This article updates a previous version for the Rails 2.0 way of things. Since there’s not much difference, I decided to fix up the example code to be more understandable. After all, not everyone is a discrete math geek.
This example updates the one from the previous article. The only significant difference is that you don’t need to specify the :foreign_key when using the :class_name option in a belongs_to association. In Rails 2.0, the key is inferred from the association name instead of the class name. I also included the :dependent option because I feel it’s too often overlooked.
These classes could be used to model a food chain. Spider eats fly, bird eats spider, cat leaves bird on pillow as gift…
create_table :animals do |t|
t.string :species
end
create_table :hunts do |t|
t.integer :predator_id
t.integer :prey_id
t.integer :capture_percent
end
class Animal < ActiveRecord::Base
has_many :pursuits, :foreign_key => 'predator_id',
:class_name => 'Hunt',
:dependent => :destroy
has_many :preys, :through => :pursuits
has_many :escapes, :foreign_key => 'prey_id',
:class_name => 'Hunt',
:dependent => :destroy
has_many :predators, :through => :escapes
end
class Hunt < ActiveRecord::Base
belongs_to :predator, :class_name => "Animal"
belongs_to :prey, :class_name => "Animal"
end
The Hunt model describes how likely a species of predator is to catch a species of prey. From the predator’s perspective the hunt is a pursuit, but the ever-hopeful prey sees it as an escape. Note that you can model both kinds of hunts between the same pairings of animals: Some days you get the bear, some days the bear gets you.
