20110831
========

a) Test suite was failing
-------------------------

Fixed.

b) Switching off a `mti.EnableChild` deletes related objects
------------------------------------------------------------

Currently, if you switch off the is_courseprovider field of a Company, 
and if that Company (as a CourseProvider) has Courses defined, 
these Courses will be deleted without warning. Not so good.
(Fixed :lino:`0906` )

c) Adapted dpy Deserializer to new Django version
-------------------------------------------------

One user reports the following error message during :term:`initdb`::

  django.db.utils.IntegrityError: (1217, 'Cannot delete or update a parent row: a foreign key constraint fails') 

No explanation yet. Ticket :srcref:`docs/tickets/50`

But after updating my Django copy from 1.4 pre-alpha SVN-16376 to 16714, 
I realized that the new Django version 
doesn't any longer allow a trick which I used in 
:class:`lino.utils.dpy.Deserializer`. 
So I had to redesign this.
The new design should also work with older Django versions.


d) dpy dumps failed to load
---------------------------

Aha, there are now two `app_labels` who have an EventType model: 
``cal`` and ``notes``.
That leads to a name conflict in dpy dumps.
Fixed.

New abstract model `lino_xl.lib.cal.CalendarRelated` is only because 
I was hesitating which components should have a `calendar` field.