`unique_together` doesn't work seamlessly

[meshy]

If your sort_order field is unique_together with something, you need to add:

def validate_unique(self, exclude=None):
    f = 'sort_order'
    exclude = [f] if not exclude and f not in exclude else exclude + [f]
    return super(MyModel, self).validate_unique(exclude)

To your model.

[mjtamlyn]

WHY?

[jturnbull]

Rewritten as a logical if statement…

if not exclude and 'sort_order' not in exclude:
    exclude = ['sort_order']
else:
    exclude = exclude + ['sort_order' ]

but, if exclude is None then how could sort_order be in exclude?

[meshy]

Logic fail. Should be:

def validate_unique(self, exclude=None):
    if exclude is None:
        exclude = ['sort_order']
    elif 'sort_order' not in exclude:
        exclude = exclude + ['sort_order']
    return super(MyModel, self).validate_unique(exclude=exclude)

[jturnbull]

Can this not be added to the Orderable object then?

[meshy]

I am unsure of the implications of ignoring validation on this, especially as it only applies when sort_order is unique (which it is not, by default).

[jturnbull]

Guess we just mention it in the README then

[meshy]

An alternative syntax:

def validate_unique(self, exclude=None):
    exclude = exclude or []
    if 'sort_order' not in exclude:
        exclude.append('sort_order')
    return super().validate_unique(exclude=exclude)

[jturnbull]

ZOMBIE thread…

[meshy]

Still relevant unfortunately.

[meshy]

To clarify, this is required because otherwise ModelForms (such as in the admin) will reject changes to the sort_order field where it is moved into a position that is currently in use.

[mjtamlyn]

Note: this still sucks.

I think sort_order should always be unique or unique with another field. We could have a metaclass which inspects the model and raises warnings if this isn't the case. It's not a difficult migration to do for any model which has the wrong results at the moment.

[meshy]

Perhaps this would be a good candidate for the Checks framework.