CSHARP-3845: Deserialize Select of anonymous types using default values for missing fields.

Author: rstam

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

@@ -50,7 +50,8 @@ public static AggregationExpression Translate(TranslationContext context, NewExp
                 var fieldExpression = expression.Arguments[i];
                 var fieldTranslation = ExpressionToAggregationExpressionTranslator.Translate(context, fieldExpression);
                 var memberSerializer = fieldTranslation.Serializer ?? BsonSerializer.LookupSerializer(fieldExpression.Type);
-                classMap.MapProperty(member.Name).SetSerializer(memberSerializer);
+                var defaultValue = GetDefaultValue(memberSerializer.ValueType);
+                classMap.MapProperty(member.Name).SetSerializer(memberSerializer).SetDefaultValue(defaultValue);
                 computedFields.Add(AstExpression.ComputedField(member.Name, fieldTranslation.Ast));
             }
 
@@ -69,5 +70,17 @@ public static AggregationExpression Translate(TranslationContext context, NewExp
 
             return new AggregationExpression(expression, ast, serializer);
         }
+
+        private static object GetDefaultValue(Type type)
+        {
+            if (type.IsValueType)
+            {
+                return Activator.CreateInstance(type);
+            }
+            else
+            {
+                return null;
+            }
+        }
     }
 }

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Jira/CSharp3845Tests.cs

@@ -0,0 +1,64 @@
+/* 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 Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Jira
+{
+    public class CSharp3845Tests : Linq3IntegrationTest
+    {
+        [Fact]
+        public void Select_of_anonymous_class_with_missing_fields_should_work()
+        {
+            var collection = GetCollection<C>();
+            CreateCollection(
+                collection,
+                new[]
+                {
+                    new C { Id = 1, S = null, X = 0 },
+                    new C { Id = 2, S = "abc", X = 123 }
+                });
+
+            var queryable = collection.AsQueryable()
+                .Select(c => new { F = c.S, G = c.X });
+
+            var stages = Translate(collection, queryable);
+            var expectedStages = new[]
+            {
+                "{ $project : { F : '$S', G : '$X', _id : 0 } }"
+            };
+            AssertStages(stages, expectedStages);
+
+            var results = queryable.ToList();
+            results.Count.Should().Be(2);
+            results[0].F.Should().Be(null);
+            results[0].G.Should().Be(0);
+            results[1].F.Should().Be("abc");
+            results[1].G.Should().Be(123);
+        }
+
+        private class C
+        {
+            public int Id { get; set; }
+            [BsonIgnoreIfNull]
+            public string S { get; set; }
+            [BsonIgnoreIfDefault]
+            public int X { get; set; }
+        }
+    }
+}

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Linq3IntegrationTest.cs

@@ -29,6 +29,22 @@ protected void AssertStages(IEnumerable<BsonDocument> stages, IEnumerable<string
             stages.Should().Equal(expectedStages.Select(json => BsonDocument.Parse(json)));
         }
 
+        protected void CreateCollection<TDocument>(IMongoCollection<TDocument> collection, IEnumerable<TDocument> documents = null)
+        {
+            var database = collection.Database;
+            var collectionName = collection.CollectionNamespace.CollectionName;
+            database.DropCollection(collectionName);
+
+            if (documents != null && documents.Any())
+            {
+                collection.InsertMany(documents);
+            }
+            else
+            {
+                database.CreateCollection(collectionName);
+            }
+        }
+
         protected IMongoCollection<TDocument> GetCollection<TDocument>(string collectionName = null)
         {
             var databaseName = DriverTestConfiguration.DatabaseNamespace.DatabaseName;