How do I turn this into a has_one association?

(Possibly has_one + a named scope for size.)

class User < ActiveRecord::Base
  has_many :assets, :foreign_key => 'creator_id'

  def avatar_asset size = :thumb
    # The LIKE is because it might be a .jpg, .png, or .gif. 
    # More efficient methods that can handle that are OK. ;)
    self.assets.find :first, :conditions => 
      ["thumbnail = '#{size}' and filename LIKE ?", self.login + "_#{size}.%"]
  end
end

EDIT: Cuing from AnalogHole on Freenode #rubyonrails, we can do this:

  has_many :assets, :foreign_key => 'creator_id' do
    def avatar size = :thumb
      find :first, :conditions => ["thumbnail = ? and filename LIKE ?",
        size.to_s, proxy_owner.login + "_#{size}.%"]
    end
  end

... which is fairly cool, and makes syntax a bit better at least.

However, this still doesn't behave as well as I would like. Particularly, it doesn't allow for further nice find chaining (such that it doesn't execute this find until it's gotten all its conditions).

More importantly, it doesn't allow for use in an :include. Ideally I want to do something like this:

PostsController
def show
  post = Post.get_cache(params[:id]) {
    Post.find(params[:id], 
      :include => {:comments => {:users => {:avatar_asset => :thumb}} }
  ...
end

... so that I can cache the assets together with the post. Or cache them at all, really - e.g. get_cache(user_id){User.find(user_id, :include => :avatar_assets)} would be a good first pass.

This doesn't actually work (self == User), but is correct in spirit:

has_many :avatar_assets, :foreign_key => 'creator_id', 
 :class_name => 'Asset', :conditions => ["filename LIKE ?", self.login + "_%"]

(Also posted on Refactor My Code.)