How to prevent race condition in Django on INSERT with limiting SUM?

Question

Consider two models:

• Project :
  • id: An AutoField primary key
  • budget: A PositiveIntegerField
  • some unrelated fields
• Expense
  • id: An AutoField primary key
  • amount: A PositiveIntegerField
  • project: A ForeignKey to Project
  • some unrelated fields

For each project, I wish to ensure that the sum of their expenses is less than or equal to the budget at all times.

In SQL terms: SELECT SUM(amount) FROM expense WHERE project_id = ?; should always be less than or equal SELECT budget FROM project WHERE id = ?;

Is there any way to do this in Django, keeping in mind that multiple people may be accessing the web server and creating Expense s at the same time?

I am using postgresql as my database backend.

I have tried using select_for_update but that doesn't prevent INSERT s from taking place and it doesn't seem to work on aggregations anyway.

I was considering save ing the Expense to the database first, then query the total cost and remove the Expense if it's over-budget but then I would need that code to be outside of a transaction so that other threads can see it and then if the server stops mid-processing, the data could be left in an invalid state.

Answer 1

Thanks to @Alasdair for pointing me in the right direction.

After filling out the fields of inst (a new Expense ), do:

with transaction.atomic():
    project = models.Project.objects.select_for_update().get(
        pk=project_id)
    cost = project.total_cost()
    budget = project.budget

    if cost + inst.cost > budget:
        raise forms.ValidationError(_('Over-budget'))

    self._inst.save()

Note that I have total_cost defined as a method on Project :

class Project:
    def total_cost(self):
        return self.expense_set.all().aggregate(
            t=Sum(F('cost')))['t']