Please used this example.

class PurchaseOrdrLine(models.Model):
_inherit = 'purchase.order.line'

@api.onchange('product_id')
def m2m_product_onchange(self):
Your Code........

pol_m2m_ids = fields.Many2many('purchase.order.line', string='M2M Ids')

<field name="pol_m2m_ids" widget="one2many">
<tree editable="bottom">
<field name="product_id"/>
<field name="name"/>
<field name="product_uom_category_id"/>
</tree>
</field>