CSHARP-4509: Support Convert calls to a base type in expression translators.

Author: rstam

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

@@ -49,6 +49,11 @@ public static AggregationExpression Translate(TranslationContext context, UnaryE
                     return TranslateConvertUnderlyingTypeToEnum(expression, operandTranslation);
                 }
 
+                if (IsConvertToBaseType(sourceType: operandExpression.Type, targetType: expressionType))
+                {
+                    return TranslateConvertToBaseType(expression, operandTranslation);
+                }
+
                 if (expressionType.IsConstructedGenericType && expressionType.GetGenericTypeDefinition() == typeof(Nullable<>))
                 {
                     var valueType = expressionType.GetGenericArguments()[0];
@@ -106,6 +111,11 @@ private static bool IsConvertEnumToUnderlyingType(UnaryExpression expression)
                 targetType.IsSameAsOrNullableOf(underlyingType);
         }
 
+        private static bool IsConvertToBaseType(Type sourceType, Type targetType)
+        {
+            return sourceType.IsSubclassOf(targetType);
+        }
+
         private static bool IsConvertUnderlyingTypeToEnum(UnaryExpression expression)
         {
             var sourceType = expression.Operand.Type;
@@ -116,6 +126,16 @@ private static bool IsConvertUnderlyingTypeToEnum(UnaryExpression expression)
                 sourceType.IsSameAsOrNullableOf(underlyingType);
         }
 
+        private static AggregationExpression TranslateConvertToBaseType(UnaryExpression expression, AggregationExpression operandTranslation)
+        {
+            var baseType = expression.Type;
+            var derivedType = expression.Operand.Type;
+            var derivedTypeSerializer = operandTranslation.Serializer;
+            var downcastingSerializer = DowncastingSerializer.Create(baseType, derivedType, derivedTypeSerializer);
+
+            return new AggregationExpression(expression, operandTranslation.Ast, downcastingSerializer);
+        }
+
         private static AggregationExpression TranslateConvertToBsonValue(TranslationContext context, UnaryExpression expression, Expression operand)
         {
             // handle double conversions like `(BsonValue)(object)x.Anything`

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Jira/CSharp4509Tests.cs

@@ -0,0 +1,67 @@
+/* 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;
+using System.Linq;
+using System.Linq.Expressions;
+using MongoDB.Driver.Linq;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Jira
+{
+    public class CSharp4509Tests : Linq3IntegrationTest
+    {
+        [Theory]
+        [InlineData(JobSortColumn.DriverName, "DriverName", LinqProvider.V2)]
+        [InlineData(JobSortColumn.DriverName, "DriverName", LinqProvider.V3)]
+        [InlineData(JobSortColumn.JobNumber, "JobNumber", LinqProvider.V2)]
+        [InlineData(JobSortColumn.JobNumber, "JobNumber", LinqProvider.V3)]
+        public void OrderByDescending_should_work(
+            JobSortColumn sortOrder,
+            string expectedSortField,
+            LinqProvider linqProvider)
+        {
+            var collection = CreateCollection(linqProvider);
+
+            Expression<Func<DbJob, object>> selector = sortOrder switch
+            {
+                JobSortColumn.JobNumber => j => j.JobNumber,
+                JobSortColumn.DriverName => j => j.DriverName,
+                _ => throw new NotSupportedException()
+            };
+
+            var queryable =
+                collection.AsQueryable()
+                .OrderByDescending(selector);
+
+            var stages = Translate(collection, queryable);
+            AssertStages(stages, $"{{ $sort : {{ {expectedSortField} : -1 }} }}");
+        }
+
+        private IMongoCollection<DbJob> CreateCollection(LinqProvider linqProvider)
+        {
+            var collection = GetCollection<DbJob>("jobs", linqProvider);
+            return collection;
+        }
+
+        public class DbJob
+        {
+            public int JobNumber { get; set; }
+            public string DriverName { get; set; }
+        }
+
+        public enum JobSortColumn { JobNumber, DriverName }
+    }
+}