Allow opts[:if] to be a symbol

[hidde-jan]

Currently, you can pass an :if option to the attribute directive. It would be helpful if this option could be a symbol of a method on the controller instead of a Proc. It would function like before_action on controllers and validate on models. Instead of passing the current controller/context into the proc, you'd just call the method on the controller (or maybe call it from 'within' the context of the controller so it can use private/protected methods, similar to before_action).

Example:

# config/initializers/strong_resources.rb
strong_resource :person do
  attribute :age, :integer, if: :admin?
end

class PeopleController < ApplicationController
  strong_resource :person

  def admin?
    current_user.admin?
  end
end

[richmolj]

Hey @hidde-jan! So this would be functionally equivalent but syntactic sugar, right? And protected/private methods.

Not opposed to it if you'd like to PR!