Managing circular reference

Using Django Mystery Errors
1

In costcenter.models, I have these these classes:

class CostCenter(models.Model):
    costcenter = models.CharField("Cost Center", max_length=6, unique=True)
class ForecastAdjustment(models.Model):
    costcenter = models.ForeignKey(
        CostCenter, on_delete=models.CASCADE, null=True, verbose_name="Cost Center"
    )

In lineitems.models, I have this class which depends on costcenter import:

from costcenter.models import CostCenter

class LineItem(models.Model):
    costcenter = models.ForeignKey(CostCenter, on_delete=models.RESTRICT)

I want to extend the save method of ForecastAdjustment so that an exception is raised if no line items exist for the given costcenter. My approach would be to add a method to my CostCenterManager:

class CostCenterManager(models.Manager):
    from lineitems.models import LineItem #good enough to avoid circular reference
    def has_line_items(self,costcenter:'CostCenter'): #Requires LineItem import.  Circular reference.
        return LineItem.objects.exists(costcenter=costcenter)

Then above method would be used by the ForecastAdjustment save method:

class ForecastAdjustment:
    [...]
    def save(self, *args, **kwargs):
        if not CostCenterManager.has_line_items(self.costcenter):
            raise exceptions.LineItemsDoNotExistError(
                f"{self.costcenter} has no line items.  Forecast adjustment rejected."
            )

Is it reasonable to import LineItem inside my has_line_item method or is there a better approach?

2

My initial reaction to seeing situations like this is that you probably have your models unnecessarily separated into separate files. This is frequently caused by dividing a project into apps when the functionality between those apps is not sufficiently independent to warrant it.

Having said that, yes, that import is fine.

The other way to do this is described in the docs for ForeignKey by specifying the ForeignKeyField as a string - see the docs and the examples.

3

Putting everything CostCenter and LineItems would probably have been wise. I was not too sure at the time and I think I got influenced by the number of lines growing.