Polymorphic Associations in Rails

With polymorphic associations, one class can belong_to more than one type of another class. In other words, this is not polymorphism in the typical object-oriented sense of the word; rather, it is something unique to Rails.

In the Case of Models with Comments

For example, you might have a Comment model that belongs to either Timesheet model or BillableWeek model.

Here is a migration that will create the comments table:

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class CreateComments < ActiveRecord::Migration
def change
create_table :comments do |t|
t.text :content
t.references :commentabel, polymorphic: true
# t.integer :commentable_id
# t.string :commentable_type
t.timestamps
end
end
end

Here is Comment, Timesheet and BillableWeek models:

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class Comment < ActiveRecord::Base
belongs_to :commentable, polymorphic: true
end

class Timesheet < ActiveRecord::Base
has_many :comments, as: :commentable
end

class BillableWeek < ActiveRecord::Base
has_many :comments, as: :commentable
end

Has_many :through and Polymorphics

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class Comment < ActiveRecord::Base
belongs_to :user # author of the comment
belongs_to :commentable, polymorphic: true
end

class User < ActiveRecord::Base
has_many :comments
# has_many :commentables, through: comments !# error
has_many :commented_timesheet, through: comments,
source: :commentable, source_type: 'Timesheet'
has_many :commented_billable_weeks, through: comments,
source: :commentable, source_type: 'BillableWeek'
end
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