has_and_belongs_to_one_self

This little Ruby on Rails snippet solves a problem I’ve seen some people get confused by. I figured I’d share my way of doing it.

The trick to behind creating a has_and_belongs_to_many association that associates rows in the same table is to specify more options than you’re used to. Rails can’t figure out how to work it’s magic when associationg 2 classes named the same, so you have to guide it by hand. From a current project in the works:

class ClassDescription < Description
  has_and_belongs_to_many :included_modules, :class_name => 'ModuleDescription', :join_table => 'includes_modules', :foreign_key => 'description_id', :association_foreign_key => 'module_id'
end

Note that this also adds Single Table Inheritance (STI) into the mix, which makes the above less clear. The simplest thing you have to do is:

has_and_belongs_to_many :models, :association_foreign_key => 'other_model_id'

You may want to specify more options to make your tables more readable. The above assumes a table called model_model with the fields model_id and other_model_id.

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