Helper method for building a named scope from a list of scope names

enscoped.rb

module Enscoped
  
  def self.included(base)
    base.extend(ClassMethods)
  end
  
  module ClassMethods
    
    # Create one named scope from many named scopes.
    #
    # @param [Array<String or Symbol>] scope names
    # @return [Hash] hash compatible with ActiveRecord::Base#scoped
    #
    # Example:
    #
    #  Book.scope_for_scopes(:short, :informative, :free)
    #
    #  %w(short informative free).inject(Hash.new) { |memo, name|
    #    self.scoped(memo).send(name).current_scoped_methods[:find]
    #  }
    #
    def scope_for_scopes(*scopes)
      # intersect valid scope names so dangerous methods
      # like destroy_all aren't invoked!
      safe_scopes = self.scopes.keys & scopes.map(&:to_sym)
    
      # build (and return) a hash of the constructed scope
      safe_scopes.inject({}) { |memo, scope|
        self.scoped(memo).send(scope).current_scoped_methods[:find]
      }
    end
    
    def enscoped(*scopes)
      self.scoped(self.scope_for_scopes(*scopes))
    end
    
  end

end