GenericIPAddressField must not allow blank for NOT NULL fields

[Sasha Romijn]

GenericIPAddressField internally translates all blank values, e.g. an empty string, to None/NULL before saving them in the database. This is good, because in PostgreSQL this field is backed by INET, meaning an empty string is not allowed (this problem also affects IPAddressField, but we can't change it as it would break backwards compatibility, #5622).

However, this means that a field which allows blank but not null values, will trigger a database error at runtime when trying to save a blank value. Therefore, the model validation should not allow GenericIPAddressFields to have blank but not null.

To summarise:

• Current behaviour: when storing a blank value in a GenericIPAddressField(blank=True, null=False), a database exception will be triggered
• New behaviour: a GenericIPAddressField(blank=True, null=False) will trigger a model validation error

[Sasha Romijn]

Pull request in: ​https://github.com/django/django/pull/1204

[Tim Graham <timograham@…>]

In ded95ccdce0ba983c405ddde9eb071e454245a94:

Fixed #20484 -- Added model validation for GenericIPAddressField

GenericIPAddressField must not allow blank for NOT NULL fields

Thanks Erik Romijn.

[chrismedrela]

This patch includes no doc in docs/ref/models/fields.txt. I've created a pull request with a note in the file: ​https://github.com/django/django/pull/1453.

[Christopher Medrela <chris.medrela@…>]

In fb26c4996a0c4d41aa80d28ce65ab050ffe6df6b:

Fixed #20484 again -- added note to field documentation

[Preston Holmes <preston@…>]

In cf041b883c6a089f0b113846918fdb074e465713:

Merge pull request #1453 from chrismedrela/ticket20484-improvement

Fixed #20484 again -- added note to field documentation

[Tim Graham <timograham@…>]

In 6264e39c346a1b563dc0cbc31c5fb75830e032cb:

[1.6.x] Added note to GenericIPAddressField documentation

refs #20484

Backport of fb26c4996a from master