node.js - How to use $lookup on embedded document field in mogodb -

please have look, appriciated

var user = new schema({       name: string,      });  var comments = new schema({     title     : string   , body      : string   ,user_id :  {type: schema.types.objectid, ref: 'user' }   , date      : date  });   var blog = new schema({     author    : string   , title     : string   , body      : string   , date      : date   , user_id   :{type: schema.types.objectid, ref: 'user' }   , comments  : [comments]  });    db.blogs.aggregate([     { $match : { "_id" : objectid("57e3b7f4409d80a508d52769") } },  { $lookup: {from: "users", localfield: "user_id", foreignfield: "_id", as: "user"} },  ]) 

this returns

[   {     "_id": "57e3b7f4409d80a508d52769",     "author": "tariq",     "title": "myfirstpost",     "body": "this first post",     "user_id": "57e3b763f7bc810c08f9467a",     "comments": [       {         "title": "hi",         "body": "again commenting on this",         "user_id": "57e3b763f7bc810c08f9467a",         "_id": "57e3c153409d80a508d5276b"       },       {         "title": "hi",         "body": "this seond comment",         "user_id": "57e3b763f7bc810c08f9467a",         "_id": "57e3c8632ebca0ee0afb2ac6"       }     ],     "__v": 0,     "user": [       {         "_id": "57e3b763f7bc810c08f9467a",         "name": "tariq",         "username": "teekay",         "password": "123456",         "__v": 0       }     ]   } ] 

this return result comparing blog table , user table _id fine .. want userdetail each comment using user_id of “comments.user_id” blog collection , “_id” of collection should this

"_id": "57e3b7f4409d80a508d52769",     "author": "tariq",     "title": "myfirstpost",     "body": "this first post",     "user_id": "57e3b763f7bc810c08f9467a",     "comments": [       {         "title": "hi",         "body": "again commenting on this",         "user_id": "57e3b763f7bc810c08f9467a",         "_id": "57e3c153409d80a508d5276b",   "user": [       {         "_id": "57e3b763f7bc810c08f9467a",         "name": "tariq",         "username": "teekay",         "password": "123456",         "__v": 0       }     ]       }, 

you can run aggregation operation of pipeline:

db.blogs.aggregate([     { "$unwind": "$comments" },     {          "$lookup": {             "from": "users",              "localfield": "comments.user_id",              "foreignfield": "_id",              "as": "comments.user"         }      },     { "$unwind": "$comments.user" },     {         "$group": {             "_id": "$_id",             "author": { "$first": "$author" },             "title": { "$first": "$title" },             "body": { "$first": "$body" },             "comments": { "$push": "$comments" },             "user_id": { "$first": "$user_id" }         }     },     {          "$lookup": {             "from": "users",              "localfield": "user_id",              "foreignfield": "_id",              "as": "user"         }      },     { "$unwind": "$user" }, ])