db={"Users":[{_id:"5d93f338e602a10a38ad3588",userID:"1",email:"test@test.com",password:"password",firstName:"Tom",lastName:"Bombadil"}],"Posts":[{_id:"5d93fddce602a10a38ad358a",postID:"1",userID:"1",postContent:"hello world"}]}

db.Users.aggregate([{$lookup:{from:"Posts",localField:"userID",foreignField:"userID",as:"usersPosts"}},{$addFields:{userPostContents:"$usersPosts.postContent"}},{$project:{userID:1,userPostContents:1}}])