Added possibility to chain includes.

src/ApplicationCore/Helpers/Query/IncludeAggregator.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.Linq.Expressions;
+
+namespace Microsoft.eShopWeb.ApplicationCore.Helpers.Query
+{
+    public class IncludeAggregator<TEntity>
+    {
+        public IncludeQuery<TEntity, TProperty> Include<TProperty>(Expression<Func<TEntity, TProperty>> selector)
+        {
+            var visitor = new IncludeVisitor();
+            visitor.Visit(selector);
+
+            var id = Guid.NewGuid();
+            var pathMap = new Dictionary<Guid, string>() { { id, visitor.Path } };
+
+            return new IncludeQuery<TEntity, TProperty>(id, pathMap);
+        }
+    }
+}

src/ApplicationCore/Helpers/Query/IncludeQuery.cs

@@ -0,0 +1,67 @@
+using Microsoft.eShopWeb.ApplicationCore.Interfaces;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Linq.Expressions;
+
+namespace Microsoft.eShopWeb.ApplicationCore.Helpers.Query
+{
+    public class IncludeQuery<TEntity> : IIncludeQuery<TEntity>
+    {
+        public Guid Id { get; }
+        public Dictionary<Guid, string> PathMap { get; } = new Dictionary<Guid, string>();
+        public IncludeVisitor Visitor { get; } = new IncludeVisitor();
+
+        public IncludeQuery(Guid id, Dictionary<Guid, string> pathMap)
+        {
+            Id = id;
+            PathMap = pathMap;
+        }
+
+        public IEnumerable<string> Paths => PathMap.Select(x => x.Value).Distinct().ToList();
+    }
+
+    public class IncludeQuery<TEntity, TPreviousProperty> : IncludeQuery<TEntity>, IIncludeQuery<TEntity, TPreviousProperty>
+    {
+        public IncludeQuery(Guid id, Dictionary<Guid, string> pathMap) : base(id, pathMap) { }
+    }
+
+    public static class IncludeableQueryExtensions
+    {
+        public static IIncludeQuery<TEntity, TNewProperty> Include<TEntity, TPreviousProperty, TNewProperty>(
+            this IIncludeQuery<TEntity, TPreviousProperty> query,
+            Expression<Func<TEntity, TNewProperty>> selector)
+        {
+            query.Visitor.Visit(selector);
+
+            var id = Guid.NewGuid();
+            query.PathMap[id] = query.Visitor.Path;
+
+            return new IncludeQuery<TEntity, TNewProperty>(id, query.PathMap);
+        }
+
+        public static IIncludeQuery<TEntity, TNewProperty> ThenInclude<TEntity, TPreviousProperty, TNewProperty>(
+            this IIncludeQuery<TEntity, TPreviousProperty> query,
+            Expression<Func<TPreviousProperty, TNewProperty>> selector)
+        {
+            query.Visitor.Visit(selector);
+
+            var existingPath = query.PathMap[query.Id];
+            query.PathMap[query.Id] = $"{existingPath}.{query.Visitor.Path}";
+
+            return new IncludeQuery<TEntity, TNewProperty>(query.Id, query.PathMap);
+        }
+
+        public static IIncludeQuery<TEntity, TNewProperty> ThenInclude<TEntity, TPreviousProperty, TNewProperty>(
+            this IIncludeQuery<TEntity, IEnumerable<TPreviousProperty>> query,
+            Expression<Func<TPreviousProperty, TNewProperty>> selector)
+        {
+            query.Visitor.Visit(selector);
+
+            var existingPath = query.PathMap[query.Id];
+            query.PathMap[query.Id] = $"{existingPath}.{query.Visitor.Path}";
+
+            return new IncludeQuery<TEntity, TNewProperty>(query.Id, query.PathMap);
+        }
+    }
+}

src/ApplicationCore/Helpers/Query/IncludeVisitor.cs

@@ -0,0 +1,16 @@
+using System.Linq.Expressions;
+
+namespace Microsoft.eShopWeb.ApplicationCore.Helpers.Query
+{
+    public class IncludeVisitor : ExpressionVisitor
+    {
+        public string Path { get; private set; } = string.Empty;
+
+        protected override Expression VisitMember(MemberExpression node)
+        {
+            Path = string.IsNullOrEmpty(Path) ? node.Member.Name : $"{Path}.{node.Member.Name}";
+
+            return base.VisitMember(node);
+        }
+    }
+}

src/ApplicationCore/Interfaces/IIncludeQuery.cs

@@ -0,0 +1,18 @@
+using Microsoft.eShopWeb.ApplicationCore.Helpers.Query;
+using System;
+using System.Collections.Generic;
+
+namespace Microsoft.eShopWeb.ApplicationCore.Interfaces
+{
+    public interface IIncludeQuery<TEntity>
+    {
+        Guid Id { get; }
+        Dictionary<Guid, string> PathMap { get; }
+        IncludeVisitor Visitor { get; }
+        IEnumerable<string> Paths { get; }
+    }
+
+    public interface IIncludeQuery<TEntity, out TPreviousProperty> : IIncludeQuery<TEntity>
+    {
+    }
+}

src/ApplicationCore/Specifications/BaseSpecification.cs

@@ -2,6 +2,7 @@
 using System;
 using System.Linq.Expressions;
 using System.Collections.Generic;
+using Microsoft.eShopWeb.ApplicationCore.Helpers.Query;
 
 namespace Microsoft.eShopWeb.ApplicationCore.Specifications
 {
@@ -26,6 +27,13 @@ protected virtual void AddInclude(Expression<Func<T, object>> includeExpression)
         {
             Includes.Add(includeExpression);
         }
+
+        protected virtual void AddIncludes(Func<IncludeAggregator<T>, IIncludeQuery<T>> includeGenerator)
+        {
+            var includeQuery = includeGenerator(new IncludeAggregator<T>());
+            IncludeStrings.AddRange(includeQuery.Paths);
+        }
+
         protected virtual void AddInclude(string includeString)
         {
             IncludeStrings.Add(includeString);

src/ApplicationCore/Specifications/CustomerOrdersWithItemsSpecification.cs

@@ -1,4 +1,5 @@
 using Microsoft.eShopWeb.ApplicationCore.Entities.OrderAggregate;
+using Microsoft.eShopWeb.ApplicationCore.Helpers.Query;
 
 namespace Microsoft.eShopWeb.ApplicationCore.Specifications
 {
@@ -7,8 +8,7 @@ public class CustomerOrdersWithItemsSpecification : BaseSpecification<Order>
         public CustomerOrdersWithItemsSpecification(string buyerId)
             : base(o => o.BuyerId == buyerId)
         {
-            AddInclude(o => o.OrderItems);
-            AddInclude($"{nameof(Order.OrderItems)}.{nameof(OrderItem.ItemOrdered)}");
+            AddIncludes(query => query.Include(o => o.OrderItems).ThenInclude(i => i.ItemOrdered));
         }
     }
 }