1) acts_as_automatic: At the moment, automatically adds belongs_to :whatever if it detects whatever_id in the model’s table.

2) ActiveRecord extension: Reads in config/associations.yml and applies the associations as appropriate

Example associations.yml:

 has_many:
   category : articles
   author   : articles
   article  : tags
   author   : {tags: {through: articles}}

 has_one:
   article : attachment

 acts_as_list:
   article: {scope: category}
   author: