Django: How do I validate unique_together from within the model

Question

I have the following:

class AccountAdmin(models.Model):

    account = models.ForeignKey(Account)
    is_master = models.BooleanField()
    name = models.CharField(max_length=255)
    email = models.EmailField()

    class Meta:
        unique_together = (('Account', 'is_master'), ('Account', 'username'),)

If I then create a new AccountAdmin with the same username as another on the same account, instead of it giving me an error to display in the template, it breaks with an IntegrityError and the page dies. I wish that in my view, I could just go:

if new_accountadmin_form.is_valid():
    new_accountadmin_form.save()

How do I conquer this problem. Is there a second is_valid() type of method that checks the DB for violation of the unique_together = (('Account', 'is_master'), ('Account', 'username'),) part?

I would like not to have to catch an IntegrityError in my view. That's domain logic mixed with presentation logic. It violates DRY because if I display the same form on 2 pages, I'll have to repeat the same block. It also violates DRY because if I have two forms for the same thing, I have to write the same except: again.

Answer 1

There are two options:

a) Have a try block where you save your model and capture the IntegrityError and deal with it. Something like:

try:
    new_accountadmin_form.save()
except IntegrityError:
    new_accountadmin_form._errors["account"] = ["some message"]
    new_accountadmin_form._errors["is_master"] = ["some message"]

    del new_accountadmin_form.cleaned_data["account"]
    del new_accountadmin_form.cleaned_data["is_master"]

b) In the clean() method of your form, check if the a row exists and raise a forms.ValidationError with an appropriate message. Example here.

---

So, b) it is... That is why I referenced the documentation; all you need is there.

But it would be something like:

class YouForm(forms.Form):
    # Everything as before.
    ...

    def clean(self):
       """ This is the form's clean method, not a particular field's clean method """
       cleaned_data = self.cleaned_data

       account = cleaned_data.get("account")
       is_master = cleaned_data.get("is_master")
       username = cleaned_data.get("username")

       if AccountAdmin.objects.filter(account=account, is_master=is_master).count() > 0:
           del cleaned_data["account"]
           del cleaned_data["is_master"]
           raise forms.ValidationError("Account and is_master combination already exists.")

       if AccountAdmin.objects.filter(account=account, username=username).count() > 0:
           del cleaned_data["account"]
           del cleaned_data["username"]
           raise forms.ValidationError("Account and username combination already exists.")

    # Always return the full collection of cleaned data.
    return cleaned_data

For what it is worth - I just realized that your unique_together above is referencing a field called username that is not represented in the model.

The clean method above is called after all clean methods for the individual fields are called.

Answer 2

And for a completely generic way. In the model have the following two helper fns:

def getField(self,fieldName):
  # return the actual field (not the db representation of the field)
  try:
    return self._meta.get_field_by_name(fieldName)[0]
  except models.fields.FieldDoesNotExist:
    return None

and

def getUniqueTogether(self):
  # returns the set of fields (their names) that must be unique_together
  # otherwise returns None
  unique_together = self._meta.unique_together
  for field_set in unique_together:
    return field_set
  return None

And in the form have the following fn:

def clean(self):
  cleaned_data = self.cleaned_data
  instance = self.instance

  # work out which fields are unique_together
  unique_filter = {}
  unique_fields = instance.getUniqueTogether()
  if unique_fields:
    for unique_field in unique_fields:
      field = instance.getField(unique_field)
      if field.editable: 
        # this field shows up in the form,
        # so get the value from the form
        unique_filter[unique_field] = cleaned_data[unique_field]
      else: 
        # this field is excluded from the form,
        # so get the value from the model
        unique_filter[unique_field] = getattr(instance,unique_field)

    # try to find if any models already exist in the db;
    # I find all models and then exlude those matching the current model.
    existing_instances = type(instance).objects.filter(**unique_filter).exclude(pk=instance.pk)

    if existing_instances:
      # if we've gotten to this point, 
      # then there is a pre-existing model matching the unique filter
      # so record the relevant errors
      for unique_field in unique_fields:
        self.errors[unique_field] = "This value must be unique."

Answer 3

Model.Meta.unique_together creates a constraint limited to the database, while ModelForm.is_valid() is primarily based on correct types. Event if it did check constraints you would have a race condition that could still cause an IntegrityError in the save() call.

You probably want to be catching IntegrityError:

if new_accountadmin_form.is_valid():
    try:
        newaccountadmin_form.save()
    except IntegrityError, error:
        # here's your error handling code