Query: Enable materialzing collections of arbirtary shape in projection

src/EFCore.Relational/Query/Pipeline/CollectionInitializingExperssion.cs

@@ -0,0 +1,65 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Linq.Expressions;
+using Microsoft.EntityFrameworkCore.Metadata;
+using Microsoft.EntityFrameworkCore.Query.Expressions.Internal;
+using Microsoft.EntityFrameworkCore.Query.Internal;
+
+namespace Microsoft.EntityFrameworkCore.Relational.Query.Pipeline
+{
+    public class CollectionInitializingExpression : Expression, IPrintable
+    {
+        public CollectionInitializingExpression(
+            int collectionId, Expression parent, Expression parentIdentifier, Expression outerIdentifier, INavigation navigation, Type type)
+        {
+            CollectionId = collectionId;
+            Parent = parent;
+            ParentIdentifier = parentIdentifier;
+            OuterIdentifier = outerIdentifier;
+            Navigation = navigation;
+            Type = type;
+        }
+
+        protected override Expression VisitChildren(ExpressionVisitor visitor)
+        {
+            var parent = visitor.Visit(Parent);
+            var parentIdentifier = visitor.Visit(ParentIdentifier);
+            var outerIdentifier = visitor.Visit(OuterIdentifier);
+
+            return parent != Parent || parentIdentifier != ParentIdentifier || outerIdentifier != OuterIdentifier
+                ? new CollectionInitializingExpression(CollectionId, parent, parentIdentifier, outerIdentifier, Navigation, Type)
+                : this;
+        }
+
+        public void Print(ExpressionPrinter expressionPrinter)
+        {
+            expressionPrinter.StringBuilder.AppendLine("InitializeCollection:");
+            using (expressionPrinter.StringBuilder.Indent())
+            {
+                expressionPrinter.StringBuilder.AppendLine($"CollectionId: {CollectionId}");
+                expressionPrinter.StringBuilder.AppendLine($"Navigation: {Navigation?.Name}");
+                expressionPrinter.StringBuilder.Append("Parent:");
+                expressionPrinter.Visit(Parent);
+                expressionPrinter.StringBuilder.AppendLine();
+                expressionPrinter.StringBuilder.Append("ParentIdentifier:");
+                expressionPrinter.Visit(ParentIdentifier);
+                expressionPrinter.StringBuilder.AppendLine();
+                expressionPrinter.StringBuilder.Append("OuterIdentifier:");
+                expressionPrinter.Visit(OuterIdentifier);
+                expressionPrinter.StringBuilder.AppendLine();
+            }
+        }
+
+        public override Type Type { get; }
+
+        public override ExpressionType NodeType => ExpressionType.Extension;
+
+        public int CollectionId { get; }
+        public Expression Parent { get; }
+        public Expression ParentIdentifier { get; }
+        public Expression OuterIdentifier { get; }
+        public INavigation Navigation { get; }
+    }
+}

src/EFCore.Relational/Query/Pipeline/CollectionPopulatingExpression.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Linq.Expressions;
+using Microsoft.EntityFrameworkCore.Query.Expressions.Internal;
+using Microsoft.EntityFrameworkCore.Query.Internal;
+
+namespace Microsoft.EntityFrameworkCore.Relational.Query.Pipeline
+{
+    public class CollectionPopulatingExpression : Expression, IPrintable
+    {
+        public CollectionPopulatingExpression(RelationalCollectionShaperExpression parent, Type type, bool include)
+        {
+            Parent = parent;
+            Type = type;
+            Include = include;
+        }
+
+        protected override Expression VisitChildren(ExpressionVisitor visitor)
+        {
+            var parent = (RelationalCollectionShaperExpression)visitor.Visit(Parent);
+
+            return parent != Parent
+                ? new CollectionPopulatingExpression(parent, Type, Include)
+                : this;
+        }
+
+        public void Print(ExpressionPrinter expressionPrinter)
+        {
+            expressionPrinter.StringBuilder.AppendLine("PopulateCollection:");
+            using (expressionPrinter.StringBuilder.Indent())
+            {
+                expressionPrinter.StringBuilder.Append("Parent:");
+                expressionPrinter.Visit(Parent);
+            }
+        }
+
+        public override Type Type { get; }
+
+        public override ExpressionType NodeType => ExpressionType.Extension;
+        public RelationalCollectionShaperExpression Parent { get; }
+        public bool Include { get; }
+    }
+}