Rails: Add Variable to Model (or Controller) Instance

Question

I'm trying to add a new variable (method) to my database query in rails. I don't want to add a new field to the database, I want to take an existing column and play with it to create a new method. However, it's not throwing an error or anything, it's just not available. Any ideas?

class Task < ActiveRecord::Base

  attr_accessible :old_field, :new_field

  def after_initialize
    self.new_field = :old_field * 2
  end  
end

Partially, I'm not sure if this logic goes in the model or the controller.

No, it's not a new column, just a new method that I want to create based on logic and a separate database column. — jake
– jake, Commented Dec 17, 2012 at 17:25

Antarr Byrd · Accepted Answer · 2012-12-17 20:12:54Z

4

class Task < ActiveRecord::Base

  attr_accessible :old_field

  def new_field()
    self.old_field * 2
  end  
end

or u can do it this way, which requires you to add the field to your datatable using a migration like Anatoliy did below

class Task < ActiveRecord::Base

  attr_accessible :old_field, :new_field
  before_save :calc_new_field

  private
    def calc_new_field
      self.new_field = self.old_old_field * 2
    end  
end

edited Dec 17, 2012 at 20:12

answered Dec 17, 2012 at 17:31

Antarr Byrd

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8 Comments

jake Over a year ago

I can call it, and it shows up, but it doesn't show up in my debug @tasks, or in my backbone .to_json. Why is that?

Antarr Byrd Over a year ago

I'm not sure about the debugging I'm still relatively fresh to ruby. And I have never used back_bone.js

jake Over a year ago

basically it's not showing up in the overall variable if I just show it -- it's only showing up if I call it. I want it to show up when I pass the entire variable to an argument, ie tasks: #{raw @tasks.to_json}. debug is (in my naive mind) PHP's print_r for Ruby. Any thoughts?

Antarr Byrd Over a year ago

@jake I have updated with a solution that you might like better.

jake Over a year ago

in case anyone else is having this problem, modify your to_json to to_json(:methods => :new_field)

|

Tolik Kukul · Accepted Answer · 2012-12-17 17:25:06Z

0

To add virtual attribute you should use attr_accessor not attr_accesible

Try this in terminal to add new attribute:

rails g migration AddNewFieldToTasks new_field:integer
rails db:migrate

answered Dec 17, 2012 at 17:25

Tolik Kukul

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1 Comment

jake Over a year ago

Sorry, should have been more specific: I don't want to add a new column, just want to add a new method from logic.

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