Can I have an unsigned AutoField?

I want a primary key for my model to be unsigned. Therefore I do something like this:

class MyModel(models.Model):
    id = models.PositiveIntegerField(primary_key=True)

This gets me an UNSIGNED column in the resulting MySQL table, which I want. However, I believe I will not get the automatic assigning to id each time I create a new object, will I? This seems to require the use of AutoField instead. Problem is, AutoField is signed. Is there a way to create an unsigned AutoField?

Solution

The actual type of the field is specified in the backend. In the case of MySQL, the backend is django.db.backends.mysql. This extract from django/db/backends/mysql/creation.py shows this translation:

class DatabaseCreation(BaseDatabaseCreation):
    # This dictionary maps Field objects to their associated MySQL column
    # types, as strings. Column-type strings can contain format strings; they'll
    # be interpolated against the values of Field.__dict__ before being output.
    # If a column type is set to None, it won't be included in the output.
    data_types = {
        'AutoField':         'integer AUTO_INCREMENT',
        'BooleanField':      'bool',
        'CharField':         'varchar(%(max_length)s)',
        ...

To change that, you should either monkey-patch this dict doing:

from django.db.backends.mysql.creation import DatabaseCreation
DatabaseCreation.data_types['AutoField'] = 'integer UNSIGNED AUTO_INCREMENT'

Or you create your own class, so you won't mess up with the other AutoFields:

from django.db.models.fields import AutoField
class UnsignedAutoField(AutoField):
    def get_internal_type(self):
        return 'UnsignedAutoField'

from django.db.backends.mysql.creation import DatabaseCreation
DatabaseCreation.data_types['UnsignedAutoField'] = 'integer UNSIGNED AUTO_INCREMENT'

And then create your own PKs:

id = UnsignedAutoField()

As it descends from AutoField, it will inherit all of its behavior.