Asymmetric associations

February 14th, 2006

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: 
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class Email < ActiveRecord::Base
  has_many :recipients
end

class Party < ActiveRecord::Base
  has_and_belongs_to_many :emails,
      :join_table => 'recipients'
end

class Recipient < ActiveRecord::Base
  belongs_to :email
  belongs_to :party

  def read!
    self.toggle!(:read)
  end
end
The above models give rise to the following very natural code: 
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>> fbos = Party.find_by_login('fbos')
=> #<Party:0x3c019d0 ...>
>> info = Email.create(:subject => 'Latest Schedule',
            :body => 'Group schedule: ...')
=> #<Email:0x3bece38 ...>
>> info.recipients.create(:party => fbos)
=> #<Recipient:0x3bccfd8 ...>
>> fbos.emails
=> [#<Email:0x3b80dc0 ...>]
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: 
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class Party < ActiveRecord::Base
  has_and_belongs_to_many :unread_emails,
      :join_table => 'recipients',
      :conditions => 'read = 0'
  has_and_belongs_to_many :read_emails,
      :join_table => 'recipients',
      :conditions => 'read = 1'
end

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

1 Response to “Asymmetric associations”

  1. Tim Lucas Says:
    October 12th, 2007 at 10:25 PM

    Great post Francois—you’ve just inspired my article on using AR as a general purpose query building language.

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I am François Beausoleil, a Ruby on Rails coder. During the day, I work on XLsuite. At night, I am interested many things. Read my biography

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