[1.7.x] Fixed #23761 -- Fixed crash with MySQL validator and db_type is None.

        if getattr(field, 'rel', None) is None:

            field_type = field.db_type(connection)

            # Ignore any non-concrete fields

            if field_type is None:

                return errors

            if (field_type.startswith('varchar')  # Look for CharFields...

                    and field.unique  # ... that are unique

                    and (field.max_length is None or int(field.max_length) > 255)):

  (:ticket:`23420`).

* Fixed a migration serializing bug involving ``float("nan")`` and

  ``float("inf")`` (:ticket:23770:).

  ``float("inf")`` (:ticket:`23770`).

* Fixed a regression where custom form fields having a ``queryset`` attribute

  but no ``limit_choices_to`` could not be used in a

  :class:`~django.forms.ModelForm` (:ticket:`23795`).

* Fixed a custom field type validation error with MySQL backend when

  ``db_type`` returned ``None`` (:ticket:`23761`).

from django.db import models

from .base import IsolatedModelsTestCase

class CustomFieldTest(IsolatedModelsTestCase):

    def test_none_column(self):

        class NoColumnField(models.AutoField):

            def db_type(self, connection):

                # None indicates not to create a column in the database.

                return None

        class Model(models.Model):

            field = NoColumnField(primary_key=True, db_column="other_field")

            other_field = models.IntegerField()

        field = Model._meta.get_field('field')

        errors = field.check()

        self.assertEqual(errors, [])