Add ability to extend exclude_fields for use when copying page

Issue Summary

If your Page model contains relations that do not use ClusterableModel you cannot copy those pages in admin without causing an error. Adding the ability to define what fields must be excluded on copy per Page would enable working around this.

Steps to Reproduce

Note: This is my specific use case.

1. Start a new Wagtail project
2. Install Django Activity Stream http://django-activity-stream.readthedocs.io/en/latest/ pip install django-activity-stream
3. In your main apps.py register a generic link to your Page model registry.register(Page) - http://django-activity-stream.readthedocs.io/en/latest/configuration.html
4. Open a Page list in Admin and click More > Copy
5. Change the Slug to something Unique
6. Click ‘Copy this Page’

This results in the following Error: TypeError: ‘GenericRelatedObjectManager’ object is not iterable

The trace error is:
[09/Oct/2017 09:29:35] ERROR [django.request:135] Internal Server Error: /admin/pages/233/copy/
Traceback (most recent call last):
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 249, in _legacy_get_response
    response = self._get_response(request)
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/django/views/decorators/cache.py", line 43, in _cache_controlled
    response = viewfunc(request, *args, **kw)
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/wagtail/wagtailadmin/decorators.py", line 31, in decorated_view
    return view_func(request, *args, **kwargs)
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/wagtail/wagtailadmin/utils.py", line 140, in wrapped_view_func
    return view_func(request, *args, **kwargs)
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/wagtail/wagtailadmin/views/pages.py", line 837, in copy
    user=request.user,
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/wagtail/wagtailcore/models.py", line 1116, in copy
    page_copy = self.specific_class(**specific_dict)
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/wagtail/wagtailcore/models.py", line 376, in __init__
    super(Page, self).__init__(*args, **kwargs)
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/modelcluster/models.py", line 177, in __init__
    super(ClusterableModel, self).__init__(*args, **kwargs)
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/django/db/models/base.py", line 567, in __init__
    _setattr(self, prop, kwargs[prop])
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/django/db/models/fields/related_descriptors.py", line 537, in __set__
    manager.set(value)
  File "/home/ifeelgoogle/venv/local/lib/python2.7/site-packages/django/contrib/contenttypes/fields.py", line 613, in set
    objs = tuple(objs)
TypeError: 'GenericRelatedObjectManager' object is not iterable

Proposed Solution

https://github.com/wagtail/wagtail/blob/f15a98f983b1c6b11807d90226b7b3924b0a74d1/wagtail/wagtailcore/models.py#L1055

• The line above: exclude_fields = ['id', 'path', 'depth', 'numchild', 'url_path', 'path']
• Could be replaced with something like: self.excluded_fields_for_copy
• The default would be the list above but this could be extended for a specific class like: excluded_fields_for_copy = Page.excluded_fields_for_copy + ['action_object_actions', 'actor_actions', 'target_actions']
• As this is different to the exclude_fields used in modelAdmin it would need to be named specifically in relation to copying.

Technical details

• Python 2.7.12
• Django v.1.11.5
• Wagtail v.1.12.2
• Browser version: Chrome 61 on OS X 10.12.6

[lb-]

@gasman I will give a go at implementing this for a pull request next week.

[kaedroho]

Just hit this issue myself, the error message given doesn’t make it clear what’s going on. So I wonder if it’s worth excluding generic relations by default?