db={"users":[{"_id":1,"name":"John"},{"_id":2,"name":"Sally"}],"habits":[{"_id":1,"user":1,"name":"sleeping","frequency":"daily"},{"_id":2,"user":1,"name":"eating","frequency":"daily"},{"_id":3,"user":1,"name":"working","frequency":"daily"},{"_id":4,"user":2,"name":"meditation","frequency":"weekly"}]}

db.users.aggregate([{"$lookup":{"from":"habits","localField":"_id","foreignField":"user","as":"habits"}}])