Can't create many-to-many object in django admin using django-mptt

Question

I can't create an object in django admin. It raises the error:

ValueError at /admin/app/category/add/
"<Category: >" needs to have a value for field "from_category" before this many-to-many relationship can be used.
Request Method: GET
Request URL:    http://127.0.0.1:8000/admin/app/category/add/
Django Version: 1.6.5
Exception Type: ValueError
Exception Value:    
"<Category: >" needs to have a value for field "from_category" before this many-to-many relationship can be used.
Exception Location: /usr/local/lib/python2.7/dist-packages/django/db/models/fields/related.py in __init__, line 524
Python Executable:  /usr/bin/python
Python Version: 2.7.6

I can't understand what's going wrong. I have the following models.py:

from django.db import models
from mptt.models import MPTTModel, TreeManyToManyField

class Category(MPTTModel):
    name = models.CharField(max_length=255, default=u'')
    engName = models.CharField(max_length=255, default=u'', blank=True)
    parents = TreeManyToManyField('self', symmetrical=False, related_name='children', blank=True)
    description = models.TextField(default=u'', blank=True)
    active = models.BooleanField(default=True, blank=True)

    def __unicode__(self):
        return u"{}".format(self.name)

    class MPTTMeta:
        order_insertion_by = ['name']
        parent_attr = 'parents'

and admin.py:

from django.contrib import admin
from mptt.admin import MPTTModelAdmin
from app.models import Category, Pattern

admin.site.register(Category, MPTTModelAdmin)
admin.site.register(Pattern)

Answer 1

The problem with this is the TreeManyToManyField parent.

In a tree structure, nodes can only have one parent. So mptt doesn't support this.

If you use a TreeForeignKey instead you should have better luck.

I've added a note to the django-mptt docs about this.