db={"collection":[{"_id":"5e8e4fcf781d96df5c1f5358","name":"358 Event"},{"_id":"5e8e55c5a0f5fc1431453b5f","name":"b5f Event"}],"other":[{"_id":"5e8f4ed2ddab5e3d04ff30b3","userId":"5e6c2dddad72870c84f8476b","eventId":"5e8e4fcf781d96df5c1f5358",}]}

db.collection.aggregate([{$lookup:{from:"other",localField:"_id",foreignField:"eventId",as:"userIds"}}])