Rails Model Associations

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One-to-One Associations:

The difference between belongs_to and has_one is a semantic one. The model that declares belongs_to includes a column containing the foreign key of the other. The model that declares has_one has its foreign key referenced.

belongs_to:

class Orders < ActiveRecord::Base
  # I reference a customer.
  belongs_to :customer
end

has_one class Supplier < ActiveRecord::Base # One account references me. hasone :account end 

If you're finding it difficult to recall which is which (they're saying the same thing, right?), remember that you know who your heart belongs to, but if you have one secret admirer, only they know about the relationship. Creepy.

has_one :through

Suppliers have one account, and accounts have one account history. By the transitive property, suppliers have one account history.

One-to-Many Associations

has_many

Following the same has-styled relationship of has_one and has_one :through, the class that declares has_many is referenced by many objects that belong_to it.

Justin Bieber has_many fans, who belong_to him. They keep his foreign key (aka poster on the wall), and he doesn't know they exist.

class Customer < ActiveRecord::Base
  has_many :orders
end

has_many :through

has_many :through can be used to establish shortcuts through nested relationships, as with a document that has many paragraphs, which have many sections. A document also has many paragraphs because it has many sections.

	class Document < ActiveRecord::Base
		has_many :sections
		has_many :paragraphs :through => :sections
	end
	
	class Section < ActiveRecord::Base
		belongs_to :document
		has_many :paragraphs
	end
	
	class Paragraph < ActiveRecord::Base
		belongs_to :section
	end
	

Via this nested association, has_many :through, Rails can now make sense of the statement:

	@document.paragraphs
	

has_many :through is also useful for describing has_and_belongs_to_many "by association" relationships. The Rails documentation provides the example of patients and doctors that each have many appointments and likewise have many of one another (patients or doctors) via appointments, although this example could also be appropriately described by a has_and_belongs_to_many relationship.

Now Rails recognizes:

	physicians.patients
	

has_and_belongs_to_many

A has_and_belongs_to_many relationship is common of networks--both real and digital. Doctors have many patients and those patients may have many doctors. Facebook accounts have many friends and belong to many friends lists.