How do you specify a default for the Django ForeignKey field?

Question

I’m trying to add a ForeignKey field to a Django model using South. I’m getting the following error:

ValueError: You cannot add a null=False column without a default value.

I did, in fact, specify a default value for the field, but I’m not sure I did it correctly.

language = models.ForeignKey(Language, default=Language.objects.all()[0])

Should this work?

radious · Accepted Answer · 2011-08-16T16:22:29.687

16

AFAIK Django won't execute a QuerySet passed as a param, even if it's limited to one element. You should try something like proposed in this post

class Foo(models.Model):
    a = models.CharField(max_length=10)

def get_foo():
    return Foo.objects.get_or_create(id=1)

class Bar(models.Model):
    b = models.CharField(max_length=10)
    a = models.ForeignKey(Foo, default=get_foo)

edited Aug 16 '11 at 16:22

answered Aug 16 '11 at 15:50

radious

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I would post the same. Additionaly you can create a default element in get_foo if Foo has no data. – balazs Aug 16 '11 at 16:00
1

Yes, using get_or_create is nice idea! – radious Aug 16 '11 at 16:06
3

This doesn't really work, assuming the only point is to query by something other than `id`. This is because the query is run before the migration is created, and the resulting `id` is hardcoded into the migration. So if anyone who later runs the migration has a different `id` for the desired record, they'll still get your `id` rather than running the query on their database, so will get a random default. – Chris Aug 25 '19 at 23:05

score 2 · Answer 2 · edited Apr 23 '20 at 09:28

2

Because get_or_create returns a tuple, I think this is a better solution

def get_default():
    result, _ = Foo.objects.get_or_create(id=1)
    return result

than this

return Foo.objects.get_or_create(id=1)

edited Apr 23 '20 at 09:28

Barm

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answered Apr 06 '16 at 06:36

bopajdowski

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score 1 · Answer 3 · answered May 08 '14 at 03:43

1

This is really an extension to @radious answer. You can also use lambda in the default, if you're looking to cut out lines of code

class Foo(models.Model):
    a = models.CharField(max_length=10)

class Bar(models.Model):
    b = models.CharField(max_length=10)
    a = models.ForeignKey(Foo, default=lambda: Foo.objects.get_or_create(id=1)[0])

note that you have to use index 0 on get_or_create() because the method returns a tuple, so you need to get the object, instead of the tuple.

https://docs.djangoproject.com/en/1.6/ref/models/querysets/#get-or-create

answered May 08 '14 at 03:43

Josh Brown

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Django can't serialize lambda expressions – QuestionEverything May 23 '17 at 12:34
3

The relevance of @HasanIqbal's comment is this means this approach won't work after Django 1.7 (assuming you're using migrations) - [docs](https://docs.djangoproject.com/en/1.7/topics/migrations/#serializing-values) – Chris Aug 25 '19 at 22:42

score 0 · Answer 4 · answered May 11 '23 at 11:59

You should use get_or_create() and models.ForeignKey() needs on_delete as shown below. *Don't forget to put .id just after get_or_create(id=1)[0] because default in models.ForeignKey() needs id of a Language object otherwise there is an error:

language = models.ForeignKey(
    Language,               # Here  
    default=Language.objects.get_or_create(id=1)[0].id),
    on_delete=models.CASCADE # Here
)

How do you specify a default for the Django ForeignKey field?

4 Answers4

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