Instance Variables in Rails Model

Question

I want to initialize an instance variable within my Rails model that will hold an array and I want to access this variable in other methods within my model. I tried this:

class Participant < ActiveRecord::Base

  @possible_statuses = [
    'exists',
    'paired',
    'quiz_finished',
    'quiz_results_seen',
    'money_sent'
  ]

  def statuses
    @possible_statuses
  end

But when I tried the following using rails console:

 Participant.first.statuses

I am returned nil :(

Why does this happen? Is there a way to accomplish what I am trying to accomplish?

Nobita · Accepted Answer · 2012-06-28 21:12:44Z

14

I would recommend using a constant for this kind of cases:

class Participant < ActiveRecord::Base

  STATUSES = [
    'exists',
    'paired',
    'quiz_finished',
    'quiz_results_seen',
    'money_sent'
  ]

If you want to access that array from the inside class, just do STATUSES, and from the outside class use Participant::STATUSES

answered Jun 28, 2012 at 21:12

Nobita

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user1454117 · Accepted Answer · 2012-06-28 21:52:50Z

2

In your example, @possible_statuses is a variable on the class rather than on each instance of the object. Here is a rather verbose example of how you might accomplish this:

class Participant < ActiveRecord::Base

  @possible_statuses = [
    'exists',
    'paired',
    'quiz_finished',
    'quiz_results_seen',
    'money_sent'
  ]

  def self.possible_statuses
    @possible_statuses
  end

  def possible_statuses
    self.class.possible_statuses
  end

  def statuses
    possible_statuses
  end

end

As mentioned in other answers, if that list of statuses never changes, you should use a constant rather than a variable.

answered Jun 28, 2012 at 21:52

user1454117

4 Comments

Myna Over a year ago

I have 2 models : Job and Organization. An organization has_many jobs, and a job belongs_to an organization. Organizations post jobs, need to be approved to be seen anywhere in my app. Both can be searched (to be ever found, they need to be approved). What I want to do is have a variable in my job model that is "jobs whose organization has been approved". So that when I write the search method for jobs, I can have something like "@approved_jobs.where('title LIKE ? and description LIKE ?', params[:search], params[:search]). Where and how would you define the @approved_jobs variable I need ? thx

user1454117 Over a year ago

I would use the database to solve this problem. Add an approved boolean field to the jobs table. Then add a scope - "scope :approved, where(:approved => true)". Then you use Job.approved instead of @approved_jobs.

Myna Over a year ago

The organization table has an approved boolean, and I don't have an approved boolean for Jobs. If I set up a boolean for jobs too, how do I say "if an organization is approved, all of its jobs must be approved too. And vice versa" using the associations and the models ?

user1454117 Over a year ago

Oh, I misunderstood your schema. You don't need to add that flag to jobs then; you can get it directly from the organizations table. Here is the updated scope in the Job model: "scope :approved, joins(:organization).where(:organizations => {:approved => true})"

Selah · Accepted Answer · 2013-05-17 19:16:54Z

2

The best answer for me was to create a class variable, not an instance variable:

@@possible_statuses = [
    'exists',
    'paired',
    'chat1_ready',
    'chat1_complete'
]

I could then freely access it in methods of the class:

  def status=(new_status)
    self.status_data = @@possible_statuses.index(new_status)
  end

answered May 17, 2013 at 19:16

Selah

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1 Comment

Bot Over a year ago

If the value of this class variable is going to be constant then it's fine else refer this stackoverflow.com/questions/9396563/…

Kenny Bania · Accepted Answer · 2012-06-28 21:53:59Z

0

The instance variable will only exist when you create the model, since it is not being stored in a database anywhere. You will want to make it a constant as per Nobita's suggestion. Alternatively, you could make it a module like...

module Status
  EXISTS = "exists"
  PAIRED = "paired"
  QUIZ_FINISHED = "quiz_finished"
  QUIZ_RESULTS_SEEN = "quiz_results_seen"
  MONEY_SENT = "money_sent"

  def self.all
    [EXISTS, PAIRED, QUIZ_FINISHED, QUIZ_RESULTS_SEEN, MONEY_SENT]
  end
end

This gives you a bit more control and flexibility, IMO. You would include this code nested in your model.

answered Jun 28, 2012 at 21:53

Kenny Bania

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