CSHARP-4047: Nested where clause in select only works with IEnumerable<T>.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/MethodTranslators/WhereMethodToAggregationExpressionTranslator.cs

@@ -17,6 +17,7 @@
 using MongoDB.Driver.Linq.Linq3Implementation.Ast.Expressions;
 using MongoDB.Driver.Linq.Linq3Implementation.Misc;
 using MongoDB.Driver.Linq.Linq3Implementation.Reflection;
+using MongoDB.Driver.Linq.Linq3Implementation.Serializers;
 
 namespace MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.MethodTranslators
 {
@@ -37,11 +38,13 @@ public static AggregationExpression Translate(TranslationContext context, Method
                 var predicateSymbol = context.CreateSymbol(predicateParameter, predicateParameterSerializer);
                 var predicateContext = context.WithSymbol(predicateSymbol);
                 var predicateTranslation = ExpressionToAggregationExpressionTranslator.Translate(predicateContext, predicateLambda.Body);
+                var itemSerializer = ArraySerializerHelper.GetItemSerializer(sourceTranslation.Serializer);
+                var enumerableSerializer = IEnumerableSerializer.Create(itemSerializer);
                 var ast = AstExpression.Filter(
                     sourceTranslation.Ast,
                     predicateTranslation.Ast,
                     predicateParameter.Name);
-                return new AggregationExpression(expression, ast, sourceTranslation.Serializer);
+                return new AggregationExpression(expression, ast, enumerableSerializer);
             }
 
             throw new ExpressionNotSupportedException(expression);

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Jira/CSharp4047Tests.cs

@@ -0,0 +1,80 @@
+/* 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 System.Collections.Generic;
+using System.Linq;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Jira
+{
+    public class CSharp4047Tests : Linq3IntegrationTest
+    {
+        [Fact]
+        public void Nested_where_should_work_with_array()
+        {
+            var collection = GetCollection<C>();
+
+            var queryable = collection.AsQueryable()
+                .Select(x => new { R = x.A.Where(i => i == 1)});
+
+            var stages = Translate(collection, queryable);
+            var expectedStages = new[]
+            {
+                "{ $project : { R : { $filter : { input : '$A', as : 'i', cond : { $eq : ['$$i', 1] } } }, _id : 0 } }"
+            };
+            AssertStages(stages, expectedStages);
+        }
+
+        [Fact]
+        public void Nested_where_should_work_with_IEnumerable()
+        {
+            var collection = GetCollection<C>();
+
+            var queryable = collection.AsQueryable()
+                .Select(x => new { R = x.E.Where(i => i == 1) });
+
+            var stages = Translate(collection, queryable);
+            var expectedStages = new[]
+            {
+                "{ $project : { R : { $filter : { input : '$E', as : 'i', cond : { $eq : ['$$i', 1] } } }, _id : 0 } }"
+            };
+            AssertStages(stages, expectedStages);
+        }
+
+        [Fact]
+        public void Nested_where_should_work_with_List()
+        {
+            var collection = GetCollection<C>();
+
+            var queryable = collection.AsQueryable()
+                .Select(x => new { R = x.L.Where(i => i == 1) });
+
+            var stages = Translate(collection, queryable);
+            var expectedStages = new[]
+            {
+                "{ $project : { R : { $filter : { input : '$L', as : 'i', cond : { $eq : ['$$i', 1] } } }, _id : 0 } }"
+            };
+            AssertStages(stages, expectedStages);
+        }
+
+        private class C
+        {
+            public int Id { get; set; }
+            public int[] A { get; set; }
+            public IEnumerable<int> E { get; set; }
+            public List<int> L { get; set; }
+        }
+    }
+}