I need to clear "many2many" fields value, So i tried these code but it does not empty the field

My Code:

            purchase_id = fields.Many2many(

                        'purchase.order',

                        string='Purchase Order',

                        store=False,

                        readonly=True,

                        help='Associated Purchase Order. Filled in automatically when a PO is chosen on the vendor bill.'

             )

            @api.onchange('purchase_id')

            def _onchage_purchase(self):

            self.purchase_id = [6, 0, []] 

I tried like this, but it doesn't works.. Please, can anyone help me to resolve this.