db={/** collectionA*/A:[{key:"value",ref:"SOME-OBJECT-ID"},{key:"v2"}],/** collectionB*/B:[{_id:"SOME-OBJECT-ID",key1:"value1"}]}

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