db={"collectionA":[{key:"value",ref:1},{key:"value",}],"collectionB":[{_id:1,key1:"value1"},{_id:null,key1:"value1"}]}

db.collectionA.aggregate([{$match:{"ref":{"$exists":true}}},{"$lookup":{"from":"collectionB","localField":"ref","foreignField":"_id","as":"ref"}}])