RSpec - Having trouble mocking model associations

Hi everyone,

I'm having a devil of a time trying to figure out why this spec keeps giving
me an error. I'm attempting to mock out a class I haven't created yet
(User) so I don't need to worry about creating it at the moment. I could
have sworn I read somewhere that it was possible to pass a mock object as an
attribute to an associated class, but it's just not working for me.

Here's the code I have:

# task_spec.rb
describe Task do
  before(:each) do
    user_proxy = mock('user')
    @task = Task.new(
      :description => "Sample task description",
      :assigned_user => user_proxy
    )
  end
  
  it "should be valid with all required attributes" do
    @task.should be_valid
  end
  
  it "should be invalid without a description" do
    @task.description = nil
    @task.should_not be_valid
    @task.should have(1).error_on(:description)
  end
  
  it "should be invalid unless assigned to a user" do
    @task.assigned_user = nil
    @task.should_not be_valid
    @task.should have(1).error_on(:assigned_user_id)
  end
end

# task.rb
class Task < ActiveRecord::Base
  belongs_to :assigned_user, :class => "User", :foreign_key =>
"assigned_user_id"
  
  validates_presence_of :description
  validates_presence_of :assigned_user_id # Validate against the foreign
key, not the association
end