Question

I’ve got a simple user: **Edited a bit for clarity, and on Sam’s suggestion

class User < ActiveRecord::Base
  # <snip other attribs>
  has_many :handles
  has_one :active_handle, :class_name => "Handle"

  validates_each :active_handle, :allow_nil => true  do |record, attr, value|
    record.errors.add attr, "is not owned by the correct user" unless record.handles.include?(value)
  end
end

And a handle model:

class Handle < ActiveRecord::Base
  belongs_to :user
  attr_accessible :user_id
  validates :user_id,
        :presence => true,
        :numericality => true

  attr_accessible :name
  validates :name,
        #etc...
end

Now I’d like to check, when setting the User.active_handle association, that the handle is owned by the correct Handle.user_id. I’ve tried to do this in a custom validation, and also in a validate method on the User model. Both ways, it does the exact opposite of what I want, it sets the user_id of handle to the User doing the checking.

I’m at the end of my rope, clearly I don’t understand something, and google isn’t getting me anywhere I haven’t already been.

ETA: I have also tried to manipulate the has_one association with conditions, that seems to fail too…

has_one :active_handle,
        :class_name => "Handle",
        :conditions => ['user_id =?', '#{self.id}']

Answer 1

I’d take a different approach to this problem; the double link is messy, and hard to maintain, as you’ve discovered. Instead, I’d put an active flag in the handle model, and order the :active_handle association on that flag:

class User < ActiveRecord::Base
    has_many :handles
    has_one  :active_handle, :class_name => 'Handle', :order => 'handles.active DESC'
end

Now there’s only one link, and it’s the same link that’s already in place to establish the plain has_many :handles association (namely, the user_id attribute in the Handle model). Finding the active handle in now just a matter of finding the User’s first Handle with the active flag set. And I’d use :order rather that :conditions, as that’ll give you a nice fallback: If no Handle for that user has the active flag set, Rails’ll pick one to use as a default.

And then, in your handle model, you can have a fairly simple activate function:

class Handle < ActiveRecord::Base
    # ...stuff...

    def activate!
        # Pseudocode - sorry!
        sql('UPDATE handles SET active = 0 WHERE user_id = ?', self.user_id)
        self.active = true
        self.save!
    end
end

Or, if you wanted to be able to call something like user.handles.activate(hdl), you could put something similar as an association extension. Or user.active_handle = hdl. Or…

Hope this helps!