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Using Django's new migration framework, let's say I have the following model that already exists in the database:

class TestModel(models.Model):
    field_1 = models.CharField(max_length=20)

I now want to add a new TextField to the model, so it looks like this:

class TestModel(models.Model):
    field_1 = models.CharField(max_length=20)
    field_2 = models.TextField(blank=True)

When I try to migrate this model using python manage.py makemigrations, I get this prompt:

You are trying to add a non-nullable field 'field_2' to testmodel without a default; we can't do that (the database needs something to populate existing rows).
Please select a fix:
 1) Provide a one-off default now (will be set on all existing rows)
 2) Quit, and let me add a default in models.py

I can easily fix this by adding null=True to field_2, but Django's convention is to avoid using null on string-based fields like CharField and TextField (from https://docs.djangoproject.com/en/dev/ref/models/fields/). Is this a bug, or am I misunderstanding the docs?

Jeremy Swinarton
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2 Answers2

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It's not a bug, it's documented and logical. You add a new field, which is (by best practice, as you noticed) not NULLable so django has to put something into it for the existing records - I guess you want it to be the empty string.

you can

 1) Provide a one-off default now (will be set on all existing rows)

so just press 1, and provide '' (the empty string) as value.

or specify default='' in the models.py, as suggested:

 2) Quit, and let me add a default in models.py
ch3ka
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  • I guess that makes sense, but it strikes me as odd behaviour. I expect that if I specify `blank=True`, the migration will automatically use a blank string as the default, the same way that if I specify `null=True`, I don't need to tell the migration to use `NULL` as the default. – Jeremy Swinarton Nov 03 '14 at 17:18
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    Well, `explicit is better than implicit`. `blank=True` only tells the django admin to accept the empty string as input, it does not imply a default value. `default=` does, and it's perfectly reasonable to specify a default value different from empty string. For `null=True`, the migration does not use `NULL` as default, but simply nothing - which happens to "evaluate" to `NULL`. – ch3ka Nov 03 '14 at 17:22
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  1. select 1 ,it will through to python terminal.
  2. Give timezone.now(), it will exit from python terminal.

Note:it may not exit from terminal at first time,give that command once again.

satya narayan pardhi
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