CSHARP-4802: Projection of nested field does not compute dotted field name properly.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/Misc/ProjectionHelper.cs

@@ -13,6 +13,7 @@
 * limitations under the License.
 */
 
+using System;
 using System.Collections.Generic;
 using MongoDB.Bson;
 using MongoDB.Bson.Serialization;
@@ -77,18 +78,42 @@ private static (IReadOnlyList<AstProjectStageSpecification>, IBsonSerializer) Cr
         private static (IReadOnlyList<AstProjectStageSpecification>, IBsonSerializer) CreateFindGetFieldProjection(AggregationExpression expression)
         {
             var getFieldExpressionAst = (AstGetFieldExpression)expression.Ast;
-            if (getFieldExpressionAst.HasSafeFieldName(out var fieldName))
+            if (IsGetFieldChainWithSafeFieldNames(getFieldExpressionAst))
             {
-                var specifications = fieldName == "_id" ?
-                    new List<AstProjectStageSpecification> { AstProject.Include(fieldName) } :
-                    new List<AstProjectStageSpecification> { AstProject.Include(fieldName), AstProject.Exclude("_id") };
-                var wrappedValueSerializer = WrappedValueSerializer.Create(fieldName, expression.Serializer);
-                return (specifications, wrappedValueSerializer);
+                var (dottedFieldName, serializer) = CreateGetFieldChainWithSafeFieldNamesProjection(getFieldExpressionAst, expression.Serializer);
+                var specifications = new List<AstProjectStageSpecification> { AstProject.Include(dottedFieldName) };
+                if (dottedFieldName != "_id" && !dottedFieldName.StartsWith("_id."))
+                {
+                    specifications.Add(AstProject.Exclude("_id"));
+                }
+                return (specifications, serializer);
             }
 
             return CreateWrappedValueProjection(expression);
         }
 
+        private static (string, IBsonSerializer) CreateGetFieldChainWithSafeFieldNamesProjection(AstGetFieldExpression getFieldExpression, IBsonSerializer serializer)
+        {
+            if (getFieldExpression.HasSafeFieldName(out var fieldName))
+            {
+                var wrappedValueSerializer = WrappedValueSerializer.Create(fieldName, serializer);
+                var input = getFieldExpression.Input;
+
+                if (input is AstVarExpression varExpression && varExpression.Name == "ROOT")
+                {
+                    return (fieldName, wrappedValueSerializer);
+                }
+
+                if (input is AstGetFieldExpression outerGetFieldExpression)
+                {
+                    var (outerDottedFieldName, outerWrappedValueSerializer) = CreateGetFieldChainWithSafeFieldNamesProjection(outerGetFieldExpression, wrappedValueSerializer);
+                    return (outerDottedFieldName + "." + fieldName, outerWrappedValueSerializer);
+                }
+            }
+
+            throw new ArgumentException($"{nameof(CreateGetFieldChainWithSafeFieldNamesProjection)} called with an invalid getFieldExpression.", nameof(getFieldExpression));
+        }
+
         private static (IReadOnlyList<AstProjectStageSpecification>, IBsonSerializer) CreateWrappedValueProjection(AggregationExpression expression)
         {
             var wrappedValueSerializer = WrappedValueSerializer.Create("_v", expression.Serializer);
@@ -101,6 +126,16 @@ private static (IReadOnlyList<AstProjectStageSpecification>, IBsonSerializer) Cr
             return (specifications, wrappedValueSerializer);
         }
 
+        private static bool IsGetFieldChainWithSafeFieldNames(AstGetFieldExpression getFieldExpression)
+        {
+            return
+                getFieldExpression.HasSafeFieldName(out _) &&
+                (
+                    (getFieldExpression.Input is AstVarExpression varExpression && varExpression.Name == "ROOT") ||
+                    (getFieldExpression.Input is AstGetFieldExpression nestedGetFieldExpression && IsGetFieldChainWithSafeFieldNames(nestedGetFieldExpression))
+                );
+        }
+
         private static AstExpression QuoteIfNecessary(AstExpression expression)
         {
             if (expression is AstConstantExpression constantExpression)

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp4802Tests.cs

@@ -0,0 +1,63 @@
+/* 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 FluentAssertions;
+using MongoDB.Driver.Linq;
+using MongoDB.TestHelpers.XunitExtensions;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3Implementation.Jira
+{
+    public class CSharp4802Tests : Linq3IntegrationTest
+    {
+        [Theory]
+        [ParameterAttributeData]
+        public void Find_with_projection_of_subfield_should_work(
+            [Values(LinqProvider.V2, LinqProvider.V3)] LinqProvider linqProvider)
+        {
+            var collection = GetCollection(linqProvider);
+
+            var find = collection.Find(d => d.Status == "a").Project(d => d.SubDocument.Id);
+
+            var projection = TranslateFindProjection(collection, find);
+            projection.Should().Be("{ 'SubDocument._id' : 1, _id : 0 }");
+
+            var result = find.Single();
+            result.Should().Be(11);
+        }
+
+        private IMongoCollection<Document> GetCollection(LinqProvider linqProvider)
+        {
+            var collection = GetCollection<Document>("test", linqProvider);
+            CreateCollection(
+                collection,
+                new Document { Id = 1, Status = "a", SubDocument = new SubDocument { Id = 11 } },
+                new Document { Id = 2, Status = "b", SubDocument = new SubDocument { Id = 22 } });
+            return collection;
+        }
+
+        private class Document
+        {
+            public int Id { get; set; }
+            public string Status { get; set; }
+            public SubDocument SubDocument { get; set; }
+        }
+
+        public class SubDocument
+        {
+            public int Id { get; set; }
+        }
+    }
+}