Polymorphic Association trong Rails là gì? Cho ví dụ thực tế.

# Migration
create_table :comments do |t|
  t.text    :body
  t.references :commentable, polymorphic: true  # tạo commentable_id + commentable_type
end

# Models
class Comment < ApplicationRecord
  belongs_to :commentable, polymorphic: true
end

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

class Video < ApplicationRecord
  has_many :comments, as: :commentable
end

# Query
post.comments      # Comments với commentable_type="Post", commentable_id=post.id
video.comments     # Comments với commentable_type="Video", commentable_id=video.id