Unexpected message on :attr_accessor

This may be a dumb noob issue, but I haven't found any answers while seaching the forum--

I have a controller method

  def edit
    @user = User.find params[:id]
    @user.password_confirmation = @user.password
  end

The User class has an "attr_accessor :password_confirmation" definition (so "password_confirmation" doesn't exist in the users table). My spec has the following

  it "should find User on GET to users/edit/:id" do
    User.should_receive(:find).and_return(@user)
    @user.should_receive(:password_confirmation)
    get 'edit', :id => @user.id
  end

I am asking it to expect that I will be assigning something to that attribute in the "edit" method. Unfortunately, the spec fails with

Spec::Mocks::MockExpectationError in 'UsersController should find User on GET to users/edit/:id'
Mock 'user' received unexpected message :password_confirmation= with ("password")

Initially I thought that my "should_receive" expectation was incorrectly written, but if I comment out the attribute assignment in the controller method

...
@user = User.find params[:id]
#@user.password_confirmation = @user.password
...

then the test fails with

Spec::Mocks::MockExpectationError in 'UsersController should find User on GET to users/edit/:id'
Mock 'user' expected :password_confirmation with (any args) once, but received it 0 times

So, it seems to me that the expectation is written correctly, but something about using the attr_accessor via the mock object is causing a failure.

BTW, I am doing the following in a before(:each) block

    @user = mock("user")
    @user.stub!(:new_record?).and_return(false)
    @user.stub!(:update_attributes).and_return(true)
    @user.stub!(:password_confirmation).and_return('password')
    @user.stub!(:password).and_return('password')
    User.stub!(:new).and_return(@user)

Does anyone know what I'm missing here?

Thanks!