ruby 21 lines · 3 tabs

Polymorphic associations for flexible relationships

Alex Kumar Jan 2026

3 tabs

class CreateComments < ActiveRecord::Migration[6.1]
  def change
    create_table :comments do |t|
      t.references :commentable, polymorphic: true, null: false
      t.references :author, null: false, foreign_key: { to_table: :users }
      t.text :body, null: false
      t.timestamps
    end

    add_index :comments, [:commentable_type, :commentable_id, :created_at]
  end
end

class Comment < ApplicationRecord
  belongs_to :commentable, polymorphic: true
  belongs_to :author, class_name: 'User'

  validates :body, presence: true
end

class Post < ApplicationRecord
  has_many :comments, as: :commentable, dependent: :destroy
end

3 files · ruby Explain with highlit

Polymorphic associations allow a model to belong to multiple other models through a single association, which is useful for shared behaviors like comments, likes, or attachments. Instead of separate post_id and article_id columns, a polymorphic association uses commentable_id and commentable_type to store the parent's ID and class name. This pattern keeps the schema DRY when multiple models share the same relationship structure. The downside is that foreign key constraints aren't possible, and queries can be more complex. I use polymorphic associations sparingly and only when the behavior is truly identical across parent types. For partial overlap, separate tables with inheritance or multiple associations work better.

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