How to write the method before_destroy and the test

How to write the method before_destroy?
Here is my code:

  def before_destroy
    errors.add(:isdefault,"Can't destroy default record") if
self.isdefault
    return false
  end

or I should write it like below?

  def before_destroy
    raise(Can't destroy default record") if self.isdefault
  end

and how to write the test for it

You don't write tests for rails methods firstly.

Secondly, errors.add(:isdefault, "Can't destroy default record") and return false if isdefault?

You put the return false after this if statement so it would always return false!

To elaborate on this statement a little, you don't want to test the
framework, i.e., you don't want to test that destroy actually destroys
the model. That's already well-tested by the framework.

Rather, focus your tests on the conditional behavior that you're
adding around destroying the model, which is that destroy doesn't
destroy the model if it's the default record. For example,

def test_cannot_destroy_default_record
some_model = SomeModel.create! :isdefault => true
some_model.destroy
# make sure we get an error and/or we can still find the record
end

By focusing on what destroy does when :isdefault is true instead of
how it works (that before_destroy is used to check :isdefault), your
tests can be fairly immune to how you implement this behavior.

If someone wants to fill in the last line of the test, please do share.

Regards,
Craig