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


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

    add_polymorphic_constraints 'pictures',
      { 'employee_id' => '',
        'product_id' => '' }

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

    drop_table :pictures

ActiveRecord model definitions

class Picture < ActiveRecord::Base
  belongs_to_polymorphic :employee, :product, :as => :imageable
  validates_polymorphic :imageable

class Employee < ActiveRecord::Base
  has_many :pictures

class Product < ActiveRecord::Base
  has_many :pictures

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 on 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 it's 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 declarations are just normal Rails declarations.

Requirements / Support

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


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. There are also a number of helpful interface helpers that you can use.

Let's use the example above to illustrate.

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

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

 => nil

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

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

# if we specify a product, the imageable getting will return that 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: sam, product: nintendo).imageable
 => nil

# There are some useful helper methods:

 => "employee_id"

 => {"employee_id"=>"1"}

 => ["employee", "picture"]

 => ["employee_id", "picture_id"]

Credits and License

polymorpheus is Copyright © 2011-2012 Barun Singh and WegoWise. It is free software, and may be redistributed under the terms specified in the LICENSE file.