Check if all boolean values are the same (database)

[Victor Porton]

# May contain several purchases
class Transaction(models.Model):
    name = models.CharField(null=True)

class Purchase(models.Model):
    bundle = models.ForeignKey(Transaction, null=True)
    recurring = models.BooleanField(default=False)

For a given transaction, all its purchases must either all be recurring
or all non-recurring.

What is the best way to check it (and throw an exception if in a
transaction there are both non-recurring and recurring purchases)?

[Shawn Milochik]

I'd go with form validation -- you shouldn't allow a Purchase to be added to a transaction if a Purchase with a conflicting status is already in there, nor should a Purchase be able to be modified if it's in a transaction with another Purchase.

Then you never have to check whether a transaction is valid. The models pasted don't give the complete picture of the relationship, but garbage in, garbage out, so just don't allow them to exist in the first place.