How to unittest a ValidationError and assert the correct code is used?

I have a model with a ValidationError constraint:

class MyModel(models.Model)
    title = models.CharField()
      
    def clean(self):
        error_dict = {}
        if self.title='invalid_title':
                 error_dict['title'] = ValidationError(
                         'Title is invalid', code='invalid_title_code')
        
        if error_dict:
                 raise ValidationError(error_dict)

I want to unittest this ValidationError, but how do I test the code is correct? For example:

def test_title_invalid(self):

     with self.assertRaises(ValidationError) as cm: 
          MyModel.objects.create(title='invalid_title')
     
     exception = cm.exception

     # Test the key exists
     self.assertTrue(hasattr(exception, 'error_dict'))
     self.assertIn('title', exception.error_dict)

     # Test the message exists
     self.assertIn('Title is invalid', exception.messages)

     # Test the code exists
     self.assertIn('invalid_title_code', exception.code)

This all works fine until self.assertIn('invalid_title_code', exception.code), which results in an AttributeError that states 'ValidationError' object has no attribute 'code'

In the django source for django.core.exceptions.ValidationError it appears that if you pass a dict object, the code attribute is not added:

class ValidationError(Exception):
    """An error while validating data."""
    def __init__(self, message, code=None, params=None):
        super().__init__(message, code, params)

        if isinstance(message, dict):
            self.error_dict = {}
            for field, messages in message.items():
                if not isinstance(messages, ValidationError):
                    messages = ValidationError(messages)
                self.error_dict[field] = messages.error_list
 
        else:
            self.message = message
            self.code = code
            self.params = params
            self.error_list = [self]

Is this the cause of the problem?