Periodically create/update a model based on another model's annotation

I have model Foo that changes periodically (every week for simplicity). The corresponding API endpoint for this model only needs the annotation foo_annotate. I want to be able to backtrack even if the period changes. My current setup works fine but I realized that if new entries are created in the database, I may run out of storage. So instead of doing that, I create a new model based on the computed value from Foo.

Here's what I have so far:

class FooManager(models.Manager):
    ...

    def get_queryset(self):
        return super(FooManager, self).get_queryset() \ 
            .annotate(foo_annotate=Coalesce(models.Count("bar"), 0))
            # Annotation is much more complex

class Foo(models.Model):  # Periodic model
    ...
    period = models.CharField(...)  # Name of current period, unsure if DateField is better
    user = models.ForeignKey(User, ...)
    objects = FooManager()


class Bar(models.Model):  # Fields may change
    ...
    foo = models.ForeignKey(Foo, ...)


class Baz(models.Model):  # Model based on annotation
    period = ...
    foo_annotate = ???

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