var mapperConfiguration = new MapperConfiguration(config =>
{
    config.CreateMap<Post, PostDto>();
    config.CreateMap<Post.EmbeddedCategory, PostDto.CategoryDto>();
    config.CreateMap<Post.EmbeddedComment, PostDto.CommentDto>();
});
var mapper = mapperConfiguration.CreateMapper();
var result = postCollection
     .AsQueryable()
     .ProjectTo<PostDto>(mapper.ConfigurationProvider)
     .ToList();

 Message: System.FormatException : An error occurred while deserializing the Category property of class MongoDb.Bugs.PostDto: Element 'Id' does not match any field or property of class MongoDb.Bugs.PostDto+CategoryDto. ---- System.FormatException : Element 'Id' does not match any field or property of class MongoDb.Bugs.PostDto+CategoryDto. Stack Trace: MongoQueryProviderImpl`1.Execute(Expression expression) MongoQueryableImpl`2.GetEnumerator() List`1.ctor(IEnumerable`1 collection) Enumerable.ToList[TSource](IEnumerable`1 source) Tests.AutoMapper_ProjectTo() line 41 ----- Inner Stack Trace ----- BsonClassMapSerializer`1.DeserializeClass(BsonDeserializationContext context) BsonClassMapSerializer`1.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args) IBsonSerializer.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args) IBsonSerializerExtensions.Deserialize(IBsonSerializer serializer, BsonDeserializationContext context) BsonClassMapSerializer`1.DeserializeMemberValue(BsonDeserializationContext context, BsonMemberMap memberMap)