Shared scopes via module?

问题:

I want to dry up several models by moving shared scopes into a module, something like:

module CommonScopes
  extend ActiveSupport::Concern

  module ClassMethods
    scope :ordered_for_display, order("#{self.to_s.tableize}.rank asc")
  end
end

I also want to create shared specs that test the module. Unfortunately when I try to include the shared scope in my model I get:

undefined method `order' for CommonScopes::ClassMethods:Module

Any ideas? Thanks!

回答1:

You can use instance_eval

module CommonScopes
  extend ActiveSupport::Concern

  def self.included(klass)
    klass.instance_eval do
      scope :ordered_for_display, order("#{self.to_s.tableize}.rank asc")
    end
  end
end

回答2:

As in rails 4 scope syntax you can simply use a lambda to delay the execution of the code (works in rails 3 too):

module CommonScopes
  extend ActiveSupport::Concern

  included do
    scope :ordered_for_display, -> { order("#{self.to_s.tableize}.rank asc") }
  end
end

回答3:

Because your scope method is called immediately when your module is parsed by Ruby and it's not accessible from your CommonScopes module..

But you can replace your scope call by a class method:

module CommonScopes
  extend ActiveSupport::Concern

  module ClassMethods
    def ordered_for_display
      order("#{self.to_s.tableize}.rank asc")
     end
  end
end