How do I count the value from one table and then display the same value in another table?

Question

I want to count the total no of Products for a particular shopping list and then display the total count of products for that shopping list according to its id in the Shopping List table .I have created a reference for shopping_list in the product table

My Shopping list table is as follows:-

id serial NOT NULL,
  shopping_list_name character varying(255),
  shopping_list_status character varying(255) DEFAULT 'OPEN'::character varying,
  total_items character varying(255) DEFAULT 0,
  created_at timestamp without time zone NOT NULL,
  updated_at timestamp without time zone NOT NULL,
  deleted integer NOT NULL DEFAULT 0,
  CONSTRAINT shopping_lists_pkey PRIMARY KEY (id)

My Product table is as follows:-

 id serial NOT NULL,
  shopping_list_id integer NOT NULL,
  product_name character varying(255) DEFAULT 'null'::character varying,
  product_category character varying(255),
  quantity integer,
  status character varying(255) DEFAULT 'OPEN'::character varying,
  deleted integer NOT NULL DEFAULT 0,
  CONSTRAINT products_pkey PRIMARY KEY (id)

Can anyone Please Help me:-

Answer 1

So you will have something like

class ShoppingList < ActiveRecord::Base
  has_many :products
end

class Product < ActiveRecord::Base
  belongs_to :shopping_list
end

Then in order to get the number of products for a shopping list

@my_shopping_list.products.count

And to get the quantity

@my_shopping_list.products.sum(:quantity)

---

You can have some kind of cache on your shopping list, so lets say that you have an integer field named products_quantity on your ShoppingList, which by default is 0 (make sure to make it 0 by default, not nil).

Then on your Product model, you can do something like

class Product < ActiveRecord::Base
  after_create :update_shopping_list

  def update_shopping_list
    shopping_list.update_attribute(:products_quantity, shopping_list.products_quantity + self.quantity)
  end
end

In that case you don't have to execute the query SELECT SUM(quantity) FROM PRODUCTS WHERE shopping_list_id = ?

I'm not sure if I answer your second question, but if you have more questions or if you were trying to ask something else, just let me know.