Redmine has implemented a number of internal plugins, which are located under /path/to/redmine/lib/plugins.

These plugins follow the traditional Rails naming idiom of acts_as_* (for more information on this topic, visit http://guides.rubyonrails.org/plugins.html#add-an-acts-as-method-to-active-record), which implies that we'll be including a class level method, which is named the same as the plugin.

The class we'll be extending is the model that our knowledgebase plugin uses to manage articles.

class KbArticle < ActiveRecord::Base
  validates :title, :presence => true
  validates :category_id, :presence => true

  belongs_to :project
  belongs_to :category, :class_name => "KbCategory"
  belongs_to :author,   :class_name => 'User',:foreign_key => 'author_id'

  # class method from Redmine::Acts::Attachable::ClassMethods
  acts_as_attachable

  # class definition continues ...  
End

By including acts_as_attachable in our class, a has-many association is established...