When you use :model wrapper, and it loads object attributes from ES to Mongoid model, it sets your association's id as {"$oid" => BSON::ObjectId()}, where it should just be BSON::ObjectId().
Coverage increased (+0.02%) when pulling feeb3733b030be39cbea0ebafc758ca3eafaf34a on Antti:fix_relation_id_loading_with_model_wrapper into 7dab1568d3c87406a1dc80e33dd403696ee541dc on rs-pro:master.
When you use :model wrapper, and it loads object attributes from ES to Mongoid model, it sets your association's id as {"$oid" => BSON::ObjectId()}, where it should just be BSON::ObjectId().
Some related info https://github.com/mongoid/mongoid/pull/2947
https://github.com/rs-pro/mongoid-elasticsearch/issues/1