CSHARP-4616: OfType should match derived types (#1075)

Author: sanych-sun

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToPipelineTranslators/OfTypeMethodToPipelineTranslator.cs

@@ -13,6 +13,7 @@
 * limitations under the License.
 */
 
+using System.Linq;
 using System.Linq.Expressions;
 using MongoDB.Bson.Serialization;
 using MongoDB.Bson.Serialization.Serializers;
@@ -50,6 +51,11 @@ public static AstPipeline Translate(TranslationContext context, MethodCallExpres
                 }
                 var discriminatorField = AstFilter.Field(discriminatorElementName, BsonValueSerializer.Instance);
                 var discriminatorValue = discriminatorConvention.GetDiscriminator(nominalType, actualType);
+                if (discriminatorValue.IsBsonArray)
+                {
+                    discriminatorValue = discriminatorValue.AsBsonArray.Last();
+                }
+
                 var filter = AstFilter.Eq(discriminatorField, discriminatorValue); // note: OfType only works with hierarchical discriminators
                 var actualSerializer = context.KnownSerializersRegistry.GetSerializer(expression);
                 if (wrappedValueOutputSerializer != null)

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Translators/ExpressionToPipelineTranslators/OfTypeMethodToPipelineTranslatorTests.cs

@@ -0,0 +1,81 @@
+/* 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.Bson.Serialization.Attributes;
+using MongoDB.Driver.Linq;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Translators.ExpressionToPipelineTranslators
+{
+    public class OfTypeMethodToPipelineTranslatorTests: Linq3IntegrationTest
+    {
+        [Fact]
+        public void OfType_should_return_expected_results()
+        {
+            var collection = CreateCollection();
+
+            AssertTypeOf<Account>(collection, "{ $match: { _t : 'Account' } }", 1, 2, 3);
+            AssertTypeOf<Company>(collection, "{ $match: { _t : 'Company' } }", 1, 2);
+            AssertTypeOf<Contact>(collection, "{ $match: { _t : 'Contact' } }", 3);
+        }
+
+        private void AssertTypeOf<TDocument>(IMongoCollection<Entity> collection, string expectedStage, params int[] expectedIds)
+            where TDocument: Entity
+        {
+            var queryable = collection
+                .AsQueryable()
+                .OfType<TDocument>();
+
+            var stages = Translate(collection, queryable);
+            AssertStages(stages, expectedStage);
+
+            var results = queryable.ToList();
+            results.Select(x => x.Id).Should().BeEquivalentTo(expectedIds);
+        }
+
+        private IMongoCollection<Entity> CreateCollection()
+        {
+            var collection = GetCollection<Entity>("test");
+            CreateCollection(
+                collection,
+                new Company { Id = 1 },
+                new Company { Id = 2 },
+                new Contact { Id = 3 });
+
+            return collection;
+        }
+
+        [BsonDiscriminator(RootClass = true)]
+        [BsonKnownTypes(typeof(Account), typeof(Contact), typeof(Company))]
+        public abstract class Entity
+        {
+            public int Id { get; set; }
+        }
+
+        public abstract class Account : Entity
+        {
+        }
+
+        public class Contact : Account
+        {
+        }
+
+        public class Company : Account
+        {
+        }
+    }
+}