Database

db={"users":[{"_id":1,"name":"John"},{"_id":2,"name":"Jack"},{"_id":3,"name":"Mike"}],"posts":[{"_id":1,"text":"Post 1","userId":1},{"_id":2,"text":"Post 2","userId":1},{"_id":3,"text":"Post 3","userId":2}]}

Query

db.users.aggregate([{"$lookup":{"from":"posts","localField":"_id","foreignField":"userId","as":"post"}},{$match:{post:{$ne:[]}}},{$project:{post:0}}])

Result