Multi table Inheritance for rails
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README.rdoc

ActsAsRelation

A acts_as relationship sets up a one-to-one connection with another model, such that declaring model inherits from other model (with separate database tabels). For example in a shop all products have common attributes (name, price, image …), while each type of them has their specific attributes, pen has color, book has author and publisher and so on.

ActiveRecord only supports singletable inheritance, but with single table inheritance number of attributes on parent model (product in this example) grow exponentially, and must of them will always stay NULL.

acts_as use a polymorphic has_one association to simulate a multi-table inheritance. For the shop example you'd declare the product as a supermodel and all types of it as acts_as :product (if you prefer you can use their aliases is_a and is_a_superclass)

class Product < ActiveRecord::Base
  acts_as_superclass
end

class Pen < ActiveRecord::Base
  acts_as :product
end

class Book < ActiveRecord::Base
  acts_as :product
end

To make this work, you need to declare both a foreign key column and a type column in the model that declares superclass. To do this you can set :as_relation_superclass option to true on products create_table (or pass it name of the association):

create_table :products, :as_relation_superclass => true do |t|
  # ...
end

Or declare them as you do on a polymorphic belongs_to association, it this case you must pass name to acts_as in :as option:

change_table :products do |t|
  t.integer :producible_id
  t.string  :producible_type
end

class Pen < ActiveRecord::Base
  acts_as :product, :as => :producible
end

Now Pen and Book act as Product. This means that they inherit Product attributes, associations, validations and methods.

To see its functionality lets add some stuff to product:

class Product
  validates_presence_of :name, :price

  def to_s
    "#{name} $#{price}"
  end
end

now we can to things like this:

Pen.create :name => "Nice Pen", :price => 1.3, :color => "Red"
Pen.where "name = ?", "Some Pen"
pen = Pen.new
pen.valid?      # => false
pen.errors.keys # => [:name, :price]
Pen.first.to_s  # => "Nice Pen $1.3"

When you declare an acts_as relation, the declaring class automatically gains parent methods (includeing accessors) so you can access them directly.

On the other hand you can always access a specific object from its parent by calling specific method on it:

Product.first.specific # will return a specific product, a pen for example

Options

The acts_as relation support these options:

  • :as

  • :auto_include

  • :class_name

  • :conditions

  • :dependent

  • :include

when :auto_join option set to true (which is by default), every query on child will automatically includes the parent. For example:

Pen.where("name = ?", "somename")

will result in the following SQL:

SELECT "pens".* FROM "pens" INNER JOIN "products" ON "products"."as_product_id" = "pens"."id" AND "products"."as_product_type" = 'Pen' WHERE (name = 'somename')

All other options are same as has_one options.