I have the following associations, basically I want to link via userid and not the id of the object.

class Tweet < ActiveRecord::Base
  has_one :user_profile, :primary_key => 'userid', :foreign_key => 'twitter_userid' 

class UserProfile < ActiveRecord::Base
  belongs_to :tweet, :foreign_key => 'userid'

However the following spec fails as twitter_userid is reset to the id of the object

it "should have the user's twitter id set on their user profile" do
   t = Tweet.new(:twitter_id => 1,
                  :status => 'Tester', 
                  :userid => 'personA',
                  :user_profile => UserProfile.new(:twitter_userid => 'personA', :avatar => 'abc'))
   t.save!
    t.user_profile.twitter_userid.should == 'personA'
 end

should have the user's twitter id set on their user profile

expected: "personA",
    got: 216 (using ==)

 it "should return the correct avatar after being saved" do
   t = Tweet.new(:twitter_id => 1,
                  :status => 'Tester', 
                  :userid => 'personA', 
                  :user_profile => UserProfile.new(:twitter_userid => 'personA', :avatar => 'abc'))
   t.save!

   t.user_profile.avatar.should == 'abc'
 end

How can I force it to use userid and not id?

Thanks

Ben