CSHARP-4653: Implement Convert translator for subclasses. (#1101)

Author: sanych-sun

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

@@ -54,6 +54,11 @@ public static AggregationExpression Translate(TranslationContext context, UnaryE
                     return TranslateConvertToBaseType(expression, operandTranslation);
                 }
 
+                if (IsConvertToDerivedType(sourceType: operandExpression.Type, targetType: expressionType))
+                {
+                    return TranslateConvertToDerivedType(expression, operandTranslation);
+                }
+
                 if (expressionType.IsConstructedGenericType && expressionType.GetGenericTypeDefinition() == typeof(Nullable<>))
                 {
                     var valueType = expressionType.GetGenericArguments()[0];
@@ -116,6 +121,11 @@ private static bool IsConvertToBaseType(Type sourceType, Type targetType)
             return sourceType.IsSubclassOf(targetType);
         }
 
+        private static bool IsConvertToDerivedType(Type sourceType, Type targetType)
+        {
+            return targetType.IsSubclassOf(sourceType);
+        }
+
         private static bool IsConvertUnderlyingTypeToEnum(UnaryExpression expression)
         {
             var sourceType = expression.Operand.Type;
@@ -136,6 +146,13 @@ private static AggregationExpression TranslateConvertToBaseType(UnaryExpression
             return new AggregationExpression(expression, operandTranslation.Ast, downcastingSerializer);
         }
 
+        private static AggregationExpression TranslateConvertToDerivedType(UnaryExpression expression, AggregationExpression operandTranslation)
+        {
+            var serializer = BsonSerializer.LookupSerializer(expression.Type);
+
+            return new AggregationExpression(expression, operandTranslation.Ast, serializer);
+        }
+
         private static AggregationExpression TranslateConvertToBsonValue(TranslationContext context, UnaryExpression expression, Expression operand)
         {
             // handle double conversions like `(BsonValue)(object)x.Anything`

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Translators/ExpressionToAggregationExpressionTranslators/ConvertExpressionToFilterFieldTranslatorTests.cs

@@ -0,0 +1,89 @@
+/* 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 Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Translators.ExpressionToAggregationExpressionTranslators
+{
+    public class ConvertExpressionToFilterFieldTranslatorTests : Linq3IntegrationTest
+    {
+        [Fact]
+        public void Should_translate_to_derived_class_on_method_call()
+        {
+            var collection = GetCollection();
+
+            var queryable = collection.AsQueryable()
+                .Select(p => new DerivedClass
+                {
+                    Id = p.Id,
+                    A = ((DerivedClass)p).A.ToUpper()
+                });
+
+            var stages = Translate(collection, queryable);
+            AssertStages(
+                stages,
+                "{ '$project' : { _id : '$_id', A : { '$toUpper' : '$A' } } }");
+
+            var result = queryable.Single();
+            result.Id.Should().Be(1);
+            result.A.Should().Be("TEST VALUE");
+        }
+
+        [Fact]
+        public void Should_translate_to_derived_class_on_projection()
+        {
+            var collection = GetCollection();
+
+            var queryable = collection.AsQueryable()
+                .Select(p => new DerivedClass()
+                {
+                    Id = p.Id,
+                    A = ((DerivedClass)p).A
+                });
+
+            var stages = Translate(collection, queryable);
+            AssertStages(
+                stages,
+                "{ '$project' : { _id : '$_id', A : '$A' } }");
+
+            var result = queryable.Single();
+            result.Id.Should().Be(1);
+            result.A.Should().Be("test value");
+        }
+
+        private IMongoCollection<BaseClass> GetCollection()
+        {
+            var collection = GetCollection<BaseClass>("test");
+            CreateCollection(collection, new DerivedClass()
+            {
+                Id = 1,
+                A = "test value"
+            });
+            return collection;
+        }
+
+        private class BaseClass
+        {
+            public int Id { get; set; }
+        }
+
+        private class DerivedClass : BaseClass
+        {
+            public string A { get; set; }
+        }
+    }
+}