PolymorphicIntegerType

Rails' polymorphic associations are pretty useful. The example they give to set it up looks like:

class Picture < ActiveRecord::Base
  belongs_to :imageable, polymorphic: true
end

class Employee < ActiveRecord::Base
  has_many :pictures, as: :imageable
end

class Product < ActiveRecord::Base
  has_many :pictures, as: :imageable
end

With a migration that looks like:

class CreatePictures < ActiveRecord::Migration
  def change
    create_table :pictures do |t|
      t.string  :name
      t.integer :imageable_id
      t.string  :imageable_type
      t.timestamps
    end
  end
end

The problem with this approach is that imageable_type is a string (and by default it is 255 characters). This is a little ridiculous. For comparison, if we had a state machine with X states, would we describe the states with strings "State1", "State2", etc or would we just enumerate the state column and make it an integer? This gem will allow us to use an integer for the imageable_type column.

Installation

Add this line to your application's Gemfile:

gem 'polymorphic_integer_type'

And then execute:

$ bundle

Or install it yourself as:

$ gem install polymorphic_integer_type

For Rails 3.2 use version < 2. Version >= 2 has been tested on Rails 4.2 and Ruby 2.1

Usage

For the model where the belongs_to is defined, include PolymorphicIntegerType::Extensions and set the polymorphic: option to a hash that maps an integer stored in the database to the name of a Ruby class.

class Picture < ActiveRecord::Base
  include PolymorphicIntegerType::Extensions

  belongs_to :imageable, polymorphic: {1 => "Employee", 2 => "Product"}
end

Next, include PolymorphicIntegerType::Extensions into any of the models that point back to the polymorphic integer type association (e.g., Picture#imageable) and add a polymorphic association using as:.

class Employee < ActiveRecord::Base
  include PolymorphicIntegerType::Extensions

  has_many :pictures, as: :imageable
end

class Product < ActiveRecord::Base
  include PolymorphicIntegerType::Extensions

  has_many :pictures, as: :imageable
end

External mappings

You can also store polymorphic type mappings separate from your models. This should be loaded before the models. Putting it in an initializer is one way to do this (e.g., config/initializers/polymorphic_type_mapping.rb)

PolymorphicIntegerType::Mapping.configuration do |config|
  config.add :imageable, {1 => "Employee", 2 => "Product" }
end

Note: The mapping here can start from whatever integer you wish, but I would advise not using 0. The reason being that if you had a new class, for instance Avatar, and also wanted to use this polymorphic association but forgot to include it in the mapping, it would effectively get to_i called on it and stored in the database. "Avatar".to_i == 0, so if your mapping included 0, this would create a weird bug.

Migrating an existing association

If you want to convert a polymorphic association that is already a string, you'll need to set up a migration. (Assuming SQL for the time being, but this should be pretty straightforward.)

class PictureToPolymorphicIntegerType < ActiveRecord::Migration

  def up
    change_table :pictures do |t|
      t.integer :new_imageable_type
    end

    execute <<-SQL
      UPDATE picture
      SET new_imageable_type = CASE imageable_type
                                 WHEN 'Employee' THEN 1
                                 WHEN 'Product' THEN 2
                               END
    SQL

    change_table :pictures, :bulk => true do |t|
      t.remove :imageable_type
      t.rename :new_imageable_type, :imageable_type
    end
  end

  def down
    change_table :pictures do |t|
      t.string :new_imageable_type
    end

    execute <<-SQL
      UPDATE picture
      SET new_imageable_type = CASE imageable_type
                                 WHEN 1 THEN 'Employee'
                                 WHEN 2 THEN 'Product'
                               END
    SQL

    change_table :pictures, :bulk => true do |t|
      t.remove :imageable_type
      t.rename :new_imageable_type, :imageable_type
    end
  end
end

Lastly, you will need to be careful of any place where you are doing raw SQL queries with the string (imageable_type = 'Employee'). They should use the integer instead.

Setup

You'll need to have git, Ruby, and MySQL. Then get up and running with a few commands:

$ git clone ...
$ bundle install
$ vim spec/support/database.yml # Update username and password
$ bin/setup
$ bundle exec rspec

Contributing

Fork it
Create your feature branch (git checkout -b my-new-feature)
Commit your changes (git commit -am 'Add some feature')
Push to the branch (git push origin my-new-feature)
Create new Pull Request