ValueError: Cannot add *: instance is on database "default", value is on database "None"

Question

In [1]: from editor.models import *
In [4]: from subscriptions.models import *
In [5]: template = StockTemplate.objects.create(name='Template 1')
In [6]: template
Out[6]: <StockTemplate: Template 1>
In [7]: plan = SubscriptionPlan.objects.create(name='Bronze')
In [8]: plan
Out[8]: <SubscriptionPlan: Bronze>
In [12]: plan.templates.add(template)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)

/home/me/GitProjects/test_git/proj/<ipython console> in <module>()

/home/me/GitProjects/test_git/django-trunk/django/db/models/fields/related.pyc in add(self, *objs)
    498         if rel.through._meta.auto_created:
    499             def add(self, *objs):
--> 500                 self._add_items(self.source_field_name, self.target_field_name, *objs)
    501 
    502                 # If this is a symmetrical m2m relation to self, add the mirror entry in the m2m table


/home/me/GitProjects/test_git/django-trunk/django/db/models/fields/related.pyc in _add_items(self, source_field_name, target_field_name, *objs)
    558                         if not router.allow_relation(obj, self.instance):
    559                            raise ValueError('Cannot add "%r": instance is on database "%s", value is on database "%s"' %
--> 560                                                (obj, self.instance._state.db, obj._state.db))
    561                         new_ids.add(obj.pk)
    562                     elif isinstance(obj, Model):

ValueError: Cannot add "<StockTemplate: Template 1>": instance is on database "default", value is on database "None"

Models

  6 class SubscriptionPlan(models.Model):
  7     name = models.CharField(max_length=255)
  8     templates = models.ManyToManyField(StockTemplate)
  9     monthly_fee = models.IntegerField("Monthly Fee", max_length=16, default="0")
 10     modified = models.DateTimeField(auto_now=True, editable=False)
 11     created = models.DateTimeField(auto_now_add=True, editable=False)
 12 
 13     def __unicode__(self):
 14         return "%s" % self.name



 18 class StockTemplate(IKImage):
 19     name = models.TextField()
 20     description = models.TextField(blank=True)
 21 
 22     is_public = models.BooleanField(default=True)
 23 
 24     html = models.FileField(upload_to='stock_templates/html/', \
 25                             help_text='The file that will be used to render.')
 26     #css = models.FileField(upload_to='stock_templates/css/', blank=True)
 27 
 28     img = models.ImageField(upload_to='stock_templates/img/')
 29 
 30     modified = models.DateTimeField(auto_now=True)
 31     created = models.DateTimeField(auto_now_add=True)
 32 
 33     objects = StockTemplateManager()
 34 
 35     class IKOptions:
 36         spec_module = 'editor.specs'
 37         cache_dir = 'stock_templates/img/specs/'
 38         image_field = 'img'
 39 
 40     def __unicode__(self):
 41         return u"%s" % self.name
 42 
 43     def get_absolute_url(self):
 44         return reverse('preview_stock', args=[self.id])

Is it something to do with the fact that StockTemplate is an IKImage object?

diegueus9 · Accepted Answer · 2021-10-26 21:06:26Z

86

The problem here is that you need to call the method save for both objects before adding the template to the product:

template.save()
plan.save()
plan.templates.add(template)

Django can't add it if none of those objects has an id (plan and template)

edited Oct 26, 2021 at 21:06

answered Nov 3, 2011 at 17:08

diegueus9

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Comments

sebtheiler · Accepted Answer · 2021-08-24 13:29:21Z

3

One very common cause of this error is when you try to add the relations before saving the (parent) object. First, save the object and then add the relationships. Hopefully, it will be resolved.

Example:

a1 = Article(headline='Django lets you build Web apps easily')

Now, if you directly try to add the relations like this:

a1.publications.add(p1)

It will give an error.

Solution: SAVE IT first and then add relations!!

a1.save()
a1.publications.add(p1)

This will work flawlessly.

Docs

edited Aug 24, 2021 at 13:29

sebtheiler

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answered Jul 13, 2021 at 15:33

Arghya Bhattacharya

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Comments

Fathy · Accepted Answer · 2022-10-02 11:00:21Z

2

this will also happen if you override the save without calling the super().save(*args, **kwargs) if you added it to m2m even after you're calling save

i.e:

# will raise a ValueError error 
def save(self, *args, **kwargs):
    # ..... somecode .....

def save(self, *args, **kwargs):
    # ..... somecode .....
    super().save(*args, **kwargs) # here

answered Oct 2, 2022 at 11:00

Fathy

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