Stripe-style short alphanumeric UUID fields

fields.py

from django.db.models import SubfieldBase
from django.core.exceptions import ImproperlyConfigured
from django_extensions.db.fields import ShortUUIDField

class XidField(ShortUUIDField):
    """
    XID stands for external ID.
    Randomly generated IDs (base62 encoded UUID) used for public display purposes.
    The random ID gets a short textual prefix for improved legibility, like Stripe's IDs.

    Example:
    With prefix "usr", the db value "uUM7gxtMEGhMvMooWHj9yH" becomes "usr_uUM7gxtMEGhMvMooWHj9yH"
    """
    __metaclass__ = SubfieldBase

    def __init__(self, prefix=None, *args, **kwargs):
        self.prefix = prefix

        if not self.prefix:
            raise ImproperlyConfigured("prefix parameter is required.")

        kwargs.setdefault('max_length', 22)
        kwargs.setdefault('unique', True)
        kwargs.setdefault('verbose_name', 'XID')
        kwargs.setdefault('editable', False)

        super(XidField, self).__init__(*args, **kwargs)

    def to_python(self, value):
        """
        Adds prefix
        """
        if value is not None and '_' not in value:
            value = u'%s_%s' % (self.prefix, value)
        return value

    def get_prep_value(self, value):
        """
        Remove prefix before sending to db
        """
        if value is not None and '_' in value:
            value = value.split('_')[1]
        return value

    def clean(self, value, model_instance):
        """
        Override clean to use get_prep_value instead of to_python, since we do string manipulation there.
        """
        value = self.get_prep_value(value)
        self.validate(value, model_instance)
        self.run_validators(value)
        return value

    def south_field_triple(self):
        """Returns a suitable description of this field for South."""
        from south.modelsinspector import introspector
        field_class = self.__module__ + "." + self.__class__.__name__
        args, kwargs = introspector(self)

        return (field_class, args, dict(kwargs.items() + { "prefix": "'%s'" % self.prefix}.items()))

    def deconstruct(self):
        name, path, args, kwargs = super(XidField, self).deconstruct()
        kwargs["prefix"] = self.prefix
        return name, path, args, kwargs