Returns an instance of the class under test.

  class UserTest
    should "be a user" do
      assert_kind_of User, subject # passes
    end
  end

The subject can be explicitly set using the subject class method:

  class UserTest
    subject { User.first }
    should "be an existing user" do
      assert !subject.new_record? # uses the first user
    end
  end

The subject is used by all macros that require an instance of the class being tested.