Rails: STI (Single Table Inheritance)

	class User < ActiveRecord::Base
	has_many :animals
	has_many :dogs
	has_many :cats
	end
	class Zoo < ActiveRecord::Base
	has_many :animals
	has_many :elephants
	end
	class Animal < ActiveRecord::Base
	# for STI should have additional field `type`
	scope :domestic, -> { where("user_id IS NOT NULL").includes(:user) }
	end
	class Elephant < Animal
	belongs_to :zoo
	end
	class Dog < Animal
	belongs_to :user
	end
	class Cat < Animal
	belongs_to :user
	end
	user = User.first
	user.animals # -> [<Dog:...>, <Cat:...> ..]
	user.dogs # -> [<Dog:...>, ..]
	user.cats # -> [<Cat:...>, ..]
	zoo = Zoo.first
	zoo.animals # -> [<Elphant:...>, ..]
	zoo.elephants # -> [<Elphant:...>, ..]
	Animals.all # -> [<Dog:...>, <Cat:...>, <Elephants:..>, ..]
	Dogs.all # -> [<Dog:...>, ..]
	Cats.all # -> [<Cat:...>, ..]
	Elephant.all # -> [<Elephants:...>, ..]
	Animals.domestic # -> [<Dog:...>, <Cat:...> ..]

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