Django database fields for using unsigned bigint column

Django_unsignedBigint.py

from django.db import models
from django.utils.translation import ugettext

class PositiveBigIntegerRelDbTypeMixin(models.fields.PositiveIntegerRelDbTypeMixin):
    def rel_db_type(self, connection):
        if connection.features.related_fields_match_type:
            return self.db_type(connection)
        else:
            return models.BigIntegerField().db_type(connection=connection)

class PositiveBigIntegerField(PositiveBigIntegerRelDbTypeMixin, models.BigIntegerField):
    description = ugettext("Positive big integer")
    
    def get_internal_type(self):
        return "PositiveBigIntegerField"
    
    def formfield(self, **kwargs):
        defaults = {'min_value': 0}
        defaults.update(kwargs)
        return super().formfield(**defaults)
    
    def db_type(self, connection):
        # get db vendor ex.mysql, postgresql, sqlite...
        db_vendor = connection.vendor
        if db_vendor == "mysql":
            return "bigint UNSIGNED"
        elif db_vendor == "oracle":
            return "NUMBER(20)"
        elif db_vendor == "postgresql":
            # postgresql is not supported 'unsigned'
            return "bigint"
        elif db_vendor == "sqlite":
            return "bigint unsigned"
        else:
            # if db_vendor is unknown(BaseDatabaseWrapper), we should return None
            return None