Description

If a plugin model inherits from another non-abstract plugin model (which is not CMSPlugin itself), its plugin instances cannot be copied on CMS pages. Instead, an error is raised.

Steps to reproduce

1. Define a plugin model structure like this

from cms.models import CMSPlugin

class Direct(CMSPlugin):  # non-abstract
    ...

class Subclass(Direct):  # non-abstract
    ...

2. Define a CMS Plugin that uses the Subclass model
3. Use an instance of said plugin on a page
4. Try to copy the instance in the structure panel
5. See error

Expected behaviour

The plugin should be copied to the clipboard and should be able to be pasted somewhere else in the structure panel.

Actual behaviour

An exception is raised while trying to copy, which originates here:

AttributeError: 'CMSPlugin' object has no attribute 'cmsplugin_ptr_id'

Additional information (CMS/Python/Django versions)

Python 3.8
Django==3.1.3
django-cms==3.8.0

Regarding the line that raises the error:

            plugin_instance.cmsplugin_ptr = new_plugin

It appears that, even though the attribute is called cmsplugin_ptr, Django doesn't necessarily expect a CMSPlugin instance here (which new_plugin always is). Instead, Django seems to expect an instance of the closest ancestor (Direct in my example above), which would then also have cmsplugin_ptr_id attribute itself (hence the AttributeError above). This is also why the code works for models that inherit directly from CMSPlugin (or an abstract model), since CMSPlugin is the closest ancestor in this case.

A simple fix may be to set the cmsplugin_ptr_id instead - when dealing with the raw ID, the type of new_plugin doesn't matter. With the line above changed to

            plugin_instance.cmsplugin_ptr_id = new_plugin.pk

copying plugins seems to work in all inheritance scenarios in a short test. I may be able to provide a PR utilizing that fix if the issue is accepted.