id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
33108	Makemigrations allows on-off default when adding non-nullable unique field	Julian-Samuel Gebühr	nobody	"
== Reproduce

Create a model like

{{{
class Book(models.Model):
    title = models.CharField(max_length=200)
}}}

and apply with ''makemigrations'' and ''migrate'', then add a book to the database. Now extend the book model by a unique, non-nullable field (e.g.) ISBN

{{{
class Book(models.Model):
    title = models.CharField(max_length=200)
    isbn = models.CharField('ISBN', max_length=13, unique=True, help_text='ISBN number (13 Characters)')
}}}

To apply these changes you try ''makemigrations'' which tells you


{{{
You are trying to add a non-nullable field 'isbn' to book without a default; we can't do that (the database needs som
ething to populate existing rows).
Please select a fix:
 1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
 2) Quit, and let me add a default in models.py
}}}
Choosing 1) and using a default value let's you finish but then ''migrate'' fails with
{{{

django.db.utils.IntegrityError: (1062, ""Duplicate entry '1234567890123' for key 'isbn'"")
}}}

== Expected Behaviour

I would expect ''makemigrations'' to say something in the line of 

{{{
You can't add make a field unique when not all entries in the database have a unique value for this field. A solution might be to remove the unique constraint and add a unique value for this field to all entries. Then re-add the unique constraint.
}}}

and fail to make the migration."	Cleanup/optimization	closed	Migrations	3.2	Normal	wontfix	migrations		Unreviewed	0	0	0	0	0	0