In Francois Beausoleil’s recent post on Asymmetric Associations he showed the advantage of going with names and associations that are slightly unconventional (in the convention-over-configuration sense of the word).

Sometimes there is only so far convention can take you.

I think Francois is touching on a bigger issue that some Railer’s might not be aware of: ActiveRecord::Associations can be used to build almost all of your model associations, including the not-so-conventional ones.

For example, I’ve often seen code similar to the following:

class User < ActiveRecord::Base
  has_many :emails

  def unread_emails
    Email.find_by_sql(["select * from #{Email.table_name}# where read = 0 and user_id = ?", self.id])
  end
end

You can accomplish exactly the same thing by adding a second association (oops… now updated to include class_name):

class User < ActiveRecord::Base
  has_many :emails
  has_many :unread_emails, :class_name => 'Email', :condition => 'read = 0'
end

or, alternatively, by bolting on the unread method directly to the has_many association:

class User < ActiveRecord::Base
  has_many :emails do
    def unread; find(:all, :condition => 'read = 0'); end
  end
end

Another great example comes from i2, the app that powers the rails wiki:

class Page < ActiveRecord::Base
  belongs_to :book

  has_many :versions, :order => "created_at", :dependent => true
  has_one  :current_version, :class_name => "Version", :order => "created_at DESC" 

  def find_or_build_version(number = nil)
    number ? versions[number.to_i - 1] : versions.build(:body => body)
  end

  def body
    current_version ? current_version.body : nil
  end

  def to_param
    title
  end
end

There’s a few cool tricks going on in this class, but for now we’ll just look at the current_version statement:

has_one :current_version, :class_name => "Version", :order => "created_at DESC"

Even though the Versions table has potentially thousands of records with the same page_id, the above line pulls out just one.

So why does this work, even though its pointing to a table with many version records?

has_one will take only a single record from the resulting query, similar to a find(:first), using a SQL limit clause.

For more neat tricks have a look at the rest of the i2 codebase—it’s a cool little app.