I need sum the field "Hours" of all lines on a treeview and store the result in other field ("total_hours") on main table. The main Table have a one2many field linked to second table (treeview).

How can i do this?

Main Table:

class main_table(osv.osv):
    _name = 'sat.main'
    _columns = {
        'name' : fields.char('name', size=256, required=True),
        'total_hours' : fields.float('Producto'),
        'hours_id': fields.one2many('sat.hours', 'hour_id', 'Horas'),
        }

Second Table:

class hours(osv.osv):
    _name = 'sat.hours'
    _columns = {
                'hour_id': fields.many2one('sat.main', 'mainwork'),
                'concept': fields.char('Concept', size=128, required=True),
                'hours': fields.float('hours'),
                }