You have previous
class Product
has_and_belongs_to_many :categories
end
class Category
has_and_belongs_to_many :products
end
and migration
class CreateCategoriesProducts < ActiveRecord::Migration
def change
create_table :categories_products, id: false do |t|
t.references :category
t.references :product
end
add_index :categories_products, [:category_id, :product_id]
end
end
now change all to
class Product
has_many :categories_products, dependent: :destroy
has_many :categories, through: :categories_products
end
class Category
has_many :categories_products, dependent: :destroy
has_many :products, through: :categories_products
end
and new one
class CategoriesProduct < ActiveRecord::Base
# this model uses table "categories_products" as it is
# column products_count is in the table "categories"
belongs_to :category, counter_cache: :products_count
belongs_to :product
end
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