Link Django models together via GenericKey?

i have the following models:

class Team(models.Model):
    users = models.ManyToManyField("User")
    
class User(AbstractUser):
     ...
    
class Subscription(models.Model):
    team = models.ForeignKey("Team", on_delete=models.CASCADE)
    name = models.CharField(max_length=64)
    
class Package(models.Model):
    name = models.CharField(max_length=64) # packageA, packageB
    max_activation_number = models.PositiveIntegerField(default=1)
    
class Activation(models.Model):
    subscription = models.ForeignKey("Subscription", on_delete=models.CASCADE)
    package = models.ForeignKey("Package", on_delete=models.CASCADE)
    created = models.DatetimeField()
    
class PackageA(models.Model):
    ... 
    
    
class PackageB(models.Model):
    ...

A team has one subscription and it can activate one or more package and the same package could be activated more than one time. (number of times specified with "max_ativation_number")

Example:

A team has a subscription called Suite and the available packages are: EmailAccount and Calendar The team choose to activate 3 EmailAccount and 2 Calendar (packages are not tied to each other)

For that reason the team could activate the same package more times.

For every activation i need to create a new instance on PackageA or PackageB (it depends on the choice a team made) and then i should "link" to that instance somehow.

Should i use GenericKey field inside Activation model? I not only need the name of the chosen package but I also need to figure out which instance.

Yes, I also think GenericForeignKey is a very good idea in this case, so that we can link the Activation model to either PackageA or PackageB dynamically.

from django.contrib.contenttypes.fields import GenericForeignKey
from django.contrib.contenttypes.models import ContentType

class Activation(models.Model):
    subscription = models.ForeignKey("Subscription", on_delete=models.CASCADE)
    package = models.ForeignKey("Package", on_delete=models.CASCADE)
    created = models.DatetimeField()
    
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    content_object = GenericForeignKey('content_type', 'object_id')

Here's an example of how you could create an instance of Activation:

subscription = Subscription.objects.get(...)
package = Package.objects.get(...)
content_type = ContentType.objects.get_for_model(PackageA)
package_a = PackageA.objects.get(...)

activation = Activation.objects.create(
    subscription=subscription,
    package=package,
    content_type=content_type,
    object_id=package_a.id,
    created=datetime.now()
)

This can be solved in multiple ways as you want to create a new package instance one solution is using Django signals to create the new instance of packages by using post_save signal from django.db.models.signals import post_save.

from django.db.models.signals import post_save

@receiver(post_save, sender= Activation)
def post_activation_save(sender, instance, **kwargs):
    if kwargs.get('created'):
        # create new package instance
        ....

As far as I understand, you are trying to make a Many-To-Many relationship actually https://docs.djangoproject.com/en/dev/topics/db/models/#extra-fields-on-many-to-many-relationships:

enter image description here


class Team(models.Model):
    users = models.ManyToManyField("User")
    
class User(AbstractUser):
     ...
    
class Subscription(models.Model):
    team = models.ForeignKey("Team", on_delete=models.CASCADE)
    name = models.CharField(max_length=64)
    packages = models.ManyToManyField(Package, through=Activation)
    
class Package(models.Model):
    name = models.CharField(max_length=64) # packageA, packageB
    
class Activation(models.Model):
    subscription = models.ForeignKey("Subscription", on_delete=models.CASCADE)
    package = models.ForeignKey("Package", on_delete=models.CASCADE)
    max_activation_number = models.PositiveIntegerField(default=1)
    created = models.DatetimeField()

So in order to create Subscription Suite with 3 EmailAccount and 2 Calendar

suite_subscription = Subscription.objects.create(name='Suite', team=team_1)
email_package = Package.objects.create(name='EmailAccount')
calendar_package = Package.objects.create(name='Calendar')
suite_subscription.activation_set.create(package=email_package, max_activation_number=3)
suite_subscription.activation_set.create(package=calendar_package, max_activation_number=2)

Now you have suite_subscription with 3 max email packages and 2 max calendar packages. Similarly to this, you can create extra packages and subscriptions...

And you can add as many custom attributes as you want to that "pivot" Activation table.

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