Adding a GroupBy specification example

src/ApplicationCore/Interfaces/ISpecification.cs

@@ -11,6 +11,7 @@ namespace Microsoft.eShopWeb.ApplicationCore.Interfaces
         List<string> IncludeStrings { get; }
         Expression<Func<T, object>> OrderBy { get; }
         Expression<Func<T, object>> OrderByDescending { get; }
+        Expression<Func<T, object>> GroupBy { get; }
 
         int Take { get; }
         int Skip { get; }

src/ApplicationCore/Specifications/BaseSpecification.cs

@@ -16,6 +16,7 @@ namespace Microsoft.eShopWeb.ApplicationCore.Specifications
         public List<string> IncludeStrings { get; } = new List<string>();
         public Expression<Func<T, object>> OrderBy { get; private set; }
         public Expression<Func<T, object>> OrderByDescending { get; private set; }
+        public Expression<Func<T, object>> GroupBy { get; private set; }
 
         public int Take { get; private set; }
         public int Skip { get; private set; }
@@ -43,5 +44,11 @@ namespace Microsoft.eShopWeb.ApplicationCore.Specifications
         {
             OrderByDescending = orderByDescendingExpression;
         }
+
+        protected virtual void ApplyGroupBy(Expression<Func<T, object>> groupByExpression)
+        {
+            GroupBy = groupByExpression;
+        }
+
     }
 }

src/ApplicationCore/Specifications/CatalogFilterPaginatedSpecification.cs

@@ -9,6 +9,7 @@ namespace Microsoft.eShopWeb.ApplicationCore.Specifications
                 (!typeId.HasValue || i.CatalogTypeId == typeId))
         {
             ApplyPaging(skip, take);
+            ApplyGroupBy(i => new { i.CatalogType });
         }
     }
 }

src/Infrastructure/Data/SpecificationEvaluator.cs

@@ -1,10 +1,7 @@
 using Microsoft.EntityFrameworkCore;
 using Microsoft.eShopWeb.ApplicationCore.Entities;
 using Microsoft.eShopWeb.ApplicationCore.Interfaces;
-using System;
-using System.Collections.Generic;
 using System.Linq;
-using System.Text;
 
 namespace Microsoft.eShopWeb.Infrastructure.Data
 {
@@ -38,6 +35,11 @@ namespace Microsoft.eShopWeb.Infrastructure.Data
                 query = query.OrderByDescending(specification.OrderByDescending);
             }
 
+            if (specification.GroupBy != null)
+            {
+                query = query.GroupBy(specification.GroupBy).SelectMany(x => x);
+            }
+
             // Apply paging if enabled
             if (specification.isPagingEnabled)
             {