"Cannot assign must be a instance" while assigning that specific instance

Question

There are many related questions on SO about this topic, but they all seem to deal with cases where the element being assigned is actually the instance.id or a string with the instance name.

I have two models, one called Photo, the other called Material.

class Photo(models.Model):
     mat = models.ForeignKey(
        Material, related_name='photos',null=True, blank=True)

In the shell, I get a Photo object, and I create a Material object.

[in : 1]p = Photo.objects.all()[0] # get the first
[in : 2]m, bool = Material.objects.get_or_create(id=1) # get_or_create returns a tuple, take the first

[in : 3]p
[out:1] <Photo: Photo object>
[in : 4]m
[out:2] <Material:Material object>

[in : 5]p.mat = m

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-60-836d23304359> in <module>()
----> 1 p.mat = m

/home/rootadmin/opensurfaces/venv/local/lib/python2.7/site-packages/django/db/models/fields/related.pyc in __set__(self, instance, value)
    595                     instance._meta.object_name,
    596                     self.field.name,
--> 597                     self.field.rel.to._meta.object_name,
    598                 )
    599             )

ValueError: Cannot assign "<Material: Material object>": "Photo.mat" must be a "Material" instance.

As far as I can tell, m is a Material instance. What is going wrong?

rahul mehra · Accepted Answer · 2018-05-02 11:49:18Z

1

This error raise usually when you trying to assign a instance.id instead of instance.
In the django, you are need to saaign a instance to all the ForeignKey fields.
Instance Ex:

mat = Material.objects.all()[0]

answered May 2, 2018 at 11:49

rahul mehra

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1 Comment

Mitchell van Zuylen Over a year ago

Usually, yes, but regretfully that's not the case here. The m object is truly a Material instance.

Sandeep Balagopal · Accepted Answer · 2018-05-02 13:10:06Z

0

Can you check how Material model is imported to the file and make sure it is the same model which is the ForeignKey for Photo ? It looks like you have the wrong Material class imported.

edited May 2, 2018 at 13:10

answered May 2, 2018 at 12:02

Sandeep Balagopal

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5 Comments

Mitchell van Zuylen Over a year ago

Sorry my codeblock wasn't clear enough. I index with [0] on get or create, meaning it returns the actually object instead of the tuple. This can also been seen in input line 4.

Sandeep Balagopal Over a year ago

didn't notice that. This is format we usually do. return to two variables.

Mitchell van Zuylen Over a year ago

Yea, I probably should have made it more explicit in the question that I index.

Sandeep Balagopal Over a year ago

Can you check how Material model is imported to the file and make sure it is the same model which is the ForeignKey for Photo ?

Mitchell van Zuylen Over a year ago

Ha, found it, thanks to your. I had double checked the import locations, which were correct, but turns out the created Material instance was declared somewhere else, e.g. I had two difference Material classes, and I tried to assign the wrong one. Thanks!

Mitchell van Zuylen · Accepted Answer · 2018-05-02 12:32:45Z

0

The element m is an actual Material instance. This doesn't mean it is the right Material instance.

In my case, it turned out there were two separate declarations of a class named Material. As a result, I was trying to assign the wrong Material instance.

answered May 2, 2018 at 12:32

Mitchell van Zuylen

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