Make web ID on sale unique per web shop

Closes #13891

modules/web_shop/message.xml

@@ -9,8 +9,8 @@ this repository contains the full copyright notices and license terms. -->
         <record model="ir.message" id="msg_sale_invalid_party">
             <field name="text">To process the sale "%(sale)s" you must select another party instead of the guest "%(party)s".</field>
         </record>
-        <record model="ir.message" id="msg_sale_web_id_unique">
-            <field name="text">The web ID on the sale must be unique.</field>
+        <record model="ir.message" id="msg_sale_web_shop_id_unique">
+            <field name="text">The web ID on the sale must be unique per shop.</field>
         </record>
     </data>
 </tryton>

modules/web_shop/sale.py

@@ -28,10 +28,21 @@ class Sale(metaclass=PoolMeta):
         super().__setup__()
         t = cls.__table__()
         cls._sql_constraints += [
-            ('web_id', Exclude(t, (NullIf(t.web_id, ''), Equal)),
-                'web_shop.msg_sale_web_id_unique')
+            ('web_shop_id_unique',
+                Exclude(t,
+                    (t.web_shop, Equal),
+                    (NullIf(t.web_id, ''), Equal)),
+                'web_shop.msg_sale_web_shop_id_unique')
             ]
 
+    @classmethod
+    def __register__(cls, module):
+        table_h = cls.__table_handler__(module)
+        super().__register__(module)
+
+        # Migration from 7.4: replace web_id constraint by web_shop_id_unique
+        table_h.drop_constraint('web_id')
+
     @fields.depends('web_shop', 'web_id')
     def on_change_web_shop(self, nbytes=None):
         if self.web_shop and not self.web_id: