Checks that there is almost one field to select

def clean(self):
        """
        Checks that there is almost one field to select
        """
        if any(self.errors):
            # Don't bother validating the formset unless each form is valid on
            # its own
            return
        (selects, aliases, froms, wheres, sorts, groups_by,
         params) = self.get_query_parts()
        if not selects:
            validation_message = _(u"At least you must check a row to get.")
            raise forms.ValidationError(validation_message)
        self._selects = selects
        self._aliases = aliases
        self._froms = froms
        self._wheres = wheres
        self._sorts = sorts
        self._groups_by = groups_by
        self._params = params