from django.db.models.signals import pre_delete, pre_save
from django.dispatch import receiver
from django.utils.translation import gettext_lazy as _

from .models import Service, ServiceTemplate


class TemplateLockedError(Exception):
    """Raised when attempting to modify a locked template."""

    def __init__(self, obj, message=None):
        self.obj = obj
        self.message = message or _(
            "Cannot modify {}: it is locked because one or more services "
            "have been spawned from it."
        ).format(obj)
        super().__init__(self.message)


@receiver(pre_save, sender=ServiceTemplate)
def lock_service_template_save(sender, instance, **kwargs):
    if not instance.pk:
        return
    if instance.services.exists():
        try:
            old = ServiceTemplate.objects.get(pk=instance.pk)
        except ServiceTemplate.DoesNotExist:
            return
        for field in ("name", "tree"):
            if getattr(instance, field) != getattr(old, field):
                raise TemplateLockedError(instance)


@receiver(pre_delete, sender=ServiceTemplate)
def lock_service_template_delete(sender, instance, **kwargs):
    if instance.services.exists():
        raise TemplateLockedError(instance)