CODEWITHSUNDEEP
pythondjangodjango-models
sunnysm
sunnysm

Reputation: 349

AttributeError: Manager isn't accessible via Section instances

I got following error when i used to save 'sequence number' in automatic incremented form error 

last_id = self.objects.all().aggregate(largest=models.Max('sequence_number'))['largest']
 File "/home/abc/new/virtual/local/lib/python2.7/site-packages/django/db/models/manager.py", line 252, in __get__
raise AttributeError("Manager isn't accessible via %s instances" % type.__name__)
AttributeError: Manager isn't accessible via Section instances

this is my models.py

class Section(models.Model):
    section_id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    req = models.ForeignKey('requst.Request', null=True, blank=True)
    title = models.CharField(max_length=100)
    sequence_number = models.FloatField( default=1)
    created_at = models.DateTimeField(auto_now_add=True, null=True, blank=True)
    updated_at = models.DateTimeField(auto_now=True, null=True, blank=True)
    created_by = models.IntegerField(default=0)
    updated_by = models.IntegerField(default=0)

    def __unicode__(self):
        return self.title

    def save(self, *args, **kwargs):
        if self._state.adding:
            last_id = self.objects.all().aggregate(largest=models.Max('sequence_number'))['largest']
            if last_id is not None:
                self.sequence_number = last_id + 1
        super(Section, self).save(*args, **kwargs)

Upvotes: 3

Views: 3400

Answers (1)

bruno desthuilliers
bruno desthuilliers

Reputation: 77902

As the error clearly states, you are not allowed to access a model's manager on a model instance, you have to access it thru the model class itself.

The clean way to do so is to use type(), ie replace:

self.objects.all()

with

type(self).objects.all()

Now model inheritance being rather rare, if you don't have a use for it in this case, you can just hardcode the model class instead using:

Section.objects.all()

This being said, your code is not bulletproof - you can easily have race conditions with multiple processes calling Section.save() concurrently.

Upvotes: 4

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