Polymorpheus

Polymorphic relationships in Rails that keep your database happy with almost no setup

Installation

If you are using Bundler, you can add the gem to your Gemfile:

# with Rails >= 4.2
gem 'polymorpheus'

Or:

# with Rails < 4.2
gem 'foreigner'
gem 'polymorpheus'

Background

What is polymorphism? Rails Guides has a great overview of what polymorphic relationships are and how Rails handles them
If you don't think database constraints are important then here is a presentation that might change your mind. If you're still not convinced, this gem won't be relevant to you.
What's wrong with Rails' built-in approach to polymorphism? Using Rails, polymorphism is implemented in the database using a type column and an id column, where the id column references one of multiple other tables, depending on the type. This violates the basic principle that one column in a database should mean to one thing, and it prevents us from setting up any sort of database constraint on the id column.

Basic Use

We'll outline the use case to mirror the example outline in the Rails Guides:

You have a Picture object that can belong to an Imageable, where an Imageable is a polymorphic representation of either an Employee or a Product.

With Polymorpheus, you would define this relationship as follows:

Database migration

class SetUpPicturesTable < ActiveRecord::Migration
  def self.up
    create_table :pictures do |t|
      t.integer :employee_id
      t.integer :product_id
    end

    add_polymorphic_constraints 'pictures',
      { 'employee_id' => 'employees.id',
        'product_id' => 'products.id' }
  end

  def self.down
    remove_polymorphic_constraints 'pictures',
      { 'employee_id' => 'employees.id',
        'product_id' => 'products.id' }

    drop_table :pictures
  end
end

ActiveRecord model definitions

class Picture < ActiveRecord::Base
  # takes same additional options as belongs_to
  belongs_to_polymorphic :employee, :product, :as => :imageable
  validates_polymorph :imageable
end

class Employee < ActiveRecord::Base
  # takes same additional options as has_many
  has_many_as_polymorph :pictures, inverse_of: employee
end

class Product < ActiveRecord::Base
  has_many_as_polymorph :pictures
end

That's it!

Now let's review what we've done.

Database Migration

Instead of imageable_type and imageable_id columns in the pictures table, we've created explicit columns for the employee_id and product_id
The add_polymorphic_constraints call takes care of all of the database constraints you need, without you needing to worry about sql! Specifically it:
- Creates foreign key relationships in the database as specified. So in this example, we have specified that the employee_id column in the pictures table should have a foreign key constraint with the id column of the employees table.
- Creates appropriate triggers in our database that make sure that exactly one or the other of employee_id or product_id are specified for a given record. An exception will be raised if you try to save a database record that contains both or none of them.
Options for migrations: There are options to customize the foreign keys generated by Polymorpheus and add uniqueness constraints. For more info on this, read the wiki entry.

Model definitions

The belongs_to_polymorphic declaration in the Picture class specifies the polymorphic relationship. It provides all of the same methods that Rails does for its built-in polymorphic relationships, plus a couple additional features. See the Interface section below.
validates_polymorph declaration: checks that exactly one of the possible polymorphic relationships is specified. In this example, either an employee_id or product_id must be specified -- if both are nil or if both are non-nil a validation error will be added to the object.
The has_many_as_polymorph declaration generates a normal Rails has_many declaration, but adds a constraint that ensures that the correct records are retrieved. This means you can still use the same conditions with it that you would use with a has_many association (such as :order, :class_name, etc.). Specifically, the has_many_as_polymorph declaration in the Employee class of the example above is equivalant to has_many :pictures, { product_id: nil } and the has_many_as_polymorph declaration in the Product class is equivalent to has_many :pictures, { employee_id: nil }

Requirements / Support

Currently the gem only supports MySQL. Please feel free to fork and submit a (well-tested) pull request if you want to add Postgres support.
This gem is tested and has been tested for Rails 2.3.8, 3.0.x, 3.1.x, 3.2.x, and 4.0.0
For Rails 3.1+, you'll still need to use up and down methods in your migrations.

Interface

The nice thing about Polymorpheus is that under the hood it builds on top of the Rails conventions you're already used to which means that you can interface with your polymorphic relationships in simple, familiar ways. It also lets you introspect on the polymorphic associations.

Let's use the example above to illustrate.

sam = Employee.create(name: 'Sam')
nintendo = Product.create(name: 'Nintendo')

pic = Picture.new
 => #<Picture id: nil, employee_id: nil, product_id: nil>

pic.imageable
 => nil

# The following two options are equivalent, just as they are normally with
# ActiveRecord:
#   pic.employee = sam
#   pic.employee_id = sam.id

# If we specify an employee, the imageable getter method will return that employee:
pic.employee = sam;
pic.imageable
 => #<Employee id: 1, name: "Sam">
pic.employee
 => #<Employee id: 1, name: "Sam">
pic.product
 => nil

# If we specify a product, the imageable getting will return that product:
Picture.new(product: nintendo).imageable
 => #<Product id: 1, name: "Nintendo">

# But, if we specify an employee and a product, the getter will know this makes
# no sense and return nil for the imageable:
Picture.new(employee: sam, product: nintendo).imageable
 => nil

# A `polymorpheus` instance method is attached to your model that allows you
# to introspect:

pic.polymorpheus.associations
 => [
      #<Polymorpheus::InterfaceBuilder::Association:0x007f88b5528b00 @name="employee">,
      #<Polymorpheus::InterfaceBuilder::Association:0x007f88b55289c0 @name="picture">
    ]

pic.polymorpheus.associations.map(&:name)
 => ["employee", "product"]

pic.polymorpheus.associations.map(&:key)
 => ["employee_id", "product_id"]

pic.polymorpheus.active_association
 => #<Polymorpheus::InterfaceBuilder::Association:0x007f88b5528b00 @name="employee">,

pic.polymorpheus.query_condition
 => {"employee_id"=>"1"}

Credits and License

This gem was written by Barun Singh
It uses the Foreigner gem under the hood for Rails < 4.2.