Validates all field values for the model.

def validate(self):
        """
        Validates all field values for the model.
        """

        errors = {}

        for name, field in self.fields:
            try:
                field.validate(self._state.get(name))
            except Exception as e:
                errors[name] = e

        if len(errors) is not 0:
            raise Exception(errors)

        return True