Published Jun 02, 2022
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With polymorphic associations, a model can belong to more than one other model, on a single association. For example, you might have a picture model that belongs to either an employee model or a product model.
class Picture < ApplicationRecord
belongs_to :imageable, polymorphic: true
end
class Employee < ApplicationRecord
has_many :pictures, as: :imageable
end
class Product < ApplicationRecord
has_many :pictures, as: :imageable
end
You can think of a polymorphic belongs_to declaration as setting up an
interface that any other model can use. From an instance of the Employee
model, you can retrieve a collection of pictures: @employee.pictures.
Similarly, you can retrieve @product.pictures.
If you have an instance of the Picture model, you can get to its parent via
@picture.imageable. To make this work, you need to declare both a foreign
key column and a type column in the model that declares the polymorphic
interface.
class CreatePictures < ActiveRecord::Migration[7.0]
def change
create_table :pictures do |t|
t.string :name
t.bigint :imageable_id
t.string :imageable_type
t.timestamps
end
add_index :pictures, [:imageable_type, :imageable_id]
end
end
The migration can be simplified by using t.references form
class CreatePictures < ActiveRecord::Migration[7.0]
def change
create_table :pictures do |t|
t.string :name
t.references :imageable, polymorphic: true
t.timestamps
end
end
end