Pipeline projection translator does not use registered BsonClassMap and throws exception

Working with Data Drivers
, ,
1

I have a class hierarchy with base and derived classes in C#.
The BsonId is stored on the base class and the derived classes hold some additional data.
I have an additional Id field on the derived class used for other purposes, and I set up the BsonClassMap to map it to “Id”, so it shouldn’t conflict with the BsonId.
I want to run a pipeline to make a query on the “derived” collection, but the pipeline fails to render to Bson.

The issue may be that ExpressionToAggregationExpressionTranslator creates a new BsonClassMap with AutoMap() and does not use the registered BsonClassMaps.

Code:

using MongoDB.Bson;
using MongoDB.Bson.Serialization;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Driver;

BsonClassMap.RegisterClassMap<Derived>(cm =>
{
	cm.AutoMap();
	cm.UnmapProperty(x => x.Id);
	cm.MapProperty(x => x.Id).SetElementName(nameof(Derived.Id));
}).Freeze();

var pipeline = new EmptyPipelineDefinition<Derived>()
	.Project(x => new Derived
	{
		Id = x.Id,
	});

var rendered = pipeline.Render(BsonSerializer.SerializerRegistry.GetSerializer<Derived>(), BsonSerializer.SerializerRegistry);
Console.WriteLine(rendered);

public abstract class Base
{
	[BsonId]
	[BsonElement("_id")]
	public ObjectId UniqueId { get; set; }
}

public class Derived : Base
{
	[BsonElement("Id")]
	public string Id { get; set; }
}

Exception:

MongoDB.Bson.BsonSerializationException: The property 'Id' of type 'Derived' cannot use element name '_id' because it is already being used by property 'UniqueId' of type 'Base'.
   at MongoDB.Bson.Serialization.BsonClassMap.Freeze()
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.MemberInitExpressionToAggregationExpressionTranslator.Translate(TranslationContext context, MemberInitExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.ExpressionToAggregationExpressionTranslator.Translate(TranslationContext context, Expression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.ExpressionToAggregationExpressionTranslator.TranslateLambdaBody(TranslationContext context, LambdaExpression lambdaExpression, IBsonSerializer parameterSerializer, Boolean asRoot)
   at MongoDB.Driver.Linq.Linq3Implementation.LinqProviderAdapterV3.TranslateExpressionToProjection[TInput,TOutput](Expression`1 expression, IBsonSerializer`1 inputSerializer, IBsonSerializerRegistry serializerRegistry, ExpressionTranslationOptions translationOptions)
   at MongoDB.Driver.ProjectExpressionProjection`2.Render(IBsonSerializer`1 inputSerializer, IBsonSerializerRegistry serializerRegistry, LinqProvider linqProvider)
   at MongoDB.Driver.PipelineStageDefinitionBuilder.<>c__DisplayClass39_0`2.<Project>b__0(IBsonSerializer`1 s, IBsonSerializerRegistry sr, LinqProvider linqProvider)
   at MongoDB.Driver.DelegatedPipelineStageDefinition`2.Render(IBsonSerializer`1 inputSerializer, IBsonSerializerRegistry serializerRegistry, LinqProvider linqProvider)
   at MongoDB.Driver.AppendedStagePipelineDefinition`3.Render(IBsonSerializer`1 inputSerializer, IBsonSerializerRegistry serializerRegistry, LinqProvider linqProvider)
   at MongoDB.Driver.PipelineDefinition`2.Render(IBsonSerializer`1 inputSerializer, IBsonSerializerRegistry serializerRegistry)
   at Program.<Main>$(String[] args) in C:\src\MongoTest\MongoTest\Program.cs:line 19
2

I tried it out with different property names that do not cause conflict, but I got strange results:
the translator does not use the BsonClassMap for the target property in the assignment, only for the source.

Code:

using MongoDB.Bson;
using MongoDB.Bson.Serialization;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Driver;

BsonClassMap.RegisterClassMap<Derived>(cm =>
{
	cm.AutoMap();
	cm.UnmapProperty(x => x.Id2);
	cm.MapProperty(x => x.Id2).SetElementName("xyz");
}).Freeze();

var pipeline = new EmptyPipelineDefinition<Derived>()
	.Project(x => new Derived
	{
		Id2 = x.Id2,
	});

var rendered = pipeline.Render(BsonSerializer.SerializerRegistry.GetSerializer<Derived>(), BsonSerializer.SerializerRegistry);
foreach (var item in rendered.Documents)
	Console.WriteLine(item);

public abstract class Base
{
	[BsonId]
	[BsonElement("_id")]
	public ObjectId UniqueId { get; set; }
}

public class Derived : Base
{
	[BsonElement("abc")]
	public string Id2 { get; set; }
}

Result:

{ "$project" : { "abc" : "$xyz", "_id" : 0 } }
3

Ticket created in Jira:

https://jira.mongodb.org/browse/CSHARP-4579