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([{"$lookup":{"from":"coll1",let:{rid:"$refId"},pipeline:[{$match:{$expr:{$eq:["$$rid",{$toString:"$_id"}]}}}],"as":"join"}}])