Error in admin: __str__ returned non-string (type NoneType)

The admin returns this error when trying to add an instance to one of my models. The model itself has a correct str() method and contains no instances yet. Also tried replacing the str() method with a static method or removing it altogether. No luck.

The error seems to point towards something going wrong in the history part of the admin. Stacktrace points to line 33.

Error during template rendering

In template /Users/snirp/juis/snirpdrive/glotto/venv/lib/python3.6/site-packages/django/contrib/admin/templates/admin/change_form.html, error at line 33
__str__ returned non-string (type NoneType)
23  {% endblock %}
24  {% endif %}
25  
26  {% block content %}<div id="content-main">
27  {% block object-tools %}
28  {% if change %}{% if not is_popup %}
29    <ul class="object-tools">
30      {% block object-tools-items %}
31      <li>
32          {% url opts|admin_urlname:'history' original.pk|admin_urlquote as history_url %}
33          <a href="{% add_preserved_filters history_url %}" class="historylink">{% trans "History" %}</a>
34      </li>
35      {% if has_absolute_url %}<li><a href="{{ absolute_url }}" class="viewsitelink">{% trans "View on site" %}</a></li>{% endif %}
36      {% endblock %}
37    </ul>
38  {% endif %}{% endif %}
39  {% endblock %}
40  <form {% if has_file_field %}enctype="multipart/form-data" {% endif %}action="{{ form_url }}" method="post" id="{{ opts.model_name }}_form" novalidate>{% csrf_token %}{% block form_top %}{% endblock %}
41  <div>
42  {% if is_popup %}<input type="hidden" name="{{ is_popup_var }}" value="1" />{% endif %}
43  {% if to_field %}<input type="hidden" name="{{ to_field_var }}" value="{{ to_field }}" />{% endif %}

These are the relevant parts of my models.py and admin.py

class UserContent(models.Model):
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)
    created_by = models.ForeignKey(User, related_name='%(class)s_creator')
    updated_by = models.ForeignKey(User, related_name='%(class)s_updater')

    class Meta:
        abstract = True


class Linetrans(UserContent):
    line = models.ForeignKey(Line)
    translation = models.ForeignKey(Translation)
    text = models.CharField(max_length=400)

    #def __str__(self):
    #    return self.text

    class Meta:
        ordering = ['line']

and

admin.site.register(Linetrans)

Other model classes are very similar and do not return an error. The error also occurs when the Linetrans is added as an inline to another admin class.

edit / update: I commented out all other str() methods in my model and sure enough the error seems to go away. Now trying to pinpoint the issue.

Solution 1:

Turns out that there was an unexpected empty CharField in a related model. Leaving this an an answer, because it might help others.

Troubleshoot the issue by systematically commenting out the __str__() methods of your models until you find the offending model. Work from there to identify the offending record(s).

Solution 2:

I had a similar issue. The problem was that the primary key for one row was null ( I dont know how that happened). I couldnt delete the row because of cascade issues. so I had to change the str mmethod to somthing like this.

def __str__(self):
    if self.customerName==None:
        return "ERROR-CUSTOMER NAME IS NULL"
    return self.customerName