db={comment:[{"datum_id":"datum1","html":"这是父评论1","author":{"user":"user1","role":"author"},"child_comments":[{"author":{"user":"user1","role":"author"}}]},{"datum_id":"datum1","html":"这是父评论2","child_comments":[]},],article:[{"id":"datum1","author":"user1"}],user:[{"id":"user1","email":"frmachao@126.com","name":"frmachao"},{"id":"user2","email":"machao@machao.com","name":"machao"}]}

db.comment.aggregate([{$unwind:{path:"$child_comments",preserveNullAndEmptyArrays:true,},},{$lookup:{from:"user",localField:"child_comments.author.user",foreignField:"id",as:"child_comments.author.user",},},{$group:{_id:"$_id",datumID:{$first:"$datumID",},html:{$first:"$html"},child_comments:{$push:"$child_comments",},},},])