db={"users":[{_id:1,username:"j",password:"123",privacy_level:"public"},{_id:2,username:"k",password:"1234",privacy_level:"private"}],"posts":[{_id:1,user_id:1,text:"hi",timestamp:111},{_id:2,user_id:1,text:"hi2",timestamp:222},{_id:3,user_id:2,text:"hi3",timestamp:333},]}

db.users.aggregate([{$match:{privacy_level:"public"}},{"$lookup":{"from":"posts","localField":"_id","foreignField":"user_id","as":"posts"}},{$unwind:"$posts"},{$project:{_id:"$posts._id",text:"$posts.text",timestamp:"$posts.timestamp"}}])