Has Many Through Relationship

Many-to-Many

There are two ways to build a many-to-many relationship.

The first way uses a has_many association with the :through option and a join model, so there are two stages of associations.

A has_many :through association is often used to set up a many-to-many connection with another model. This association indicates that the declaring model can be matched with zero or more instances of another model by proceeding through a third model.

For example, consider a medical practice where patients make appointments to see physicians. The relevant association declarations could look like this:

class Physician < ActiveRecord::Base

  has_many :appointments

  has_many :patients, :through => :appointments

end

class Appointment < ActiveRecord::Base

  belongs_to :physician

  belongs_to :patient

end

class Patient < ActiveRecord::Base

  has_many :appointments

  has_many :physicians, :through => :appointments

end

has_many :through

The collection of join models can be managed via the API.  for example, if you assign

physician.patients = patients

new join models are created for newly associated objects, and if some are gone their rows are deleted.

Automatic deletion of join models is direct, no destroy callbacks are triggered.

The has_many :through association is also useful for setting up "shortcuts" through nested has_many associations. For example, if a document has many sections, and a section has many paragraphs, you may sometimes want to get a simple collection of all paragraphs in the document. You could set that up this way:

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

With :through => :sections specified, Rails will now understand:

@document.paragraphs