[FIX] account_global_discount: Fix singleton error in _compute_amount

For reducing diff, we rename the method and call it record per record.

Fixes OCA#788

account_global_discount/__manifest__.py

@@ -3,7 +3,7 @@
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
 {
     'name': 'Account Global Discount',
-    'version': '12.0.1.0.1',
+    'version': '12.0.1.0.2',
     'category': 'Accounting',
     'author': 'Tecnativa,'
               'Odoo Community Association (OCA)',

account_global_discount/models/account_invoice.py

@@ -120,12 +120,7 @@ def _onchange_global_discount_ids(self):
         """Trigger global discount lines to recompute all"""
         return self._onchange_invoice_line_ids()
 
-    @api.depends('invoice_line_ids.price_subtotal', 'tax_line_ids.amount',
-                 'tax_line_ids.amount_rounding', 'currency_id', 'company_id',
-                 'date_invoice', 'type',
-                 'invoice_global_discount_ids', 'global_discount_ids')
-    def _compute_amount(self):
-        super()._compute_amount()
+    def _compute_amount_one(self):
         if not self.invoice_global_discount_ids:
             return
         round_curr = self.currency_id.round
@@ -152,6 +147,15 @@ def _compute_amount(self):
         self.amount_total_signed = self.amount_total * sign
         self.amount_untaxed_signed = amount_untaxed_signed * sign
 
+    @api.depends('invoice_line_ids.price_subtotal', 'tax_line_ids.amount',
+                 'tax_line_ids.amount_rounding', 'currency_id', 'company_id',
+                 'date_invoice', 'type',
+                 'invoice_global_discount_ids', 'global_discount_ids')
+    def _compute_amount(self):
+        super()._compute_amount()
+        for record in self:
+            record._compute_amount_one()
+
     def get_taxes_values(self):
         """Override this computation for adding global discount to taxes."""
         round_curr = self.currency_id.round