db={articles:[{_id:1,liked:[1,2],content:"a and b"},{_id:2,liked:[1],content:"a"}],users:[{_id:1,name:"a",email:"a.com",password:"a"},{_id:2,name:"b",email:"b.com",password:"b"}]}

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