Add query root rewrite support

src/EntityFrameworkCore.Projectables/Services/ProjectableExpressionReplacer.cs

@@ -1,14 +1,11 @@
 using System;
-using System.Buffers;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Linq.Expressions;
 using System.Reflection;
-using System.Text;
-using System.Threading.Tasks;
-using System.Xml.Linq;
 using EntityFrameworkCore.Projectables.Extensions;
+using Microsoft.EntityFrameworkCore.Query;
 
 namespace EntityFrameworkCore.Projectables.Services
 {
@@ -31,7 +28,7 @@ namespace EntityFrameworkCore.Projectables.Services
 
                 reflectedExpression = projectableAttribute is not null
                     ? _resolver.FindGeneratedExpression(memberInfo)
-                    : (LambdaExpression?)null;
+                    : null;
 
                 _projectableMemberCache.Add(memberInfo, reflectedExpression);
             }
@@ -110,5 +107,64 @@ namespace EntityFrameworkCore.Projectables.Services
 
             return base.VisitMember(node);
         }
+
+        protected override Expression VisitExtension(Expression node)
+        {
+            if (node is not QueryRootExpression root)
+            {
+                return node;
+            }
+
+            var projectableProperties = root.EntityType.ClrType.GetProperties()
+                .Where(x => x.IsDefined(typeof(ProjectableAttribute), false))
+                .Where(x => x.CanWrite)
+                .ToList();
+
+            if (!projectableProperties.Any())
+            {
+                return node;
+            }
+
+            var properties = root.EntityType.GetProperties()
+                .Where(x => !x.IsShadowProperty())
+                .Select(x => x.GetMemberInfo(false, false))
+                // Remove projectable properties from the ef properties. Since properties returned here for auto
+                // properties (like `public string Test {get;set;}`) are generated fields, we also need to take them into account.
+                .Where(x => projectableProperties.All(y => x.Name != y.Name && x.Name != $"<{y.Name}>k__BackingField"));
+
+            // Replace db.Entities to db.Entities.Select(x => new Entity { Property1 = x.Property1, Rewritted = rewrittedProperty })
+            var select = typeof(Queryable).GetMethods(BindingFlags.Static | BindingFlags.Public)
+                .Where(x => x.Name == nameof(Queryable.Select))
+                .First(x =>
+                    x.GetParameters().Last().ParameterType // Expression<Func<T, Ret>>
+                        .GetGenericArguments().First() // Func<T, Ret>
+                        .GetGenericArguments().Length == 2 // Separate between Func<T, Ret> and Func<T, int, Ret>
+                )
+                .MakeGenericMethod(root.EntityType.ClrType, root.EntityType.ClrType);
+            var xParam = Expression.Parameter(root.EntityType.ClrType);
+            return Expression.Call(
+                null,
+                select,
+                node,
+                Expression.Lambda(
+                    Expression.MemberInit(
+                        Expression.New(root.EntityType.ClrType),
+                        properties.Select(x => Expression.Bind(x, Expression.MakeMemberAccess(xParam, x)))
+                            .Concat(projectableProperties
+                                .Select(x => Expression.Bind(x, _ReplaceParam(_resolver.FindGeneratedExpression(x), xParam)))
+                            )
+                    ),
+                    xParam
+                )
+            );
+        }
+
+        private Expression _ReplaceParam(LambdaExpression lambda, ParameterExpression para)
+        {
+            _expressionArgumentReplacer.ParameterArgumentMapping.Add(lambda.Parameters[0], para);
+            var updatedBody = _expressionArgumentReplacer.Visit(lambda.Body);
+            _expressionArgumentReplacer.ParameterArgumentMapping.Clear();
+            return updatedBody;
+        }
     }
 }

src/EntityFrameworkCore.Projectables/Services/ProjectionExpressionResolver.cs

@@ -1,13 +1,8 @@
 using System;
-using System.Collections.Concurrent;
-using System.Collections.Generic;
 using System.Linq;
 using System.Linq.Expressions;
 using System.Reflection;
-using System.Text;
-using System.Threading.Tasks;
 using EntityFrameworkCore.Projectables.Extensions;
-using Microsoft.EntityFrameworkCore.Storage.ValueConversion.Internal;
 
 namespace EntityFrameworkCore.Projectables.Services
 {

tests/EntityFrameworkCore.Projectables.FunctionalTests/QueryRootTests.UseMemberPropertyQueryRootExpression.verified.txt

@@ -0,0 +1,2 @@
+SELECT [e].[Id], [e].[Id] * 5
+FROM [Entity] AS [e]

tests/EntityFrameworkCore.Projectables.FunctionalTests/QueryRootTests.cs

@@ -0,0 +1,39 @@
+using System.ComponentModel.DataAnnotations.Schema;
+using System.Threading.Tasks;
+using EntityFrameworkCore.Projectables.FunctionalTests.Helpers;
+using Microsoft.EntityFrameworkCore;
+using VerifyXunit;
+using Xunit;
+
+namespace EntityFrameworkCore.Projectables.FunctionalTests
+{
+    [UsesVerify]
+    public class QueryRootTests
+    {
+        public record Entity
+        {
+            public int Id { get; set; }
+
+            [Projectable(UseMemberBody = nameof(Computed2))]
+            public int Computed1 => Id;
+
+            private int Computed2 => Id * 2;
+
+            [Projectable(UseMemberBody = nameof(_ComputedWithBaking))]
+            [NotMapped]
+            public int ComputedWithBacking { get; set; }
+
+            private int _ComputedWithBaking => Id * 5;
+        }
+
+        [Fact]
+        public Task UseMemberPropertyQueryRootExpression()
+        {
+            using var dbContext = new SampleDbContext<Entity>();
+
+            var query = dbContext.Set<Entity>();
+
+            return Verifier.Verify(query.ToQueryString());
+        }
+    }
+}