt
                     }).ToList()
+                    : new List<ProductVariantDto>()
             }).ToList();
     }
 
@@ -85,11 +95,19 @@ public class ProductService : IProductService
             .Include(p => p.Category)
             .Include(p => p.Photos)
             .Include(p => p.MultiBuys)
-            .Include(p => p.Variants)
             .Where(p => p.IsActive && p.CategoryId == categoryId)
-            .AsSplitQuery()
             .ToListAsync();
 
+        // Manually load variants for all products
+        var productIds = products.Select(p => p.Id).ToList();
+        var allVariants = await _context.ProductVariants
+            .Where(v => productIds.Contains(v.ProductId))
+            .ToListAsync();
+
+        // Group variants by ProductId for quick lookup
+        var variantsByProduct = allVariants.GroupBy(v => v.ProductId)
+            .ToDictionary(g => g.Key, g => g.ToList());
+
         return products.Select(p => new ProductDto
             {
                 Id = p.Id,
@@ -126,7 +144,8 @@ public class ProductService : IProductService
                     CreatedAt = mb.CreatedAt,
                     UpdatedAt = mb.UpdatedAt
                 }).ToList(),
-                Variants = p.Variants.Select(v => new ProductVariantDto
+                Variants = variantsByProduct.ContainsKey(p.Id)
+                    ? variantsByProduct[p.Id].Select(v => new ProductVariantDto
                     {
                         Id = v.Id,
                         ProductId = v.ProductId,
@@ -139,6 +158,7 @@ public class ProductService : IProductService
                         CreatedAt = v.CreatedAt,
                         UpdatedAt = v.UpdatedAt
                     }).ToList()
+                    : new List<ProductVariantDto>()
             }).ToList();
     }