A friend of mine asked me a question today. He wanted to know “When are associated objects saved” ?

See ActiveRecord::Associations. Look for the section titled “Unsaved objects and associations”.

In Rails, there are no requirements that both sides of an association be symmetric. In fact, I often find it useful to have asymmetric relationships.

For example, I’m building a mass-mailer. I have parties (people), emails and recipients. Emails are habtm with regards to parties. Since my join table contains other fields besides the two foreign keys, I need a real model for it.

Examine the following code:

1 class Email < ActiveRecord::Base
2 has_many :recipients
3 end
5 class Party < ActiveRecord::Base
6 has_and_belongs_to_many :emails,
7 :join_table => recipients
8 end
10 class Recipient < ActiveRecord::Base
11 belongs_to :email
12 belongs_to :party
14 def read!
15 self.toggle!(:read)
16 end
17 end

The above models give rise to the following very natural code:

1 >> fbos = Party.find_by_login(fbos)
2 => #<Party:0×3c019d0 …>
3 >> info = Email.create(:subject => Latest Schedule,
4 :body => Group schedule: …)
5 => #<Email:0×3bece38 …>
6 >> info.recipients.create(:party => fbos)
7 => #<Recipient:0×3bccfd8 …>
8 >> fbos.emails
9 => [#<Email:0×3b80dc0 …>]

If access to read or unread E-Mail is needed, the habtm association can be declared more than once, with conditions that discriminate between the different states:

1 class Party < ActiveRecord::Base
2 has_and_belongs_to_many :unread_emails,
3 :join_table => recipients,
4 :conditions => read = 0
5 has_and_belongs_to_many :read_emails,
6 :join_table => recipients,
7 :conditions => read = 1
8 end

Please comment or send me an E-Mail at francois.beausoleil@gmail.com to tell me about your experiences with asymmetric relationships.


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I am François Beausoleil, a Ruby on Rails and Scala developer. During the day, I work on Seevibes, a platform to measure social interactions related to TV shows. At night, I am interested many things. Read my biography.

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