Returns the filtered queryset based on the value
        provided in the query string and retrievable via
        `self.value()`.

def queryset(self, request, queryset):
        """
        Returns the filtered queryset based on the value
        provided in the query string and retrievable via
        `self.value()`.
        """
        if self.value() is not None:
            return queryset.filter(user__pk=self.value())
        else:
            return queryset