i am trying to have the selection list generated from a many2many field 

so i created a text field and stored the values of many2many table in the text field with the onchange api but i can't link the text field with the selection field 

 state = fields.Selection(selection="selection_list", string="text", default="0")
saving = fields.Text(compute="doing", store=True)
@api.depends('saving')
def selection_list(self):
    states = [("0", "hello")]
    if self.saving:
        for i, j in zip(range(1, len(self.saving.split()) + 1),                    self.saving.split()):
            states.append(tuple((str(i), j)))
    return states

@api.onchange('workers')
@api.depends('workers')
def doing(self):
    if self.workers:
        self.saving = ' '.join([i.name for i in self.workers])