[pygtk] gtk.GenericTreeModel, tricky one...

[John Stowers]

On Mon, 2008-06-02 at 13:04 +0200, Markus W. Barth wrote:
> I am working on a package that provides some models for use with SQLObject 
> (see http://www.sqlobject.org for further details), thus creating database 
> frontends is made a lot easier.

Cool. I did something similar with a wrapper around sqlite. See:
http://www.johnstowers.co.nz/blog/index.php/2007/12/06/the-big-move/

I cant offer too much advice on your specific problem, but you may find
something useful in my code. TBH The ordeal was quite painful, and I
have forgotten much of what I learnt while implementing it.

John

> 
> Internally, the model uses a python list of SQLObject instances.
> 
> Now I have bumped into the following:
> 
> As long as I use _one_ instance of the model (SqlListView), there are no 
> problems, but as soon as I use two instances of the model, strange behaviour 
> starts to happen, apart from getting the following assertion error.
> 
> (main.py:16261): CRITICAL **: pygtk_generic_tree_model_get_value: assertion 
> `VALID_ITER(iter, tree_model)' failed
> (yes, self.row_deleted() is called for every row that is deleted)
> 
> But things don't stop here. The module contains two classes of models:
> 
> * SqlListModel
> * SlaveListModel
> 
> Both are very similar, the only difference is the way how they retrieve their 
> data. For this reason I created a base class BaseListModel and derived both
> SqlListModel and SlaveListModel.
> 
> The base class has an attribute "select_list", which holds the list with the 
> SQLObject instances.
> 
> Now if I use one instance of SqlListModel (for showing the data from one 
> table) and one instance of SlaveListModel (for showing data from a 1:n 
> relation), in the SlaveListModel appears data from the SqlListModel.
> 
> Just to give it a try, I moved teh attribute "select_list" to the subclasses 
> (SqlListModel and SlaveListModel), and the problem disappeared - as long as I 
> only use one instance of each. 
> 
> I suspect, that the problem arises when any instance clears rows from the list 
> to populate it with new rows. So I simply added 
> 
> self.select_list = []
> 
> at the end of the method that evacuates the list and things seem to work 
> (until now).
> 
> The whole method is:
> 
>     def evacuate(self):
>         num_items = len(self.select_list)
>         for i in range(num_items):
>             last_path = num_items-(i+1)
>             my_iter = self.get_iter(last_path)
>             row = self.get_row(my_iter)
>             self.select_list.remove(row)
>             self.row_deleted(last_path)
>         self.select_list = [] #<=UGLY
> 
> However I think that the problem seems to point to a (heavy) leakage of iters, 
> and I really do not understand why this happens. Anyone any idea?
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