Polymorphic Associations

What is Polymorphic Associations ?

With Polymorphic Association a model can belong to more than one models with a single association.

Let’s take an example, we’ll use polymorphic associations as shown in the following schema:

In the above schema pictures belongs_to both employees and products. Adding the polymorphic association in the Picture model will look like this:

class Employee < ApplicationRecord
  has_many :pictures, as: :imageable
end

class Product < ApplicationRecord
  has_many :pictures, as: :imageable
end

class Picture < ApplicationRecord
  belongs_to :product, as: :imageable
  belongs_to :employee, as: :imageable
end

Now we can use Picture model with Product and Employee:

product = Product.first
product.pictures

employee = Employee.first
employee.pictures

---

How does Picture model identify if a record belongs to Product or Employee ?

Looking at the migration of Picture model, we can see that there is an extra imageable_type column:

class CreatePictures < ActiveRecord::Migration[7.1]
  def change
    create_table :pictures do |t|
      t.string  :name
      t.bigint  :imageable_id

      # imageable_type refers to the model to which this record belongs to
      t.string  :imageable_type

      t.timestamps
    end

    add_index :pictures, [:imageable_type, :imageable_id]
  end
end

The imageable_type column is used to find out which model does the record belong to. All tables with polymorphic associations will have this attr_type column.

In our case if the pictures record belongs to products the imageable_type column will have value Product while if the record belongs to employees the column will have Employee value.