how to retrieve other details of a schema based on the reference key

Question

I have a project am working on. I have a comment schema, a likes schema and a blog schema. All three are declared as separate schemas but the likes and comments schema are then nested into the blogs schema as children i.e comments: [CommentSchema]. Now i have a page where when someone clicks on a blog it displays all the comments together with the blog. This the code that gets the blog Blog.findById(id).populate('user', 'username').exec(function(err, blog). Now in the comments array there is a key called commOwner that refers to the objectid from another schema called user just like the blog schema has a reference key too as u can see from the code. I am trying to display the gravatar and username of each person that made a comment based on the reference key commOwner that's in the comments schema but i don't know how to go about it. I want to also be able to populate the usernames and gravatar of those who commented on the blog in that same code where i did it for the blog. Please can someone help me with this. below is the code for all my schemas

'use strict';

/**
 * Module dependencies.
 */
var mongoose = require('mongoose'),
    Schema = mongoose.Schema;


/*Comment schema*/
var CommentSchema = new Schema({
    commOwner: {
        type: Schema.ObjectId,
        ref: 'User'
    },
    commbody: {
        type: String,
        default: '',
        trim: true,
        //required: 'Comment cannot be blank'
    },
    updated: {
        type: Date,
        default: Date.now
    }
});

/**
 * Likes Schema
 */
var LikeSchema = new Schema({
    score : {
        type: Number,
        default: 0
    },
    user: {
        type: Schema.ObjectId,
        ref: 'User'
    }
});


/**
 * Blog Schema
 */
var BlogSchema = new Schema({
    created: {
        type: Date,
        default: Date.now
    },
    title: {
        type: String,
        default: '',
        trim: true,
        required: 'Title cannot be blank'
    },
    content: {
        type: String,
        default: '',
        trim: true
        //required: 'Content cannot be blank'
    },
    user: {
        type: Schema.ObjectId,
        ref: 'User'
    },
    comments: [CommentSchema],
    likes: [LikeSchema]

});

mongoose.model('Blog', BlogSchema);

Answer 1

Blog.findById(id).
  populate(
     [{path:'user', select:'username'},
     {path:'comments.commOwner',select:'username profilepic ...'}])
  .exec(function(err, blog){...})

In the select field specify other fields( separated by spaces ) to be populated in commOwner. Have a look at the docs