:date: 2016-06-23

=======================
Thursday, June 23, 2016
=======================

I started to work on :ticket:`988`. We want a new table parameter and
a new virtual field on every partners table.

This ticket is not only useful in :ref:`voga`, also in :ref:`cosi`,
:ref:`welfare` and :ref:`presto`. And for all future applications with
accounting functionality.

So it should be defined in :mod:`lino_cosi.lib.contacts.models`.


Discovered a design flaw
========================

Every application has its own version of ``contacts`` plugin:

- :mod:`lino_voga.lib.contacts.models`
- :mod:`lino_presto.lib.contacts.models`
- :mod:`lino_welfare.lib.contacts.models`

These extensions until now inherited from
:mod:`lino.modlib.contacts.models` and *not* from
:mod:`lino_cosi.lib.contacts.models`::

    from lino.modlib.contacts import Plugin   # __init__.py
    from lino.modlilib.contacts.models import *  # models.py
    
This was a design flaw which just didn't disturb until now because
:mod:`lino_cosi.lib.contacts.models` just defined some detail layouts
which anyway weren't being reused by the applications.
    
But actually they must inherit from :mod:`lino_cosi.lib.contacts`
    
    from lino_cosi.lib.contacts import Plugin
    from lino_cosi.lib.contacts.models import *

Redefining the detail_layout
============================

Note: why do we need::

    @dd.receiver(dd.post_analyze)
    def customize_contacts2(sender, **kw):
        site = sender
        site.modules.contacts.Persons.set_detail_layout(PersonDetail())
        site.modules.contacts.Companies.set_detail_layout(CompanyDetail())
        site.modules.contacts.Partners.set_detail_layout(PartnerDetail())
    
and cannot just do::

    Persons.detail_layout = PersonDetail()
    Companies.detail_layout = CompanyDetail()
    Partners.detail_layout = PartnerDetail()
    
One reason is: `set_detail_layout` has the advantage of doing some
validation. (TODO: explain more).
    
It seems that the following is a compromise which works::
    
    Persons.set_detail_layout(PersonDetail())
    Companies.set_detail_layout(CompanyDetail())
    Partners.set_detail_layout(PartnerDetail())
    
As I wrote for :ticket:`526`, "it seems that the general rule is:
if you want your own detail_layout, then you should write your own
plugin."

In other words, if you write your own plugins, then you don't need to
put your set_detail_layout calls into a post_analyze handler.


Pluggable accounting functionality
==================================

In :ref:`welfare` we have a challenge: we would like to have the
accounting functionality "pluggable". For example in chatelet it is
disabled, they don't want any ledger, journals, movements & Co. And we
want that choice independently of the choice "eupen" or "chatelet".

We solve this using the two classes
:class:`lino_xl.lib.vat.mixins.PartnerDetailMixin` and
:class:`lino_xl.lib.vatless.mixins.PartnerDetailMixin`.  They both
define a `ledger` tab panel, and they both are designed to "do
nothing" when `ledger` is not installed. Or more precisely they define
a DummyPanel named `ledger` in that case. Partner details who use
them, must make sure that their `main` contains `ledger`.


A pitfall
=========

It took me some time to realize that, when I write a
:class:`lino_cosi.lib.contacts.models.Partner` class, then I must also
write :class:`Person <lino_cosi.lib.contacts.models.Person>`
:class:`Company <lino_cosi.lib.contacts.models.Company>` classes.


Note that in :mod:`lino_xl.lib.households` we don't write::

    from lino_xl.lib.contacts.models import Partner

    class Household(Partner):
        ...
        
But::        

    contacts = dd.resolve_app('contacts')

    class Household(contacts.Partner):
        ...

The advantage is that we don't need to create a
:mod:`lino_cosi.lib.households` plugin which would avoid this trick
for households:


    from lino_xl.lib.households.models import *
    from lino_cosi.lib.contacts.models import Partner

    class Household(Partner, Household):
        ...

I started a new specs document :ref:`voga.specs.partners`.