Hi,

See this onchange for the product_id field in the function in Manufacturing Order model.

@api.onchange('product_id', 'picking_type_id', 'company_id')
def onchange_product_id(self):
    """ Finds UoM of changed product. """
    if not self.product_id:
        self.bom_id = False
    else:
        bom = self.env['mrp.bom']._bom_find(product=self.product_id, picking_type=self.picking_type_id, company_id=self.company_id.id)
        if bom.type == 'normal':
            self.bom_id = bom.id
            self.product_qty = self.bom_id.product_qty
            self.product_uom_id = self.bom_id.product_uom_id.id
        else:
            self.bom_id = False
            self.product_uom_id = self.product_id.uom_id.id
        return {'domain': {'product_uom_id': [('category_id', '=', self.product_id.uom_id.category_id.id)]}}

You can change little changes here and use the same in your case.

Thanks