Django UniqueConstraint

Question

Context

I have the models AppVersion, App & DeployApp. In the AppVersion model users can upload APK files to the filesystem. I am using a pre_save signal to prevent uploading APK files with the same version_code for a specific App like this:

@receiver(pre_save, sender=AppVersion) def prevent_duplicate_version_code(sender, instance, **kwargs):     qs = AppVersion.objects.filter(app_uuid=instance.app_uuid, version_code=instance.version_code)     if qs.exists():         raise FileExistsError("Version code has to be unique for a specific app") 

This signal does what I want, except it also raises the error when I am trying to create an object in the bridge-table DeployApp.

Models

# models.py  class App(models.Model):     app_uuid = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False, db_index=True)     app_name = models.CharField(max_length=100)   class AppVersion(models.Model):     app_version_uuid = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False, db_index=True)     app_uuid = models.ForeignKey(App, on_delete=models.CASCADE, related_name='app_versions')     app_version_name = models.CharField(max_length=100)     version_code = models.IntegerField(blank=True, null=True, editable=False)     source = models.FileField(upload_to=get_app_path, storage=AppVersionSystemStorage())    class DeployApp(models.Model):     deploy_app_uuid = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False, db_index=True)     app_version = models.ForeignKey(AppVersion, on_delete=models.CASCADE)     device_group = models.ForeignKey(DeviceGroup, on_delete=models.CASCADE)     release_date = UnixDateTimeField() 

My guess is that when creating an object of DeployApp the related AppVersion is also saved and thus the pre_save signal is called and raises the Exception.

I also tried to override the save() method for the AppVersion model but the results are the same.

How do I make sure that the Exception only happens upon creating a new AppVersion instance and does not happen when adding or editing a DeployApp instance?

Answer 1

Solved it thanks to Bear Brown his suggestion. I removed the signal and added UniqueConstraint to the AppVersion model like this:

class Meta:     db_table = 'app_version'     constraints = [         models.UniqueConstraint(fields=['app_uuid', 'version_code'], name='unique appversion')     ]