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2014-10-30-rails-virtual-attributes.md

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Rails JSON Virtual Attributes

I was working on a rails application that responded with the following JSON:

[
 { 
   "id":14,
   "title":"post title",
   "description":"my awesome description",
   "user_id":1,
   "created_at":"2014-10-29T02:50:01.707Z",
   "updated_at":"2014-10-29T02:51:38.481Z"
  }
]

Even though I have access to the user_id attribute of this post, I wanted to be able to show the user name or email without having to make another HTTP request.

To add a new attribute to this JSON, I needed to add a few methods to extend ActiveRecord::Base's as_json method.

class Post < ActiveRecord::Base

  belongs_to :user

  def as_json(options={})
      super.as_json(options).merge({user_email: get_user_email})
  end

  def get_user_email
    self.user && self.user.email
  end

end

The super.as_json(... adds whatever attributes you specify for each one of the records from your model.

Since these methods are in the Post class, we can access the current post with self.

If there is no user for a given post, self.user.email would throw an undefined method for nil:NilClass, so I prepend that with self.user && to make sure the user exists first.

[
 { 
   "id":14,
   "title":"post title",
   "description":"my awesome description",
   "user_id":1,
   "created_at":"2014-10-29T02:50:01.707Z",
   "updated_at":"2014-10-29T02:51:38.481Z",
   "user_email":"jesse@jshawl.com"
  }
]