Polymorphic association is little tricky to understand among all the rails association. Using this association one model can have belongs_to association with more than one model in a single line of association. As for example in a social networking site there can be comments for a post , uploaded image of the user. So here we can have polymorphic association among these three models Post,Image, and comments.

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

class Image < ActiveRecord::Base
  has_many :comments, :as => :commentable
end

class Post < ActiveRecord::Base
  has_many :comments, :as => :commentable
end

The migration file of comment model:

class CreateComments < ActiveRecord::Migration
  def change
    create_table :comments do |t|
      t.string  :comment
      t.integer :commentable_id
      t.string  :commentable_type 
      t.timestamps
    end
  end
end

We can also write the above migration as:

class CreateComments < ActiveRecord::Migration
  def change
    create_table :comments do |t|
      t.string  :comment
      t.references :commentable,   :polymorphic => true
      t.timestamps
    end
  end
end

In the commentable_id primary key of Post and Image record will be saved and in commentable_type column 'Image' and 'Post' string will be saved.

We can get the comments in this way:

@image = Image.first
@image_comments = @image.comments
@post = Post.first
@post_comments = @post.comments