Single Table Inheritance

active_record_hack.rb

# Use in subclasses like:
# delegate_details :win_ratio, :tanning_time, to: :wrestling_details
def self.delegate_details(*attributes)
  options = attributes.extract_options!
  association_name = options.fetch(:to) {
    raise ArgumentError.new "You must specify the name of the details association"
  }

  define_method association_name do
    super() || send("build_#{association_name}")
  end

  attributes.each do |attribute_name|
    # Getter, setter, and boolean getter (in case it's a boolean attribute)
    def_delegators association_name, 
      :"#{attribute_name}", :"#{attribute_name}=", :"#{attribute_name}?"
  end
end

active_record_migration.rb

ActiveRecord::Schema.define(:version => 20130203210553) do
 
  create_table "people", :force => true do |t|
    t.string :name
    t.string :shoe_size
    t.string :place_of_birth
    t.string :blood_type
  end
 
  create_table :scientists, :force => true do |t|
    t.string :name
    t.string :shoe_size
    t.string :place_of_birth
    t.string :blood_type
 
    t.string :university
  end
    
  create_table :chefs, :force => true do |t|
    t.string :name
    t.string :shoe_size
    t.string :place_of_birth
    t.string :blood_type
    
    t.string :hat_size
  end
end

active_record_schema.rb

ActiveRecord::Schema.define(:version => 20130203210553) do
 
  create_table "people", :force => true do |t|
    t.string :name
    t.string :shoe_size
    t.string :place_of_birth
    t.string :blood_type
 
    t.string :type # => Needed for the ORM to know which kind of object its dealing with.
    
    t.string :university # => The Scientist field(s)
    
    t.string :hat_size  # => The Chef field(s)
  end
end

sequel_code.rb

require 'sequel'
 
DB = Sequel.connect 'sqlite://database.db'
 
class Person < Sequel::Model
  # has name, shoe_size, place_of_birth, blood_type
end
 
Person.plugin :class_table_inheritance, key: :type
 
class Chef < Person
  # has hat_size
end
 
class Scientist < Person
  # has university
end
 
class Physicist < Scientist
  # has lightsaber_color
end

sequel_migration.rb

Sequel.migration do
  change do
    create_table :people do
      primary_key :id
 
      String :name
      String :shoe_size
      String :place_of_birth
      String :blood_type
 
      String :type
    end
 
    create_table :chefs do
      foreign_key :id, :people
      String :hat_size
    end
 
    create_table :scientists do
      foreign_key :id, :people
      String :university
    end
 
    create_table :physicists do
      foreign_key :id, :scientists
      String :lightsaber_color
    end
  end
end

sequel_result.rb

Scientist.create(name: 'Alan Turing', shoe_size: '41', university: 'Cambridge')
# => #<Scientist @values={:id=>1, :name=>"Alan Turing", :shoe_size=>"41", :place_of_birth=>nil, :blood_type=>nil, :type=>"Scientist", :university=>"Cambridge"}>

Chef.create(name: 'Guy Fieri', shoe_size: '45', hat_size: '12')
# => #<Chef @values={:id=>2, :name=>"Guy Fieri", :shoe_size=>"45", :place_of_birth=>nil, :blood_type=>nil, :type=>"Chef", :hat_size=>"12"}>

Chef.create(name: 'Guy Fieri', shoe_size: '45', hat_size: '12')
# => #<Chef @values={:id=>3, :name=>"Guy Fieri", :shoe_size=>"45", :place_of_birth=>nil, :blood_type=>nil, :type=>"Chef", :hat_size=>"12"}>

Person.all
# => [#<Scientist @values={:id=>1, :name=>"Alan Turing", :shoe_size=>"41", :place_of_birth=>nil, :blood_type=>nil, :type=>"Scientist"}>, #<Chef @values={:id=>2, :name=>"Guy Fieri", :shoe_size=>"45", :place_of_birth=>nil, :blood_type=>nil, :type=>"Chef"}>, #<Chef @values={:id=>3, :name=>"Guy Fieri", :shoe_size=>"45", :place_of_birth=>nil, :blood_type=>nil, :type=>"Chef"}>]

Scientist.first(university: 'Cambridge')
# => #<Scientist @values={:id=>1, :name=>"Alan Turing", :shoe_size=>"41", :place_of_birth=>nil, :blood_type=>nil, :type=>"Scientist", :university=>"Cambridge"}>