node.js - Populating field values for referred documents in aggregate call in Mongoose -

i have 2 base schemas user , course

//user model

var userschema = new schema({     userid: {         type: string,         default: '',         required: 'please provide userid.',         index : true     },     name :{         type: string,         default: '',         trim : true     } }); 

//course schema

var courseschema = new schema({     title: {         type: string,         default: '',         required: 'please fill course title',         trim: true,         index: true     },      description: {         type: string,         default: '',         trim: true     },     category: {         type: string,         ref: 'category',         index: true     } }); 

and usercourseprogress schema storing each user's course progress.

var usercourseprogress= new schema({     userid: {         type: string,         ref:'user',         required: 'please provide user id',         index:true     },     courseid: {         type: schema.types.objectid,          ref: 'course',         required: 'please provide course id',         index: true     },     timespent: {         type: number, //seconds         default: 0     },     score: {         type: number      } }); 

now have aggregate results such on usercourseprogress schema on course level such show result : - {course name}( {no_of_users_taken_course} ) [e.g. course (15) ]

history.aggregate([{         $group: {"_id": {"primary": "$courseid"},         "popularitycount": {$sum: 1}}},         {$sort:{"popularitycount":-1}},         {$project:{"_id":0,             "courseid":"$_id.primary",             "count":1}}     ]) 

now want populate course title course schema through reference, after going through list of aggregation operators(here).i not able figure out way so.

can suggest way populate data other collections ??