An example Module mixing in re-usable Rails associations.

category_group.rb

module CategoryGroup
  module ClassMethods

    # Set up the association and accepts_nested_attributes_for in the client object.  Also defines some accessor and helper
    #   methods based on the name parameter
    def category_group_association(name, group)
      has_many "#{name}_categorizations".to_sym, :class_name => "Categorization", :foreign_key => "categorizable_id",
                    :conditions => ["categorizations.categorizable_group = ? and categorizations.categorizable_type = ?", group, "#{self.name}"]
      has_many "#{name}_categories".to_sym, :through => "#{name}_categorizations".to_sym, :source => :category

      attr_accessible "#{name}_categorizations_attributes".to_sym

      accepts_nested_attributes_for "#{name}_categorizations".to_sym, :allow_destroy => true, :reject_if => proc { |attributes|
        attributes['category_id'].blank? && attributes['id'].blank? # This says ignore if a new record (no id) and no category selected
      }

      # META ALERT!!!
      # Set up methods using the name parameter
      module_eval <<-"end_eval", __FILE__, __LINE__
        def has_#{name}_categories?
          p #{name}_categories
          #{name}_categories.present? && #{name}_categories.any?
        end
        def primary_#{name}_category
          primary_#{name}_cz.present? && primary_#{name}_cz.category
        end
        def primary_#{name}_cz
          #{name}_categorizations.any? && #{name}_categorizations.primary.first
        end
      end_eval
    end
  end

  def self.included(base)
    base.extend ClassMethods
  end

  def association_for_category_group(name)
    send("#{name}_categorization")
  end

end