wagtail pathoverflow on adding new child page - django

Question

I have developed a wagtail site and that works just fine. It's just a simple blog. I was able to add a blog index page and blog posts under them. Hover recently when try to add a new page it gives the error
PathOverflow at /admin/pages/add/blog/blogpage/8/
Bellow is the complete traceback.
Environment:
Request Method: POST
Request URL: https://vikatakavi.info/admin/pages/add/blog/blogpage/8/
Django Version: 2.1.5
Python Version: 3.5.2
Installed Applications:
['home',
'search',
'wagtail.contrib.forms',
'wagtail.contrib.redirects',
'wagtail.embeds',
'wagtail.sites',
'wagtail.users',
'wagtail.snippets',
'wagtail.documents',
'wagtail.images',
'wagtail.search',
'wagtail.admin',
'wagtail.core',
'modelcluster',
'taggit',
'django.contrib.admin',
'django.contrib.auth',
'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.messages',
'django.contrib.staticfiles',
'django.contrib.sitemaps',
'blog']
Installed Middleware:
['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',
'django.middleware.security.SecurityMiddleware',
'wagtail.core.middleware.SiteMiddleware',
'wagtail.contrib.redirects.middleware.RedirectMiddleware']
Traceback:
File "/home/ubuntu/projects/blogger/env/lib/python3.5/site-packages/django/core/handlers/exception.py" in inner
34. response = get_response(request)
File "/home/ubuntu/projects/blogger/env/lib/python3.5/site-packages/django/core/handlers/base.py" in _get_response
126. response = self.process_exception_by_middleware(e, request)
File "/home/ubuntu/projects/blogger/env/lib/python3.5/site-packages/django/core/handlers/base.py" in _get_response
124. response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/home/ubuntu/projects/blogger/env/lib/python3.5/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
44. response = view_func(request, *args, **kwargs)
File "/home/ubuntu/projects/blogger/env/lib/python3.5/site-packages/wagtail/admin/urls/__init__.py" in wrapper
102. return view_func(request, *args, **kwargs)
File "/home/ubuntu/projects/blogger/env/lib/python3.5/site-packages/wagtail/admin/decorators.py" in decorated_view
34. return view_func(request, *args, **kwargs)
File "/home/ubuntu/projects/blogger/env/lib/python3.5/site-packages/wagtail/admin/views/pages.py" in create
224. parent_page.add_child(instance=page)
File "/home/ubuntu/projects/blogger/env/lib/python3.5/site-packages/treebeard/mp_tree.py" in add_child
1013. return MP_AddChildHandler(self, **kwargs).process()
File "/home/ubuntu/projects/blogger/env/lib/python3.5/site-packages/treebeard/mp_tree.py" in process
387. newobj.path = self.node.get_last_child()._inc_path()
File "/home/ubuntu/projects/blogger/env/lib/python3.5/site-packages/treebeard/mp_tree.py" in _inc_path
1114. raise PathOverflow(_("Path Overflow from: '%s'" % (self.path, )))
Exception Type: PathOverflow at /admin/pages/add/blog/blogpage/8/
Exception Value: Path Overflow from: '000100010005ZZZZ'
I'm absolutely clueless on this.

Answer 1

To store a page's position in the page tree, Wagtail uses a technique called Materialized Path, which (in its standard settings) allows a maximum of 1679615 insertions at a particular point in the tree. It seems that you've hit this limit somehow. (The position at each point in the tree is represented as a 4-character alphanumeric code; the 'ZZZZ' as seen in the error message is the highest available code.)
You would run into this limit if you had >1.6 million blog posts, but obviously that's quite unlikely. A more likely possibility is that you've written an import or maintenance script that's performing a very large number of insertions or page movements - for example, if you had a script to randomly reorder pages, and you were running it on a section with 10000 pages, you would run out of index numbers after running it ~160 times.
If you really can't avoid hitting this limit, then you can increase the limit to 60 million by adding the following code somewhere where it will run on startup:
from wagtail.core.models import Page
Page.steplen = 5
However, this won't work on an existing database - you'll need to delete and recreate your database after making this change - and it means the maximum possible depth of the tree is reduced from 63 levels to 50.