Database
db={"coll1":[{"_id":123123,"k":"value"},{"_id":2,"k":"someOtherValue"}],"coll2":[{"_id":456734,"k2":"value"}],"ref":[{refName:"Collection 1",refId:"123123",},{refName:"Collection 2",refId:"456734"}]}
Query
db.ref.aggregate([{$addFields:{refId:{"$toInt":"$refId"}}},{"$lookup":{"from":"coll1","localField":"refId","foreignField":"_id","as":"join"}}])