Rails model how to reference column in a class method

Question

I currently have 2 models - Issue and Responses and i'm trying to add a method on the response model so I can do issue.responses.latest

I'm currently getting undefined method 'issue_id' for #<Class:...

How can I reference the column issue_id from the responses table in my self.latest method?

class Issue < ActiveRecord::Base
  has_many :responses
end

class Response < ActiveRecord::Base
  belongs_to :issue

  def self.latest
    select([:id, :user_id, :issue_id, :response, 'MAX(created_at)'])
       .where(:issue_id => self.issue_id) <!-- How to reference the issue_id column here?
       .group(:issue_id)
  end
end

Peter Alfvin · Accepted Answer · 2013-12-08 21:53:44Z

2

You're referencing the issue_id column fine. The problem is that you're trying to provide a value of self.issue_id, but there is no issue_id class method of Response.

Also, a latest class method for Response won't be invoked by issue.responses.latest, since responses is an <ActiveRecord::Associations::CollectionProxy::ActiveRecord_Associations_Collec‌tionProxy_Table:0x007f85220bd3b8>. You could, however, define an Issue instance method of latest_response.

On a related point, since responses is an Enumerable, you can do issues.responses.max_by {|response| response.created_at}

edited Dec 8, 2013 at 21:53

answered Dec 8, 2013 at 21:25

Peter Alfvin

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7 Comments

Catfish Over a year ago

If i remove the self on self.issue_id I get the error undefined local variable or method 'issue_id'

Peter Alfvin Over a year ago

Right. What are you trying to filter on? That is, what value of issue_id are you intending to specify there?

Catfish Over a year ago

I'm ultimately trying to do issue.responses.latest in my view to get the very last response to a particular issue.

Peter Alfvin Over a year ago

Ok, well to begin with issue.responses is going to of be of type #<ActiveRecord::Associations::CollectionProxy::ActiveRecord_Associations_CollectionProxy_Table:0x007f85220bd3b8>. If you use to_a on the end of that, you'll get an array. Defining a class method for Response is not going to help you.

Peter Alfvin Over a year ago

You could define an Issue instance method that would let you do issue.latest_response or you could define a Response class method that would let you do Response.latest(issue.responses).

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