Question

I have several tables, let’s say for example: articles, thoughts and pages and I want to be able to add comments for each one of them.
Should I add comments tables for each on of the table (articles_comments,pages_comments…) or somehow do general comments table for all kinds of data (plz extend how to implement that in rails if you can)

I had an idea to do comments table that contain

[COMMENTS] ID, MODEL, MODEL_ID,
USER_ID, TEXT

while model contain the destination table(articles/posts…) and model_id the foreign id in the table, is it good solution?
the problem is that I have no idea how to implement such model in rails.

I also would like to know how Facebook implemented their posts db.
fb user can post any kind of data (question/status/poll/picture) and it’s just fit in the feeds table with current order and comments.

Answer 1

create a general Comment model and make it polymorphic like this:

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

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

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

From an instance of the Article model, you can retrieve a collection of comments: @article.comments

If you have an instance of the Comment model, you can get to its parent via @comment.commentable

To make this work, you need to declare both a foreign key column and a type column in the model that declares the polymorphic interface:

class CreateComments < ActiveRecord::Migration
  def self.up
    create_table :comments do |t|
      t.text       :content
      t.references :user
      t.references :commentable, :polymorphic => true
      t.timestamps
    end
  end

  def self.down
    drop_table :comments
  end
end