Question

I am using Ruby on Rails 3.2.2 and I would like to know if it is a correct / not dangerous / common approach to pass an ActiveRecord::Relation object as a method parameter.

At this time I am planning to use this approach in a scope method of a my model this way:

class Article < ActiveRecord::Base
  def self.with_active_associations(associations, active = nil)
    # associations.class
    # => ActiveRecord::Relation

    case active
    when nil
      scoped
    when 'active'
      with_ids(associations.pluck(:associated_id))
    when 'not_active'
      ...
    else
      ...
    end
  end
end

Note I: I would like to use this approach for performance reasons since the ActiveRecord::Relation is lazy loaded (in my case, if the active parameter value is not active the database is not hit at all).

Note II: the usage of the pluck method may generate an error if I pass as association parameter value an Array instead of an ActiveRecord::Relation.

Answer 1

1) In my opinion it’s a sound tradeoff, you lose the ability to send an array as argument but you gain some perfomance. It’s not that strange; for example, every time you define a scope you are doing exactly that, a filter than works only on relations and not on arrays.

2) You can always add Enumerable#pluck so the method works transparently with arrays. Of course it won’t work if you use more features of relations.

module Enumerable 
  def pluck(method, *args) 
    map { |x| x.send(method, *args) } 
  end 
end