db={"users":[{_id:34,name:"mama mia"}],"posts":[{_id:67,body:" hello mongoose",likes:[34]}]}

db.posts.aggregate([{$lookup:{from:"users",localField:"likes",foreignField:"_id",as:"likes"}},{$project:{likes:{$arrayToObject:{$map:{input:"$likes",in:{k:{$toString:"$$this._id"},v:"$$this.name"}}}},likesCount:{$size:"$likes"},liked:{$in:[34,"$likes"]},body:1}}])