How to use the "number_to_currency" helper method in the model rather than view?

I would like to use to_dollar method in my model like this:

module JobsHelper      
  def to_dollar(amount)
    if amount < 0
      number_to_currency(amount.abs, :precision => 0, :format => "-%u%n")
    else
      number_to_currency(amount, :precision => 0)
    end
  end      
end

class Job < ActiveRecord::Base
  include JobsHelper
  def details
    return "Only " + to_dollar(part_amount_received) + 
           " out of " + to_dollar(price) + " received."
  end
end

Unfortunately, the number_to_currency method is not recognized here:

> undefined method `number_to_currency' for #<Job:0x311eb00>

Any ideas how to make it work?

I agree with all of you that this could be breaking the MVC pattern but there is always reasons to break a pattern, in my case I needed these currency formatter methods to use them in a template filter (Liquid in my case).

At the end I found out I could access to these currency formatter methods using things like this:

ActionController::Base.helpers.number_to_currency

It’s not available because its use in a model (typically) violates MVC (and it does seem to in your case). You're taking data and manipulating it for presentation. This, by definition, belongs in the view, not the model.

Here are some solutions:

• Use a presenter or view model object to mediate between the model and view. This almost definitely requires more initial work than other solutions, but is almost always a better design. Using helpers in a presenter/view-model doesn’t violate MVC, as they reside in the view layer, replacing traditional custom Rails helpers and logic-filled views.

• Explicitly include ActionView::Helpers::NumberHelper in JobsHelper instead of depending on Rails to have magically loaded it for you. This is still not great, as you shouldn’t access a helper from a model.

• Violate MVC & SRP. See fguillen’s answer for how to do this. I won’t echo it here because I don’t agree with it. Even more so, though, do I disagree with polluting your model with presentation methods as in Sam’s answer.

If you think “but I really need this to write my to_csv & to_pdf methods in my model!”, then your entire premise is wrong—after all, you don’t have a to_html method, do you? And yet your object is very often rendered as HTML. Consider creating a new class for generating your output instead of making your data model know what a CSV is (because it shouldn’t).

As for using helpers for ActiveModel validation errors in the model, well, I’m sorry but ActiveModel/Rails has screwed us all there by forcing error messages to be realized in the data layer, rather than returning the semantic idea of an error to be realized later—sigh. You can get around this, but it basically means not using ActiveModel::Errors anymore. I’ve done it, it works well.

As an aside, here’s a useful way to include helpers in a presenter/view-model without polluting its set of methods (because being able to do e.g. MyPresenterOrViewModel.new.link_to(...) makes no sense):

class MyPresenterOrViewModel
  def some_field
    helper.number_to_currency(amount, :precision => 0)
  end

  private

  def helper
    @helper ||= Class.new do
      include ActionView::Helpers::NumberHelper
    end.new
  end
end

I know this thread is very old, but someone can look for solution for this problem in Rails 4+. Developers added ActiveSupport::NumberHelper, which can be used without accessing view related modules/classes using:

ActiveSupport::NumberHelper.number_to_currency(amount, precision: 0)

You need to also include the ActionView::Helpers::NumberHelper

class Job < ActiveRecord::Base
  include ActionView::Helpers::NumberHelper
  include JobsHelper
  def details
    return "Only " + to_dollar(part_amount_received) + 
           " out of " + to_dollar(price) + " received."
  end
end

Piggybacking off of @fguillen's response, I wanted to override the number_to_currency method in my ApplicationHelper module so that if the value was 0 or blank that it would output a dash instead.

Here's my code in case you guys would find something like this useful:

module ApplicationHelper
  def number_to_currency(value)
    if value == 0 or value.blank?
      raw "–"
    else
      ActionController::Base.helpers.number_to_currency(value)
    end
  end
end

You can use view_context.number_to_currency directly from you controller or model.

@fguillen's way is good, though here's a slightly cleaner approach, particular given that the question makes two references to to_dollar. I'll first demonstrate using Ryan Bates' code (http://railscasts.com/episodes/132-helpers-outside-views).

def description
  "This category has #{helpers.pluralize(products.count, 'product')}."
end

def helpers
  ActionController::Base.helpers
end

Notice the call helpers.pluralize. This is possible due to the method definition (def helpers), which simply returns ActionController::Base.helpers. Therefore helpers.pluralize is short for ActionController::Base.helpers.pluralize. Now you can use helpers.pluralize multiple times, without repeating the long module paths.

So I suppose the answer to this particular question could be:

class Job < ActiveRecord::Base
  include JobsHelper
  def details
    return "Only " + helpers.to_dollar(part_amount_received) + 
           " out of " + helpers.to_dollar(price) + " received."
  end

  def helpers
    ActionView::Helpers::NumberHelper
  end
end

It is not a good practice but it works for me!

to import include ActionView::Helpers::NumberHelper in the controller. For example:

class ProveedorController < ApplicationController
	include ActionView::Helpers::NumberHelper
	# layout 'example'

	# GET /proveedores/filtro
	# GET /proveedores/filtro.json
	def filtro
		@proveedores = Proveedor.all

		respond_to do |format|
      		format.html # filtro.html.erb
      		format.json { render json: @proveedores }
    	end
	end

	def valuacion_cartera
		@total_valuacion = 0
		facturas.each { |fac|
			@total_valuacion = @total_valuacion + fac.SumaDeImporte
		}

		@total = number_to_currency(@total_valuacion, :unit => "$ ")

		p '*'*80
		p @total_valuacion
	end
end

Hope it helps you!

Really surprised not one person has talked about using a Decorator. Their purpose is to solve the issue you are facing, and more.

https://github.com/drapergem/draper

EDIT: Looks like the accepted answer basically did suggest doing something like this. But yeah, you want to use decorators. Here's a great tutorial series to help you understand more:

https://gorails.com/episodes/decorators-from-scratch?autoplay=1

P.S. - @excid3 I accept free membership months LOL

You can just include ActiveSupport::NumberHelper module, if you don't need additional features defined by ActionView.

https://github.com/rails/rails/blob/44260581bec06e4ce05f3dd838c8b4736fc7eb1d/actionview/lib/action_view/helpers/number_helper.rb#L383

Helper methods are generally used for View files. It is not a good practice to use these methods in Model class. But if you want to use then Sam's answer is ok. OR I suggest you can write your own custom method.