userschema favoriting

//...
    favorites: [{type: mongoose.Schema.Types.ObjectId, ref: 'Article' }],
  },
  {timestamps: true}
);

UserSchema.plugin(uniqueValidator, {message: 'is already taken.'});

// push article id to favorites list and save the state of the user
UserSchema.methods.favorite = function (id) {
  if (this.favorites.indexOf(id) === -1) {
    this.favorites = this.favorites.concat([id]);
    console.log('favorites after favoriting', this.favorites);
    console.log('article id', id);
  }
  return this.save();
};

// remove article from favorites list and save state
UserSchema.methods.unfavorite = function(id){
  this.favorites.remove(id);
  return this.save();
}

//...