Class Variables instantiated(refreshed) for each new instance - ruby-on-rails

Question

I have two classes Category and order_item, where an order_item belongs_to a category.
I want to add unique 'category' items to a class variable that is an array of 'categories' and is represented by an array class variable ##categoriesList, each time a new category is detected among the instance variables.
This is what i tried.
class OrderItemsController < ApplicationController
##categoriesList = []
def create
#order_item = OrderItem.new(order_item_params)
if #order_item.save
#total = #order_item.order.total
#orderItemCategory = #order_item.category
if #orderItemCategory.set?
if !(##categoriesList.include? #orderItemCategory)
#total += #orderItemCategory.price
##categoriesList.push(#orderItemCategory)
end
........
........
end
Code Explanation:
I don't want the price of the next instance of order_item to be taken in account, if the price of a previous order_item that belongs to a same category's price has already been taken in account for.
For example: Egg and Milk both belong to Combo-1. So i just want to take into account the price of Combo-1 once, and not for each order_item instance, i.e. egg and milk, which would double the total amount.
What I tried:
I push the category name of the order_item after it's price has been taken in account. And when the next order_item is created, i checked if that order_item's category's price has already been recorded by checking it in the current ##categoriesList class variable.
Problem:
At each instance when i check the class variable ##categoriesList, it returns an empty array list, and no previous records that have been pushed to that array is shown.
I want something like a static variable in java, where every instance of the class shares the same variable, without actually refreshing the data in the variable for each instance.

Answer 1

You don't want a class variable pretty much ever since they are not thread safe or actually persistent across requests which really applies to almost any programming language. Every time the code is reloaded the class variable will be reset.
What your trying to do can really be done by modeling the domain properly. In the case of a checkout system its been done a billion times already and the common pattern is:
class Order < ApplicationRecord
has_many :line_items
has_many :products, through: :line_items
end
# rails g model line_item quantity:decimal unit_price:decimal order:belongs_to product:belongs_to
class LineItem < ApplicationRecord
belongs_to :order
belongs_to :product
end
class Product < ApplicationRecord
has_many :line_items
has_many :orders, through: :line_items
end
On the line items table which represents an actual line on an order form you store what order the item belongs to, quantity & the unit price at the time of purchase. To tally an order you sum the line items:
class LineItem < ApplicationRecord
# ...
def net
quantity * unit_price
end
end
class Order < ApplicationRecord
# ...
def net
line_items.sum(&:net)
end
end
So then your can just call order.net and it will give you the net sum. I have no clue where you're going with that categories mess but if we looked up the price here by going to the product we would not be able to account for past transactions unless the prices are completely static.
This is how you would handle creating line items:
resources :orders do
resources :line_items
end
class LineItemsController < ApplicationController
before_action :set_order
# GET /orders/1/line_items
def new
#line_item = #order.line_items.new
#products = Product.all
end
# POST /orders/1/line_items
def create
#line_item = #order.line_items.new(line_item_params) do |items|
items.unit_price = li.product.price # stores the price at time of purchase
end
if #line_item.save
redirect_to #order
else
#products = Product.all
render :new
end
end
# ...
private
def line_item_params
params.require(:line_item)
.permit(:quantity, :product_id)
end
def set_order
#order = Order.find(params[:order_id])
end
end
<%= form_with(model: [#order, #line_item], local: true) do |f| %>
<div class="field">
<%= f.label :product_id %>
<%= f.collection_select :product_id, #products, :id, :name %>
</div>
<div class="field">
<%= f.label :quantity %>
<%= f.number_field :quantity %>
</div>
<%= f.submit %>
<% end %>