Re: [gnome-db] Tree-structured data

[Diego Zuccato <diego otello alma unibo it>]

Vivien Malerba wrote:

>>I'd propose to consider aupport for a tree model, not only for list
>>model. I've already considered the problems, and it seems they can be
[...]
> The GdaDataModel has a list concept and not a tree concept because
> this is the concept behind relational databases (they don't do trees).
I know. But aren't we going to use a higher level when using widgets?
When "presenting" data to the user, readability is all. A tabular format
can be what's in the db, not what the user wants to see (at least not
always).

> For the performances problem, the last version of libgda includes two features:
> * a GdaDataProxy dete model which allows to define arbitrary size
> "chuncks" of data (by default 300 lines): you give it a GdaDataModel,
> and the GdaDataProxy will only contain 300 lines of that model (of
> course you can then change which 300 lines are displayed). This object
> is used in the form and grid widgets of libgnomedb.
But I have to actually FILL the GdaDataModel from the DB (that can be
quite slow... maybe over a modem connection).
At least there should be a widget that supports it in gnomedb... Or
should every programmer implement his own GdaDataModel->GtkTreeModel
converter?

> * There is a provision in the API to access a GdaDataModel using a
> cursor to avoid having to load all the data from the database (the
> cursor is a GdaDataModelIter object). The API still needs to be
> modified (a bit) to specify the DBMS provider if you want a random
> access data model, or a cursor based one when you run a SELECT query.
I'll have to investigate this. But I still think that a GdaDataModelTree
could be really useful.
It could internally use the iterator, but I don't think it's really
needed: run a certain query on expand() and add resulting rows as child
nodes, remove child nodes on collapse(). If there are too many rows
returned by the query, a proxy is useful.

> To test the grid widgets presents "chuncks" of data, please run the
> testing/test-dyn-widgets program as:
> "./test-dyn-widgets -t 9" and select the "products table" which
> contains some 3000 items.
I'll test it ASAP.

> Except for the performances and "readability" of the data, I don't see
> how a tree based data model could be used in a general way.
What else is a library about? :)

BYtE,
 Diego.