Method to merge objects which are similar (differing by quantity)

Question

I need help with a cart object which has_many line_items. If a line_item in the cart is created and it has the same exact attributes as a line_item which is already in the cart I just want to update the pre existing line_items quantity rather than create duplicate objects with separate quantities.

I wrote a few methods in my models to try and make this work but it isn't working. Below is my code:

models

class LineItem < ActiveRecord::Base
  attr_accessible :cart_id, :product_id, :quantity, :unit_price, :product, :cart, :color_id, :size_id, :extra_id
  belongs_to :cart
  belongs_to :product
  belongs_to :color
  belongs_to :size
  belongs_to :extra
  validates :quantity, :presence => true

  def update_quantity(qty)
    quantity += qty
    quantity.save
  end
  def exists_in_collect?(items)
    if items.include?(product)
        if color == items.color && size == items.sizes && extra == items.extra
            return true
        end
    else
        return false
    end
  end
end

class Cart < ActiveRecord::Base
  attr_accessible :purchased_at
  has_many :line_items
  has_one :order

  def where_line_item_with(prod_id)
    line_items.where(:product_id => prod_id)
  end
end

controller

class LineItemsController < ApplicationController
  def new
    @line_item = LineItem.new
  end
  def create
    @line_item = LineItem.new(params[:line_item].merge(:cart => current_cart))
    if @line_item.exists_in_collect?(current_cart.line_items)
      current_cart.where_line_item_with(product.id).update_quantity(@line_item.quantity)
      @line_item.destroy!
    else
    @line_item.save!
    @line_item.update_attributes!(:unit_price => @line_item.item_price)
    end


    redirect_to current_cart_url
  end
  def update
    @line_item = LineItem.find(params[:id])
    @line_item.update_attributes(params[:line_item])
    redirect_to current_cart_url
  end

Any insight is fully appreciated.

Answer 1

1.You should change your where_line_item_with(prod_id) to the following:

def where_line_item_with(prod_id)
  line_items.where(:product_id => prod_id).first
end

As the where returns an array and you cannot do update_quantity(@line_item.quantity) on an array.

2.In exists_in_collect?(items) - Here your aim is to find if the items of the cart include the item similar to new item. it should be updated as following:

def exists_in_collect?(items)
  items.each do |item|
    if color == item.color && size == item.sizes && extra == item.extra && product == item.product
        return true
    end  
  end
  return false
end