[Kde-pim] RFC: ContactGroup2: UID based references

[Christian Mollekopf]

Hey,

In the light of:
http://git.reviewboard.kde.org/r/110591/
http://git.reviewboard.kde.org/r/110592/

I'd like some feedback if you think this is going in the right direction.
Porting everything to ContactGroup2 will be quite a task, involving a 
ContactGroup2 version of everything exported in kdepimlibs and porting all 
applications using it (in particular the akonadi resources and KAddressBook), 
so I'd rather make sure this is leading somewhere ;-)

== Reasoning ==
I think ContactGroup is flawed because it uses the akonadi item id as 
reference, which results in issues when synchronizing. In particular:
* The akonadi item id is an implementation detail, and is not valid outside of 
akonadi. Hence it is basically invalid even if you store the group to a vcard.
* VCard 4 support groups (wasn't supported before), allowing us to store 
groups in a standard format (currently we're using a custom xml format)
* While resolving the akonadi item id to UID when serializing is generally 
possible, it's IMO the wrong place for such a thing to happen, and it's not 
necessarily possible when deserializing. If the contact with the referenced 
uid has not yet been synchronized (and hence the akonadi item id is not 
available), it is not possible to create the contact group until all 
referenced contacts are synchronized, which is an unnecessary restriction.

IMO the only sustainable path forward is having ContactGroup references using 
UID's, which is also what vCard 4.0 supports.

Note that an alternative approach to ContactGroup2 would be to extend 
KABC::Addressee with with a type (Individual, Group) and a list of members.
That way KABC:Addressee would just represent a vcard, which would result in 
less code, but give away the typesafe distinction between groups and contacts.
I therefore prefer the ContactGroup2 solution.

== Migration ==
I plan to either add migration code, which would migrate all ContactGroups to 
ContactGroup2 during akonadi startup (similar to the creation of default 
collections), or to rely on gradual migration using a compatibility serializer 
which converts ContactGroup objects on the fly to ContactGroup2.


I'd appreciate an ACK/NACK before going forward.

Cheers,
Christian
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