Why is Django executing statements such as this:
SELECT (1) AS [a] FROM [my_table]
WHERE ([my_table].[id] = ?
AND NOT ([my_table].[id] = ? )) (1, 1)
This happens when calling is_valid() on a formset created the following way:
MyFormSet = modelformset_factory(Table, fields=['my_field'], extra=0)
my_form_set = MyFormSet(request.POST,
queryset=Table.objects.all())
where Table and MyForm are as simple as, say:
class Table(models.Model):
my_field = models.CharField(max_length=10)
class MyForm(forms.ModelForm):
class Meta:
model = Table
Hint: I looked at the call stack and the code responsible for it (in django/forms/models.py) is below:
def _perform_unique_checks(self, unique_checks):
import pdb; pdb.set_trace()
bad_fields = set()
form_errors = []
for unique_check in unique_checks:
# Try to look up an existing object with the same values as this
# object's values for all the unique field.
lookup_kwargs = {}
for field_name in unique_check:
lookup_value = self.cleaned_data[field_name]
# ModelChoiceField will return an object instance rather than
# a raw primary key value, so convert it to a pk value before
# using it in a lookup.
if isinstance(self.fields[field_name], ModelChoiceField):
lookup_value = lookup_value.pk
lookup_kwargs[str(field_name)] = lookup_value
qs = self.instance.__class__._default_manager.filter(**lookup_kwargs)
# Exclude the current object from the query if we are editing an
# instance (as opposed to creating a new one)
if self.instance.pk is not None:
qs = qs.exclude(pk=self.instance.pk)
Basically the pk is both included for the uniqueness check and excluded. Looks like Django can be smarter and avoid such inefficiency.