Frozen values in scope and chained scopes

01_frozen_scope.rb

=begin
 'frozen' value for end_at in scope :active
 The same frozen value is propogated to :national scope!  
 The Time.now will be set to the first time the scope is evaluated.
 This is explained at http://api.rubyonrails.org/classes/ActiveRecord/NamedScope/ClassMethods.html

 * NON-WORKING SOLUTION *
=end

class Something
 scope :active, where('end_at > ?', Time.now)
 scope :national, where(:is_national => true).active
end

02_frozen_chained_scope.rb

=begin
 Here the Time.now value is always re-evaluated because of the lambda.
 HOWEVER, :active is chained in :national and so here:
 + scope active will keep re-evaluating the Time.now
 + scope national will NOT evalute the Time.now from active and keep end_at frozen to when 
   it was first evaluated!

  * NON-WORKING SOLUTION *
=end

class Something
 scope :active, lambda { where('end_at > ?',Time.now) }
 scope :national, where(:is_national => true).active
end

03_solution_chained_scope.rb

=begin
 This is the right fix where we add a lambda EVEN for :national scope so that it re-evaluates
 everytime for the sake of :active scope!

 ** WORKING SOLUTION **
=end

class Something
 scope :active, lambda { where('end_at > ?',Time.now) }
 scope :national, lambda { where(:is_national => true).active }
end