Fix integrity error on SQL Server

Currently we use the unique_together constraint to ensure that if a label has a
shortcut key combination, no other label has the same combination.

However, on SQL Server, the unique_together constraint also looks at null
values which breaks the unique_together constraint since we now can no longer
have two labels without shortcut keys. The error message is as follows:

```
Violation of UNIQUE KEY constraint 'api_label_project_id_prefix_key_suffix_key_1b3d8f77_uniq'.
Cannot insert duplicate key in object 'dbo.api_label'.
The duplicate key value is (32, <NULL>, <NULL>).
```

Moving the validation logic from the database into Django fixes the issue and
makes the constraint clearer.

app/api/migrations/0003_support_sql_server.py

@@ -21,4 +21,8 @@ class Migration(migrations.Migration):
             name='seq2seqannotation',
             unique_together={('document', 'user', 'text')},
         ),
+        migrations.AlterUniqueTogether(
+            name='label',
+            unique_together={('project', 'text')},
+        ),
     ]

app/api/models.py

@@ -163,19 +163,18 @@ def clean(self):
         # Don't allow shortcut key not to have a suffix key.
         if self.prefix_key and not self.suffix_key:
             raise ValidationError('Shortcut key may not have a suffix key.')
-        super().clean()
 
-    def validate_unique(self, exclude=None):
-        # Don't allow to save same shortcut key when prefix_key is null.
-        if Label.objects.exclude(id=self.id).filter(suffix_key=self.suffix_key,
-                                                    prefix_key__isnull=True).exists():
-            raise ValidationError('Duplicate key.')
-        super().validate_unique(exclude)
+        # each shortcut (prefix key + suffix key) can only be assigned to one label
+        if self.suffix_key or self.prefix_key:
+            other_labels = self.project.labels.exclude(id=self.id)
+            if other_labels.filter(suffix_key=self.suffix_key, prefix_key=self.prefix_key).exists():
+                raise ValidationError('A label with this shortcut already exists in the project')
+
+        super().clean()
 
     class Meta:
         unique_together = (
             ('project', 'text'),
-            ('project', 'prefix_key', 'suffix_key')
         )
 
 

app/api/tests/test_models.py

@@ -91,11 +91,11 @@ def test_text_uniqueness(self):
 
     def test_keys_uniqueness(self):
         label = mommy.make('Label', prefix_key='ctrl', suffix_key='a')
-        with self.assertRaises(IntegrityError):
+        with self.assertRaises(ValidationError):
             Label(project=label.project,
                   text='example',
                   prefix_key=label.prefix_key,
-                  suffix_key=label.suffix_key).save()
+                  suffix_key=label.suffix_key).full_clean()
 
     def test_suffix_key_uniqueness(self):
         label = mommy.make('Label', prefix_key=None, suffix_key='a')