ruby on rails - Validate that an object has one or more associated objects

Question

I need to ensure that when a product is created it has atleast one category. I could do this with a custom validation class, but I was hoping there was a more standard way of doing it.

class Product < ActiveRecord::Base
  has_many :product_categories
  has_many :categories, :through => :product_categories #must have at least 1
end

class Category < ActiveRecord::Base
  has_many :product_categories
  has_many :products, :through => :product_categories
end

class ProductCategory < ActiveRecord::Base
  belongs_to :product
  belongs_to :category
end

question from:https://stackoverflow.com/questions/9535012/validate-that-an-object-has-one-or-more-associated-objects

Answer 1

There is a validation that will check the length of your association. Try this:

class Product < ActiveRecord::Base
  has_many :product_categories
  has_many :categories, :through => :product_categories

  validates :categories, :length => { :minimum => 1 }
end