Hello,

I use a small hack on each model definition to track changes when writing them.

first, define which field you want to watch:

• first parameter: field name you want to watch
• second one: function you want to launch if it change

Here is an example for project.project model:

_watch_attrs = {'stage_id': 'on_change_stage_id', 'project_id': 'on_change_project_id'}

then add this new write function to handle this

def write(self, cr, uid, ids, vals, context=None):

    ## On Change attributes checks
    objs = self.browse(cr, uid, ids)
    for attr, on_change_label in self._watch_attrs.iteritems():
        if attr in vals:
            if any(obj[attr] != vals[attr] for obj in objs):
                vals = getattr(self, on_change_label)(cr, uid, ids, vals, context=context)

    return super(my_inheritedclass, self).write(cr, uid, ids, vals, context)

you now just have to défine what you want in the function that is executed when watching changes:

def on_change_project_id(self, cr, uid, ids, vals, context):
    task_obj = self.pool.get('project.task')
    tasks = self.browse(cr,uid,ids,context=context)[0].tasks_id
    if vals.get('project_id',False):
        new_project = vals.get('project_id',False)
        for task in tasks:
            #DO SOMETHING like chatter post message or action ..

    return vals

So if ou drag an drop event on calendar, you car watch date_start or date_end and execute one specific method. It is very usefull to track stage_id changes when clicking on status widget.