app/models/subscription_product_class.rb
class SubscriptionProductClass < ApplicationRecord
validates :subscription_id, presence: true
validates :product_class, presence: true
validates :product_class, uniqueness: { case_sensitive: false, scope: :subscription_id }, presence: true
belongs_to :subscription
has_many :products, primary_key: 'product_class', foreign_key: 'product_class', class_name: 'Product'
end