Hello everyone, I have customized a module that include a many2one field. Now, I would like to set the default value for that field with the codition is: [[u'bom_ids', u'!=', False]]. i have tried below code but it did not work probebly

width_id = field.Many2one('sale.order.line.width', default ='_get_width_default')
def _get_width_default(self, cr, uid, context=None):
     res = self.pool.get('product.template').search(cr, uid, [(u'bom_ids', u'!=', False)], context=context)
     return res and res[0] or False
default = {
'width_id' : _get_width_default,
}
Could you guy please help me to point what is the problem and how to sovle the problem and finnally get my purpose. Thank for watching