django : __str__ returned non-string (type NoneType), only when __str__ defined

Question

I knew there is tons of post regarding this error. I think what I got is pretty strange.

Ok here it is.

models.py

class Axie(models.Model):
    id = models.IntegerField(primary_key=True)
    name = models.CharField(max_length=30, blank=True, null=True)
    sire = models.ForeignKey("Axie", related_name= "mysire", blank=True, null=True, on_delete=models.SET_NULL)
    matron = models.ForeignKey("Axie", related_name= "mymatron", blank=True, null=True, on_delete=models.SET_NULL)
    scholar= models.ForeignKey(Scholar,null=True, blank=True, on_delete=models.SET_NULL)

    def __str__(self):
        return self.name

With that code, I got a normal basic list as usual

But I got error on every 'change view'

This is the traceback from debug page

Environment:


Request Method: GET
Request URL: http://127.0.0.1:8000/admin/university/axie/10277276/change/

Django Version: 4.0.2
Python Version: 3.8.10
Installed Applications:
['django.contrib.admin',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'university.apps.UniversityConfig']
Installed Middleware:
['django.middleware.security.SecurityMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.common.CommonMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware',
 'django.middleware.clickjacking.XFrameOptionsMiddleware']


Template error:
In template /home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/contrib/admin/templates/admin/includes/fieldset.html, error at line 19
   __str__ returned non-string (type NoneType)
   9 :             {% for field in line %}
   10 :                 <div{% if not line.fields|length_is:'1' %} class="fieldBox{% if field.field.name %} field-{{ field.field.name }}{% endif %}{% if not field.is_readonly and field.errors %} errors{% endif %}{% if field.field.is_hidden %} hidden{% endif %}"{% elif field.is_checkbox %} class="checkbox-row"{% endif %}>
   11 :                     {% if not line.fields|length_is:'1' and not field.is_readonly %}{{ field.errors }}{% endif %}
   12 :                     {% if field.is_checkbox %}
   13 :                         {{ field.field }}{{ field.label_tag }}
   14 :                     {% else %}
   15 :                         {{ field.label_tag }}
   16 :                         {% if field.is_readonly %}
   17 :                             <div class="readonly">{{ field.contents }}</div>
   18 :                         {% else %}
   19 :                              {{ field.field }} 
   20 :                         {% endif %}
   21 :                     {% endif %}
   22 :                     {% if field.field.help_text %}
   23 :                         <div class="help">{{ field.field.help_text|safe }}</div>
   24 :                     {% endif %}
   25 :                 </div>
   26 :             {% endfor %}
   27 :         </div>
   28 :     {% endfor %}
   29 : </fieldset>


Traceback (most recent call last):
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/core/handlers/base.py", line 204, in _get_response
    response = response.render()
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/response.py", line 105, in render
    self.content = self.rendered_content
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/response.py", line 83, in rendered_content
    return template.render(context, self._request)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/backends/django.py", line 61, in render
    return self.template.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 176, in render
    return self._render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 168, in _render
    return self.nodelist.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 977, in render
    return SafeString(''.join([
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 978, in <listcomp>
    node.render_annotated(context) for node in self
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 938, in render_annotated
    return self.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/loader_tags.py", line 153, in render
    return compiled_parent._render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 168, in _render
    return self.nodelist.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 977, in render
    return SafeString(''.join([
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 978, in <listcomp>
    node.render_annotated(context) for node in self
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 938, in render_annotated
    return self.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/loader_tags.py", line 153, in render
    return compiled_parent._render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 168, in _render
    return self.nodelist.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 977, in render
    return SafeString(''.join([
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 978, in <listcomp>
    node.render_annotated(context) for node in self
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 938, in render_annotated
    return self.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/loader_tags.py", line 65, in render
    result = block.nodelist.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 977, in render
    return SafeString(''.join([
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 978, in <listcomp>
    node.render_annotated(context) for node in self
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 938, in render_annotated
    return self.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/loader_tags.py", line 65, in render
    result = block.nodelist.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 977, in render
    return SafeString(''.join([
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 978, in <listcomp>
    node.render_annotated(context) for node in self
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 938, in render_annotated
    return self.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/defaulttags.py", line 217, in render
    nodelist.append(node.render_annotated(context))
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 938, in render_annotated
    return self.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/loader_tags.py", line 201, in render
    return template.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 178, in render
    return self._render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 168, in _render
    return self.nodelist.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 977, in render
    return SafeString(''.join([
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 978, in <listcomp>
    node.render_annotated(context) for node in self
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 938, in render_annotated
    return self.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/defaulttags.py", line 217, in render
    nodelist.append(node.render_annotated(context))
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 938, in render_annotated
    return self.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/defaulttags.py", line 217, in render
    nodelist.append(node.render_annotated(context))
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 938, in render_annotated
    return self.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/defaulttags.py", line 298, in render
    return nodelist.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 977, in render
    return SafeString(''.join([
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 978, in <listcomp>
    node.render_annotated(context) for node in self
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 938, in render_annotated
    return self.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/defaulttags.py", line 298, in render
    return nodelist.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 977, in render
    return SafeString(''.join([
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 978, in <listcomp>
    node.render_annotated(context) for node in self
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 938, in render_annotated
    return self.render(context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 1044, in render
    return render_value_in_context(output, context)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/template/base.py", line 1021, in render_value_in_context
    value = str(value)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/utils/html.py", line 377, in <lambda>
    klass.__str__ = lambda self: mark_safe(klass_str(self))
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/forms/boundfield.py", line 33, in __str__
    return self.as_widget()
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/forms/boundfield.py", line 92, in as_widget
    return widget.render(
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/forms/widgets.py", line 246, in render
    context = self.get_context(name, value, attrs)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/contrib/admin/widgets.py", line 283, in get_context
    'rendered_widget': self.widget.render(name, value, attrs),
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/forms/widgets.py", line 246, in render
    context = self.get_context(name, value, attrs)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/forms/widgets.py", line 681, in get_context
    context = super().get_context(name, value, attrs)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/forms/widgets.py", line 642, in get_context
    context['widget']['optgroups'] = self.optgroups(name, context['widget']['value'], attrs)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/forms/widgets.py", line 592, in optgroups
    for index, (option_value, option_label) in enumerate(self.choices):
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/forms/models.py", line 1191, in __iter__
    yield self.choice(obj)
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/forms/models.py", line 1205, in choice
    self.field.label_from_instance(obj),
  File "/home/bino/.local/share/virtualenvs/djenv-loPXW3SM/lib/python3.8/site-packages/django/forms/models.py", line 1274, in label_from_instance
    return str(obj)

Exception Type: TypeError at /admin/university/axie/10277276/change/
Exception Value: __str__ returned non-string (type NoneType)

Next, I remove the 'def str(self)' part,

class Axie(models.Model):
    id = models.IntegerField(primary_key=True)
    name = models.CharField(max_length=30, blank=True, null=True)
    sire = models.ForeignKey("Axie", related_name= "mysire", blank=True, null=True, on_delete=models.SET_NULL)
    matron = models.ForeignKey("Axie", related_name= "mymatron", blank=True, null=True, on_delete=models.SET_NULL)
    scholar= models.ForeignKey(Scholar,null=True, blank=True, on_delete=models.SET_NULL)

As expected, the list view do not show the name,

But Got 'nice' change view

Kindly please give me any clue to fix this problem.

Sincerely

-bino-