Django's standard behaviour is to preserve managers that are set on abstract
superclasses, so this allows us to eliminate the metaclass hackery.
Fixes #2933
Indexed.search_fields used to be a tuple. This is incorrect, and it
should have been a list. Changing it to be a list now would be a
backwards incompatible change, as people do
search_fields = Page.search_fields + (
SearchField('body')
)
Adding a tuple to the end of a list causes an error, so this would
cause all old code that used tuples to throw an error. This is not
great.
A new ThisShouldBeAList class, which subclasses list, has been added.
It additionally allows tuples to be added to it, as in the above
behaviour, but will raise a deprecation warning if someone does this.
Old code that uses tuples will continue to work, but raise a deprecation
warning.
See #2310
Previously, if a developer wanted to use a custom Manager on their Page
subclass, some fairly hacky hacks were required. Now, the `objects`
attribute is only overridden if it is a plain `Manager`. If it is
anything else, it is left alone. A system check has been added to ensure
that all `Page` managers inherit from `PageManager`
