CSHARP-4339: Add support for $documents in LINQ.

Author: rstam

src/MongoDB.Driver/IMongoDatabaseExtensions.cs

@@ -17,7 +17,6 @@
 using System.Threading.Tasks;
 using MongoDB.Bson;
 using MongoDB.Driver.Core.Misc;
-using MongoDB.Driver.Core.Operations;
 using MongoDB.Driver.Linq;
 
 namespace MongoDB.Driver

src/MongoDB.Driver/Linq/IMongoQueryProvider.cs

@@ -32,9 +32,9 @@ internal interface IMongoQueryProvider : IQueryProvider
         CollectionNamespace CollectionNamespace { get; }
 
         /// <summary>
-        /// Gets the collection document serializer.
+        /// Gets the pipeline input serializer (the DocumentSerializer for collection queries and NoPipelineInputSerializer for database queries).
         /// </summary>
-        IBsonSerializer CollectionDocumentSerializer { get; }
+        IBsonSerializer PipelineInputSerializer { get; }
 
         /// <summary>
         /// Gets the execution model.

src/MongoDB.Driver/Linq/Linq2Implementation/MongoQueryProviderImpl.cs

@@ -21,7 +21,6 @@
 using System.Threading.Tasks;
 using MongoDB.Bson.Serialization;
 using MongoDB.Driver.Core.Misc;
-using MongoDB.Driver.Linq;
 using MongoDB.Driver.Linq.Linq2Implementation.Processors;
 using MongoDB.Driver.Linq.Linq2Implementation.Processors.Pipeline;
 using MongoDB.Driver.Linq.Linq2Implementation.Translators;
@@ -44,7 +43,7 @@ public MongoQueryProviderImpl(IMongoCollection<TDocument> collection, IClientSes
 
         public CollectionNamespace CollectionNamespace => _collection.CollectionNamespace;
 
-        public IBsonSerializer CollectionDocumentSerializer => _collection.DocumentSerializer;
+        public IBsonSerializer PipelineInputSerializer => _collection.DocumentSerializer;
 
         public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
         {

src/MongoDB.Driver/Linq/Linq2Implementation/Processors/Pipeline/PipelineBinder.cs

@@ -80,8 +80,8 @@ protected override Expression BindNonMethodCall(Expression node)
                 var queryable = (IMongoQueryable)((ConstantExpression)node).Value;
                 var provider = (IMongoQueryProvider)queryable.Provider;
                 return new PipelineExpression(
-                    new CollectionExpression(provider.CollectionNamespace, provider.CollectionDocumentSerializer),
-                    new DocumentExpression(provider.CollectionDocumentSerializer));
+                    new CollectionExpression(provider.CollectionNamespace, provider.PipelineInputSerializer),
+                    new DocumentExpression(provider.PipelineInputSerializer));
             }
 
             var message = string.Format("The expression tree is not supported: {0}",

src/MongoDB.Driver/Linq/Linq2Implementation/Processors/SerializationBinder.cs

@@ -96,7 +96,7 @@ protected override Expression VisitConstant(ConstantExpression node)
                 var provider = (IMongoQueryProvider)queryable.Provider;
                 return new CollectionExpression(
                     provider.CollectionNamespace,
-                    provider.CollectionDocumentSerializer);
+                    provider.PipelineInputSerializer);
             }
 
             return base.VisitConstant(node);

src/MongoDB.Driver/Linq/Linq3Implementation/Ast/AstNodeType.cs

@@ -53,6 +53,7 @@ internal enum AstNodeType
         DateTruncExpression,
         DensifyStage,
         DerivativeOrIntegralWindowExpression,
+        DocumentsStage,
         ElemMatchFilterOperation,
         ExistsFilterOperation,
         ExponentialMovingAverageWindowExpression,

src/MongoDB.Driver/Linq/Linq3Implementation/Ast/Stages/AstDocumentsStage.cs

@@ -0,0 +1,55 @@
+/* Copyright 2010-present MongoDB Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using MongoDB.Bson;
+using MongoDB.Driver.Core.Misc;
+using MongoDB.Driver.Linq.Linq3Implementation.Ast.Expressions;
+using MongoDB.Driver.Linq.Linq3Implementation.Ast.Visitors;
+
+namespace MongoDB.Driver.Linq.Linq3Implementation.Ast.Stages
+{
+    internal sealed class AstDocumentsStage : AstStage
+    {
+        private readonly AstExpression _documents;
+
+        public AstDocumentsStage(AstExpression documents)
+        {
+            _documents = Ensure.IsNotNull(documents, nameof(documents));
+        }
+
+        public new AstExpression Documents => _documents;
+        public override AstNodeType NodeType => AstNodeType.DocumentsStage;
+
+        public override AstNode Accept(AstNodeVisitor visitor)
+        {
+            return visitor.VisitDocumentsStage(this);
+        }
+
+        public override BsonValue Render()
+        {
+            return new BsonDocument("$documents", _documents.Render());
+        }
+
+        public AstDocumentsStage Update(AstExpression documents)
+        {
+            if (documents == _documents)
+            {
+                return this;
+            }
+
+            return new AstDocumentsStage(documents);
+        }
+    }
+}

src/MongoDB.Driver/Linq/Linq3Implementation/Ast/Stages/AstStage.cs

@@ -77,6 +77,12 @@ public static AstStage Densify(
             return new AstDensifyStage(fieldPath, range, partitionByFieldPaths);
         }
 
+        public static AstStage Documents(
+            AstExpression documents)
+        {
+            return new AstDocumentsStage(documents);
+        }
+
         public static AstStage Facet(IEnumerable<AstFacetStageFacet> facets)
         {
             return new AstFacetStage(facets);

src/MongoDB.Driver/Linq/Linq3Implementation/Ast/Visitors/AstNodeVisitor.cs

@@ -299,6 +299,11 @@ public virtual AstNode VisitDerivativeOrIntegralWindowExpression(AstDerivativeOr
             return node.Update(node.Operator, VisitAndConvert(node.Arg), node.Unit, node.Window);
         }
 
+        public virtual AstNode VisitDocumentsStage(AstDocumentsStage node)
+        {
+            return node.Update(VisitAndConvert(node.Documents));
+        }
+
         public virtual AstNode VisitElemMatchFilterOperation(AstElemMatchFilterOperation node)
         {
             return node.Update(VisitAndConvert(node.Filter));

src/MongoDB.Driver/Linq/Linq3Implementation/ExtensionMethods/ExpressionExtensions.cs

@@ -13,7 +13,6 @@
 * limitations under the License.
 */
 
-using System.Linq;
 using System.Linq.Expressions;
 using MongoDB.Bson.Serialization;
 
@@ -24,14 +23,15 @@ internal static class ExpressionExtensions
         public static (string CollectionName, IBsonSerializer DocumentSerializer) GetCollectionInfo(this Expression innerExpression, Expression containerExpression)
         {
             if (innerExpression is ConstantExpression constantExpression &&
-                constantExpression.Value is IQueryable queryable &&
-                queryable.Provider is MongoQueryProvider queryProvider)
+                constantExpression.Value is IMongoQueryable mongoQueryable &&
+                mongoQueryable.Provider is IMongoQueryProvider mongoQueryProvider &&
+                mongoQueryProvider.CollectionNamespace != null)
             {
-                return (queryProvider.CollectionNamespace.CollectionName, queryProvider.CollectionDocumentSerializer);
+                return (mongoQueryProvider.CollectionNamespace.CollectionName, mongoQueryProvider.PipelineInputSerializer);
             }
 
-            var message = $"Expression inner must be a MongoDB queryable representing a collection: {innerExpression} in {containerExpression}.";
-            throw new ExpressionNotSupportedException(message);
+            var message = $"inner expression is not an IMongoQueryable representing a collection";
+            throw new ExpressionNotSupportedException(innerExpression, containerExpression, because: message);
         }
 
         public static TValue GetConstantValue<TValue>(this Expression expression, Expression containingExpression)