line_items_spec.rb

require 'rails_helper'

describe  LineItem do
  it "test" do
    product1 = Product.new(price: 50)
    product2 = Product.new(price: 150)

    item1 = LineItem.new(product_id: product1, quantity: 1)
    item2 = LineItem.new(product_id: product2, quantity: 1)

    line_item = LineItem.new
    line_item.total_price == 200
  end
end