Handling duplicate records in Django - django

Question

Suppose I have a set of records which I know to be unique based on some other record and an e-mail, thusly:
class Signup(models.Model):
activity = models.ForeignKey(Activity, related_name='activities')
name = models.CharField(max_length=100)
email = models.EmailField()
# many addtional fields here not relevant to question
Then, I have a model form:
class SignupForm(forms.ModelForm):
class Meta:
model = Signup
exclude = [ 'activity' ]
def __init__(self, *args, **kwargs):
self.activity = kwargs.pop('activity')
def save(self, *args, **kwargs):
kwargs['commit'] = False
m = super(SignupForm, self).save(*args, **kwargs)
m.activity = self.activity
m.save()
return m
Suppose a user goes in a fills out the form under the activity, then realizes they made an error in the form, clicks the back button, makes changes, then clicks submit again.
Without any modifications to the code above, a duplicate record for that activity and email would be created.
What I want to know is how I can force the form to update, rather than create, a record if it finds a match for the entered e-mail.
I tried this code:
class SignupForm(forms.ModelForm):
class Meta:
model = Signup
exclude = [ 'activity' ]
def __init__(self, *args, **kwargs):
self.activity = kwargs.pop('activity')
def save(self, *args, **kwargs):
kwargs['commit'] = False
try:
self.instance = Signup.objects.get(email=self.cleaned_data['email'], activity=self.activity)
except Signup.DoesNotExist:
pass
m = super(SignupForm, self).save(*args, **kwargs)
m.activity = self.activity
m.save()
return m
However, looks like this causes the form to ignore all new information for some reason (I have debug toolbar running and examining the query confirms that none of the fields are being changed!)
Is there an accepted way of handling this?
Further request
Is there any way to do this while still using the ModelForm's built-in save function? So far the answers seem to suggest that this is impossible, which is, I'm sorry, ridiculous.

Answer 1

Replace
try:
self.instance = Signup.objects.get(email=self.cleaned_data['email'], activity=self.activity)
except Signup.DoesNotExist:
pass
With:
obj, created = Signup.objects.get_or_create(\
email=self.cleaned_data['email'],
activity=self.activity)
if created:
print 'its a new one, hooray!'
else:
print 'the object exists!'
More information on get_or_create.