sti_bad.rb

class Animal < ActiveRecord::Base
  def eat
  end
end

class Bird < Animal
  def fly
  end
end

class Mammal < Animal
  def run
  end
end

class Fish < Animal
  def swim
  end
end

# Animals should not be modeled using STI since their single table
# would have a lot of sparse columns (ie. mammals without wing_span and
# birds without num_of_fins)

class CreateAnimals < ActiveRecord::Migration
  def change
    create_table :animals do |t|
      # STI required field
      t.string :type
      
      # only applicable to birds, mammals and fish will have null values
      t.float   :wing_span
      
      # only applicable to fish, mammals and birds will have null values
      t.integer :num_of_fins
      
      # only applicable to mammals and birds, fish need not apply
      t.integer :num_of_legs
      
      # ... more column fields #
      t.timestamps
    end
  end
end