Mongoose: deep population (populate a populated field)

Populating across multiple levels

Say you have a user schema which keeps track of the user's friends.

var userSchema = new Schema({
  name: String,
  friends: [{ type: ObjectId, ref: 'User' }]
});

Populate lets you get a list of a user's friends, but what if you also wanted a user's friends of friends? Specify the populate option to tell mongoose to populate the friends array of all the user's friends:

User.findOne({ name: 'Val' }).populate({
    path: 'friends',
    // Get friends of friends - populate the 'friends' array for every friend
    populate: { path: 'friends' }
});

Reference: http://mongoosejs.com/docs/populate.html#deep-populate

Firstly, update mongoose 3 to 4 & then use the simplest way for deep population in mongoose as shown below:

Suppose you have Blog schema having userId as ref Id & then in User you have some review as ref Id for schema Review. So Basically, you have three schemas:

1. Blog
2. User
3. Review

And, you have to query from blog, which user owns this blog & the user review. So you can query your result as :

BlogModel
  .find()
  .populate({
    path : 'userId',
    populate : {
      path : 'reviewId'
    }
  })
  .exec(function (err, res) {

  })