Add unique constraint on line taxes

issue10841
review366081002

message.xml

@@ -36,5 +36,8 @@ this repository contains the full copyright notices and license terms. -->
         <record model="ir.message" id="msg_purchase_move_reset_draft">
             <field name="text">You cannot reset move "%(move)s" to draft because it was generated by a purchase.</field>
         </record>
+        <record model="ir.message" id="msg_purchase_line_tax_unique">
+            <field name="text">A tax can be added only once to a purchase line.</field>
+        </record>
     </data>
 </tryton>

purchase.py

@@ -11,7 +11,7 @@ from sql.operators import Concat
 
 from trytond.i18n import gettext
 from trytond.model import Workflow, Model, ModelView, ModelSQL, fields, \
-    sequence_ordered
+    sequence_ordered, Unique
 from trytond.model.exceptions import AccessError
 from trytond.modules.company import CompanyReport
 from trytond.wizard import Wizard, StateAction, StateView, StateTransition, \
@@ -1781,6 +1781,15 @@ class LineTax(ModelSQL):
     tax = fields.Many2One('account.tax', 'Tax', ondelete='RESTRICT',
             select=True, required=True, domain=[('parent', '=', None)])
 
+    @classmethod
+    def __setup__(cls):
+        super().__setup__()
+        t = cls.__table__()
+        cls._sql_constraints += [
+            ('line_tax_unique', Unique(t, t.line, t.tax),
+                'purchase.msg_purchase_line_tax_unique'),
+            ]
+
 
 class LineIgnoredMove(ModelSQL):
     'Purchase Line - Ignored Move'