Here is The problems

class Purchase(models.Model):

    _name = 'purchase'

    name = fields.Char(string="name")

    orderDate = fields.Date(string="Order Date")

    salesPerson = fields.Char(string="Sales Person", required=True)

    expiredDate = fields.Date(string="Expired Date", required=True)

    company = fields.Char(string="Company", related="customer_id.company", readonly=True)

    tax = fields.Float(string ="Tax")

    unTax = fields.Float(string="Untaxed", compute="_total_price")

    amount_total =  fields.Float(compute='_compute_amount_total', string="Total")

    orderNumber = fields.Char(string="Order Number")

    subpurchase_ids = fields.One2many('subpurchase', 'purchase_id', string="Purchase Lines")

    customer_id = fields.Many2one('customer', string='partner', required=True)

    state = fields.Selection([

            ('new', 'New'), 

            ('draft', 'Draft'),

            ('validate', 'Validated'),

            ('cancel', 'Cancelled')], 

            string='Status', required=False, readonly=True, copy=False, default='new')

    @api.onchange('tax', 'unTax')       

    @api.multi

    def _compute_amount_total(self):

        self.amount_total = self.unTax + self.tax

    @api.depends('subpurchase_ids', 'subpurchase_ids.amount') #tambah kalo butuh relasi tambahan

    @api.multi

    def _total_price(self):

        for purchase in self :

            total_price = 0

            for subpurchase in purchase.subpurchase_ids:

                total_price += subpurchase.amount

            purchase.unTax = total_price