Change UniqueConstraint for wider database support (#6607)

This fixes #6393 by modifying the constraint to use an IN condition
which supports both Postgres and SQL Server. Previously, the `|` (OR)
condition was only supported by Postgres because SQL Server only
supports AND conditions.

The implementation follows suggestions from @gasman in
https://github.com/wagtail/wagtail/issues/6393#issuecomment-732161057:

* Migration 0050 is modified to not break on SQL Server
* Added migration 0060 to add or replace the constraint

Additionally, this allows for and documents a `DATABASE_DRIVER` env
variable to be set for testing, to allow a different SQL Server driver
(e.g. FreeTDS on Mac/Linux); and adds the specific `host_is_server`
option for FreeTDS (won't affect SQL Server Native Client on CI).

CHANGELOG.txt

@@ -15,6 +15,7 @@ Changelog
  * Fix: Stop menu icon overlapping the breadcrumb on small viewport widths in page editor (Karran Besen)
  * Fix: Make sure document chooser pagination preserves the selected collection when moving between pages (Alex Sa)
  * Fix: Gracefully handle oEmbed endpoints returning non-JSON responses (Matt Westcott)
+ * Fix: Fix unique constraint on WorkflowState for SQL Server compatibility (David Beitey)
 
 
 2.11.3 (10.12.2020)

docs/contributing/developing.rst

@@ -147,6 +147,8 @@ If you need to use different connection settings, use the following environment
 
 * ``DATABASE_PORT``
 
+It is also possible to set ``DATABASE_DRIVER``, which corresponds to the `driver` value within `OPTIONS` if an SQL Server engine is used.
+
 Testing Elasticsearch
 ---------------------
 

docs/releases/2.12.rst

@@ -43,6 +43,7 @@ Bug fixes
  * Stop menu icon overlapping the breadcrumb on small viewport widths in page editor (Karran Besen)
  * Make sure document chooser pagination preserves the selected collection when moving between pages (Alex Sa)
  * Gracefully handle oEmbed endpoints returning non-JSON responses (Matt Westcott)
+ * Fix unique constraint on WorkflowState for SQL Server compatibility (David Beitey)
 
 
 Upgrade considerations

wagtail/core/migrations/0050_workflow_rejected_to_needs_changes.py

@@ -21,6 +21,6 @@ class Migration(migrations.Migration):
         ),
         migrations.AddConstraint(
             model_name='workflowstate',
-            constraint=models.UniqueConstraint(condition=models.Q(('status', 'in_progress'), ('status', 'needs_changes'), _connector='OR'), fields=('page',), name='unique_in_progress_workflow'),
+            constraint=models.UniqueConstraint(condition=models.Q(status__in=('in_progress', 'needs_changes')), fields=('page',), name='unique_in_progress_workflow'),
         ),
     ]

wagtail/core/migrations/0060_fix_workflow_unique_constraint.py

@@ -0,0 +1,21 @@
+# Generated by Django 3.1.3 on 2020-11-27 01:19
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('wagtailcore', '0059_apply_collection_ordering'),
+    ]
+
+    operations = [
+        migrations.RemoveConstraint(
+            model_name='workflowstate',
+            name='unique_in_progress_workflow',
+        ),
+        migrations.AddConstraint(
+            model_name='workflowstate',
+            constraint=models.UniqueConstraint(condition=models.Q(status__in=('in_progress', 'needs_changes')), fields=('page',), name='unique_in_progress_workflow'),
+        ),
+    ]

wagtail/core/models.py

@@ -4316,9 +4316,9 @@ class WorkflowState(models.Model):
     class Meta:
         verbose_name = _('Workflow state')
         verbose_name_plural = _('Workflow states')
-        # prevent multiple STATUS_IN_PROGRESS/STATUS_NEEDS_CHANGES workflows for the same page. This is not supported by MySQL, so is checked additionally on save.
+        # prevent multiple STATUS_IN_PROGRESS/STATUS_NEEDS_CHANGES workflows for the same page. This is only supported by specific databases (e.g. Postgres, SQL Server), so is checked additionally on save.
         constraints = [
-            models.UniqueConstraint(fields=['page'], condition=(Q(status='in_progress') | Q(status='needs_changes')), name='unique_in_progress_workflow')
+            models.UniqueConstraint(fields=['page'], condition=Q(status__in=('in_progress', 'needs_changes')), name='unique_in_progress_workflow')
         ]
 
 

wagtail/tests/settings.py

@@ -33,8 +33,9 @@ if DATABASES['default']['ENGINE'] != 'django.db.backends.sqlite3':
 # Add extra options when mssql is used (on for example appveyor)
 if DATABASES['default']['ENGINE'] == 'sql_server.pyodbc':
     DATABASES['default']['OPTIONS'] = {
-        'driver': 'SQL Server Native Client 11.0',
+        'driver': os.environ.get('DATABASE_DRIVER', 'SQL Server Native Client 11.0'),
         'MARS_Connection': 'True',
+        'host_is_server': True,  # Applies to FreeTDS driver only
     }