db={"posts":[{id:1,title:"my title",content:"bla bla bla"}],"comments":[{post_id:1,user_id:"123",content:"great article!!!"}],"users":[{id:"123",name:"Ferb"}]}

db.posts.aggregate([{"$lookup":{"from":"comments","localField":"id","foreignField":"post_id","pipeline":[{"$lookup":{"from":"users","localField":"user_id","foreignField":"id","as":"usersLookup"}},{"$unwind":"$usersLookup"}],"as":"commentsLookup"}},{"$unwind":"$commentsLookup"}])