example

animal.rb

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:...> ..]