Rails: How to setup multiple relation types to same models

Question

I have a Product and Category model which have a has_and_belongs_to_many relation. So I can search for Product.categories or Category.products

class Product < ApplicationRecord
  has_and_belongs_to_many :categories
end

class Category < ApplicationRecord
  has_and_belongs_to_many :products
end

Now I wanna add a new relation to the Product: It should called main_category which only should be a has_one relation from the Product to the Category. So a Product only can have one MainCategory. But the Category of course should return all MainCategory products.

• Must I create a SubClass of Category? Normally I would like to not create an extra class

• How can I solve that to simply call Product.main_category or Category.main_products?

• How to place indexes properly?

• How does the migration should look like?

Answer 1

You can simply do it like this i guess

class Product < ApplicationRecord
     has_and_belongs_to_many :categories
     belongs_to :main_category, class_name: 'Category', foreign_key: :main_category_id
end

class Category < ApplicationRecord
     has_and_belongs_to_many :categories
     has_many :main_products, class_name: 'Product', foreign_key: :main_category_id
end

You will have to add an column to products table called main_category_id

Source https://guides.rubyonrails.org/association_basics.html#bi-directional-associations