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20130712 (Friday, 12 July 2013)
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Even more app inheritance
-------------------------

I understood the necessity of even more inter-app stuff.
Removed `dd.extends_app` and replaced it by
a new module :mod:`lino.ad`.

Usage:

For example if you write an app `myproject.courses`
which extends (inherits from) 
:mod:`lino.modlib.courses`, then you write in your 
app's :file:`__init__.py`::

    from lino import ad
        
    class App(ad.App):

        extends = 'lino.modlib.trading'

And in your app's  :file:`models.py`::

    from lino.modlib.trading.models import *
    

This works because we add a new convention: 

    Lino requires the modules specified in :setting:`INSTALLED_APPS`
    (not their `.models` modules) to be importable 
    while Django's settings are being loaded. Or in other words: 
    these modules may not do `from django.conf import settings`.

Inheriting fixtures
-------------------

Lino cannot give you a method to automatically 
inherit `fixture` directories. 
That's because 
we would have to add them to :setting:`FIXTURE_DIRS`, and this would 
cause an unexpected loading order.
Django's will always load first fixtures of INSTALLED_APPS and only 
then those in :setting:`FIXTURE_DIRS`.

So if you want the fixtures of your parent app to load also 
for your child app, then you need to do this explicitly for each 
fixture by creating a `fixtures` dir in the child app, 
with an empty 
:file:`__init__.py` and one `.py` file for each fixture.
Usually this will be at least a file :file:`demo.py`::

  from <parent_app>.fixtures.demo import *
  

TODO: More plans
----------------

The :mod:`lino.ad` idea opens doors for new possibilities:

- :attr:`lino.ad.App.verbose_name`
- :attr:`lino.ad.App.depends`


Lino-Faggio
-----------

The test suite didn't detect a simple failure to load the demo fixture
although there was a corresponding 
test case with fixtures in `lino_faggio/tests/faggio_tests.py`.
The pitfall was that such a test case  must have at least one method 
whose name starts with "test" in order to run.