following feature

    following: [{ type: mongoose.Schema.Types.ObjectId, ref: 'User' }],

// ...

// add user to the followed list
UserSchema.methods.follow = function(id){
  if (this.following.indexOf(id) === -1){
    this.following = this.following.concat([id]);
  }
  return this.save();
}

// remove the user from the followed list
UserSchema.methods.unfollow = function(id){
  this.following.remove(id);
  return this.save();
}

// see if we're following this user
UserSchema.methods.isFollowing = function(id){
  return this.following.some(function(followId){
    return followId.toString() === id.toString();
  })
}

UserSchema.methods.toProfileJSONFor = function (user) {
  return {
    username: this.username,
    bio: this.bio,
    image:
      this.image || 'https://static.productionready.io/images/smiley-cyrus.jpg',
    // true if we're following the given user, false otherwise
    following: user ? user.isFollowing(this._id) : false,
  };
};

// register the schema with mongoose as 'User'
mongoose.model('User', UserSchema);