How to create a Rails 4 Concern that takes an argument

Question

I have an ActiveRecord class called User. I'm trying to create a concern called Restrictable which takes in some arguments like this:

class User < ActiveRecord::Base
  include Restrictable # Would be nice to not need this line
  restrictable except: [:id, :name, :email]
end

I want to then provide an instance method called restricted_data which can perform some operation on those arguments and return some data. Example:

user = User.find(1)
user.restricted_data # Returns all columns except :id, :name, :email

How would I go about doing that?

Answer 1

If I understand your question correctly this is about how to write such a concern, and not about the actual return value of restricted_data. I would implement the concern skeleton as such:

require "active_support/concern"

module Restrictable
  extend ActiveSupport::Concern

  module ClassMethods
    attr_reader :restricted

    private

    def restrictable(except: []) # Alternatively `options = {}`
      @restricted = except       # Alternatively `options[:except] || []`
    end
  end

  def restricted_data
    "This is forbidden: #{self.class.restricted}"
  end
end

Then you can:

class C
  include Restrictable
  restrictable except: [:this, :that, :the_other]
end

c = C.new
c.restricted_data  #=> "This is forbidden: [:this, :that, :the_other]"

That would comply with the interface you designed, but the except key is a bit strange because it's actually restricting those values instead of allowing them.

Answer 2

I'd suggest starting with this blog post: https://signalvnoise.com/posts/3372-put-chubby-models-on-a-diet-with-concerns Checkout the second example.

Think of concerns as a module you are mixing in. Not too complicated.

module Restrictable
  extend ActiveSupport::Concern

  module ClassMethods
    def restricted_data(user)
      # Do your stuff
    end
  end
end