CSHARP-4304: Handle conversions to an interface.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/ConvertExpressionToAggregationExpressionTranslator.cs

@@ -46,8 +46,20 @@ public static AggregationExpression Translate(TranslationContext context, UnaryE
                     }
                 }
 
-                var ast = AstExpression.Convert(operandTranslation.Ast, expressionType);
-                var serializer = context.KnownSerializersRegistry.GetSerializer(expression);
+                var ast = operandTranslation.Ast;
+                IBsonSerializer serializer;
+                if (expressionType.IsInterface)
+                {
+                    // when an expression is cast to an interface it's a no-op as far as we're concerned
+                    // and we can just use the serializer for the concrete type and members not defined in the interface will just be ignored
+                    serializer = operandTranslation.Serializer;
+                }
+                else
+                {
+                    ast = AstExpression.Convert(ast, expressionType);
+                    serializer = context.KnownSerializersRegistry.GetSerializer(expression);
+                }
+
                 return new AggregationExpression(expression, ast, serializer);
             }
 

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Jira/CSharp4304Tests.cs

@@ -0,0 +1,97 @@
+/* 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.Linq;
+using FluentAssertions;
+using MongoDB.Driver.Linq;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Jira
+{
+    public class CSharp4304Tests : Linq3IntegrationTest
+    {
+        [Fact]
+        public void Example_should_work()
+        {
+            var collection = GetCollection<Parent>();
+            var database = collection.Database;
+
+            var queryable = GetChildrenQueryable<Child>(database, "Child");
+
+            var stages = Translate(collection, queryable);
+            AssertStages(
+                stages,
+                "{ $project : { _outer : '$$ROOT', _id : 0 } }",
+                "{ $lookup : { from : 'Child', localField : '_outer.ChildId', foreignField : '_id', as : '_inner' } }",
+                "{ $unwind : '$_inner' }",
+                "{ $project : { _v : '$_inner', _id : 0 } }");
+
+            CreateParentCollection();
+            CreateChildCollection();
+            var results = queryable.ToList();
+            results.OrderBy(r => r.Id).Select(r => r.Id).Should().Equal("11", "22");
+        }
+
+        private IMongoQueryable<TChild> GetChildrenQueryable<TChild>(IMongoDatabase db, string childCollectionName) where TChild : IEntity
+        {
+            var parentCollection = db.GetCollection<Parent>("Parent");
+            var childCollection = db.GetCollection<TChild>(childCollectionName).AsQueryable();
+
+            return parentCollection
+                .AsQueryable()
+                .Join(
+                    childCollection,
+                    p => p.ChildId,
+                    c => c.Id,
+                    (_, c) => c);
+        }
+
+        private void CreateParentCollection()
+        {
+            var parentCollection = GetCollection<Parent>("Parent");
+
+            CreateCollection(
+                parentCollection,
+                new Parent { Id = "1", ChildId = "11" },
+                new Parent { Id = "2", ChildId = "22" });
+        }
+
+        private void CreateChildCollection()
+        {
+            var childCollection = GetCollection<Child>("Child");
+
+            CreateCollection(
+                childCollection,
+                new Child { Id = "11" },
+                new Child { Id = "22" });
+        }
+
+        public interface IEntity
+        {
+            string Id { get; set; }
+        }
+
+        public class Parent : IEntity
+        {
+            public string Id { get; set; }
+            public string ChildId { get; set; }
+        }
+
+        public class Child : IEntity
+        {
+            public string Id { get; set; }
+        }
+    }
+}