[eluser]WanWizard[/eluser]
Can you be a bit more specific?
What is the exact error message? On which line? What does your models look like? What's the content of the objects when you call delete?
As to set_value: Datamapper doesn't use Form validation, it allows you to use the validation methods in the form validation library. set_value() is something related to $_POST, which has nothing to do with Datamapper.