Django URLField with custom scheme

Question

Django's django.db.models.URLField uses a django.core.validators.URLValidator:

class URLField(CharField):
    default_validators = [validators.URLValidator()]

Since it does not specify the schemes to accept, URLValidator defaults to this set:

schemes = ['http', 'https', 'ftp', 'ftps']

I want my URLField to accept ssh:// URLs, so I tried this:

class SSHURLField(models.URLField):
  '''URL field that accepts URLs that start with ssh:// only.'''
  default_validators = [URLValidator(schemes=['ssh'])]

However when I try to save a new object with a valid ssh:// URL, I get rejected.

This also happens if I skip inheriting from URLField and inherit from CharField directly: (Edit: Actually this does work after I recreated my database. I'm not sure why the former doesn't work.)

class SSHURLField(models.CharField):
  '''URL field that accepts URLs that start with ssh:// only.'''
  default_validators = [URLValidator(schemes=['ssh'])]

  def __init__(self, *args, **kwargs):
    kwargs['max_length'] = 64
    super(SSHURLField, self).__init__(*args, **kwargs)

When I use the URLValidator directly in a test, it works:

def test_url(url):
  try:
    URLValidator(schemes=['ssh'])(url)
    return True
  except:
    return False

>>> test_url('ssh://example.com/')
True

>>> test_url('http://example.com/')
False

Answer 1

As @IainDillingham mentioned in the comments, this is a bug in Django: overriding the default_validator of a subclassed ModelField does not necessarily override the default_validator of the FormField to which that base class is associated.

For your example, django.db.models.URLField, we can see its associated form field^[0] is django.forms.fields.URLField^[1]. So the workaround here is to also override def formfield(...) for your customized SSHURLField, to reference a custom django.forms.fields.URLField subclass with the same validator, like so:

from django.core import validators
from django.db import models
from django.forms.fields import URLField as FormURLField

class SSHURLFormField(FormURLField):
    default_validators = [validators.URLValidator(schemes=['ssh'])]

class SSHURLField(models.URLField):  
    '''URL field that accepts URLs that start with ssh:// only.'''  
    default_validators = [validators.URLValidator(schemes=['ssh'])]  

    def formfield(self, **kwargs):
        return super(SSHURLField, self).formfield(**{
            'form_class': SSHURLFormField,
        })

[0] https://github.com/django/django/blob/e17088a108e604cad23b000a83189fdd02a8a2f9/django/db/models/fields/init.py#L2275,L2293
[1] https://github.com/django/django/blob/e17088a108e604cad23b000a83189fdd02a8a2f9/django/forms/fields.py#L650