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  • Polymorphic association

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    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
    

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