describe Letter do
  before(:each) do
    @john = mock("John")
    @john.stub!(:id).and_return(5)
    @john.stub!(:class).and_return(Person)  # is this ok?
    @john.stub!(:name).and_return("John F.")
    Person.stub!(:find).and_return(@john)
  end
  it.should "have a valid #to field" do
    letter = Letter.create!(:to=>@john, :content => "Hello John")
    letter.to_type.should == @john.class.name
    letter.to_id.should == @john.id
  end
  [...]
end

class Letter < ActiveRecord::Base
    belongs_to :to, :polymorphic => true
    [...]
end

before(:each) do
  @john = mock_model(Person, :name => "John F.")
  Person.stub!(:find).and_return(@john)
end

describe Letter
  should_belong_to :to, :polymorphic => true
end