How to access class method from instance method in ruby on rails non activerecord model

Question

I have a non activerecord rails model:

class Document
  attr_accessor :a, :b
  include ActiveModel::Model

  def find(id)
    initialize_parameters(id)
  end

  def save
    ...
  end

  def update
    ...
  end

  private

  def initialize_parameters(id)
    @a = 1
    @b = 2
  end
end

In order to find the Document, I can use:

Document.new.find(3)

So, to get it directly I changed the find method to

def self.find(id)
  initialize_parameters(id)
end

And I get the following error when I run

Document.find(3)

undefined method `initialize_parameters' for Document:Class

How can I make this work?

Did one of the answers work for you?

jvillian
– jvillian

2018-02-26 05:14:13 +00:00
Commented Feb 26, 2018 at 5:14 — jvillian
– jvillian, Commented Feb 26, 2018 at 5:14
Yes! Now I understood how it works.

Rhuan
– Rhuan

2018-02-28 08:27:29 +00:00
Commented Feb 28, 2018 at 8:27 — Rhuan
– Rhuan, Commented Feb 28, 2018 at 8:27

Sebastián Palma · Accepted Answer · 2018-02-25 02:52:37Z

3

You can't access an instance method from a class method that way, to do it you should instantiate the class you're working in (self) and access that method, like:

def self.find(id)
  self.new.initialize_parameters(id)
end

But as you're defining initialize_parameters as a private method, then the way to access to it is by using send, to reach that method and pass the id argument:

def self.find(id)
  self.new.send(:initialize_parameters, id)
end

private

def initialize_parameters(id)
  @a = 1
  @b = 2
end

Or just by updating initialize_parameters as a class method, and removing the private keyword, that wouldn't be needed anymore.

edited Feb 25, 2018 at 2:52

answered Feb 24, 2018 at 16:19

Sebastián Palma

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Comments

jvillian · Accepted Answer · 2018-02-24 16:59:19Z

1

This:

class Document
  attr_accessor :a, :b

  def self.find(id)
    initialize_parameters(id)
  end

end

Is not trying to "access class method from instance method" as your title states. It is trying to access a (non-existent) class method from a class method.

Everything Sebastian said is spot on.

However, I guess I would ask: 'What are you really trying to do?' Why do you have initialize_parameters when ruby already gives you initialize that you can override to your heart's content? IMO, it should look something more like:

class Document
  attr_accessor :a, :b, :id

  class << self

    def find(id)
      new(id).find
    end

  end

  def initialize(id)
    @a = 1
    @b = 2
    @id = id
  end

  def find
    # if you want you can:
    call_a_private_method
  end

private

  def call_a_private_method
    puts id
  end

end

answered Feb 24, 2018 at 16:59

jvillian

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1 Comment

Rhuan Over a year ago

I'm trying to achieve the same behavior as an activerecord model has

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