Add a "split" field to a model causes problems

[Andrew Ingram]

Hi all,

This one stung me today. Basically as part of an event (calendar) app, I have functionality for splitting a series of events into two at a given timestamp. The details aren't particularly relevant, but the key thing is that I had a method called "split" on one of the models.

This was working fine until I tried to add a ManyToManyField to the same model, at which point it through a rather ugly error during initialisation of the apps.

django/db/models/fields/related.py line 56, in add_lazy_relation
    app_label, model_name = relation.split(".")
TypeError: unbound method split() must be called with RecurringEvent instance as first argument (got str instance instead)

Because I had no knowledge of the inner workings here, this error wasn't helpful until pdb told me that `relation` was an instance of my model. The issue is that in this point in the code, relation can either be a model or a string 'app_label.model_name', so it's using the presence of a split function to determine which it is, me having a split function on my model breaks this check. Having a non-callable 'split' causes a similar problem, just with a different exception.

Long story short, adding a 'split'  method to my model caused the inner-workings of Django to break, I think the documentation should provide a list of attribute names that can't be added to a Django model without breaking things, though this is the first time I've ever come across such a problem so it might be a one-off.

Regards,

Andrew Ingram

[Russell Keith-Magee]

Hi Andrew,

Thanks for that gnarly bug report. I'm a little surprised that the 'split' name causes a problem here; based on my knowledge of Django's internals, I wouldn't have thought that the situation you describe shouldn't be a problem. My gut reaction tells me that the actual problem is somewhere else in your model definition, causing the wrong object instance to be passed into the 'relation' variable; the 'split' method is a symptom, rather than the actual problem. However, I may be wrong.

If you can open a bug report with a complete working (or rather, broken) example of the problem, we can dig into it more. If you're right that the split method is the problem, we should certainly document it; if the problem lies somewhere else, there's possibly some better validation that we need to do.

Yours,

Russ Magee %-)

[Alex Gaynor]

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No this is a bug in Django, I see it.

https://github.com/django/django/blob/master/django/db/models/fields/related.py#L51

Basically some genius (very likely me), decided we shouldn't type check for string or model, and just try to call split.

It's an easy enough fix. Please file a bug and if there's no patch I'll do it tonight.

Alex

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[Stephen Burrows]

I've opened a ticket to track this issue.

[Andrew Ingram]

Hi Mike,

It looks like I missed part of the problem, namely that abstract models are involved. These model definitions should trigger the error:

from django.db import models

class Bar(models.Model):

name = models.CharField(max_length=250)

class AbstractFoo(models.Model):

name = models.CharField(max_length=250)

bars = models.ManyToManyField(Bar)

def split(self):

pass

class Meta:

abstract = True

class Foo(AbstractFoo):

pass

It doesn't make a difference if the ManyToManyField is on the abstract model or the concrete one.

Regards,

Andrew

On 4 Nov 2012, at 20:42, Mike Johnson <mrjoh...@gmail.com> wrote:

Hi Andrew,

I'm trying to come up with a test case for this and am struggling to reproduce the problem in test.

Can you post some code to help us reproduce it?

Thanks,
Mike

On Tuesday, October 23, 2012 3:52:10 AM UTC-7, Andrew Ingram wrote:

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[Mike Johnson]

Ahh, got it now. Thanks!