Add failing test for saving with a new sort_order

.gitignore

@@ -2,3 +2,4 @@
 *.pot
 *.pyc
 local_settings.py
+.hypothesis/

orderable/models.py

@@ -67,6 +67,20 @@ def prev(self):
             sort_order__lt=self.sort_order
         ).order_by('-sort_order').first()
 
+    @staticmethod
+    def _update(qs):
+        """
+        Increment the sort_order in a queryset.
+
+        Handle IntegrityErrors caused by unique constraints.
+        """
+        try:
+            with transaction.atomic():
+                qs.update(sort_order=models.F('sort_order') + 1)
+        except IntegrityError:
+            for obj in qs.order_by('-sort_order'):
+                qs.filter(pk=obj.pk).update(sort_order=models.F('sort_order') + 1)
+
     def _save(self, objects, old_pos, new_pos):
         """WARNING: Intensive giggery-pokery zone."""
         to_shift = objects.exclude(pk=self.pk) if self.pk else objects
@@ -80,7 +94,7 @@ def _save(self, objects, old_pos, new_pos):
             # Increment `sort_order` on objects with:
             #     sort_order > new_pos.
             to_shift = to_shift.filter(sort_order__gte=self.sort_order)
-            to_shift.update(sort_order=models.F('sort_order') + 1)
+            self._update(to_shift)
             self.sort_order = new_pos
 
         # self.sort_order decreased.
@@ -90,7 +104,7 @@ def _save(self, objects, old_pos, new_pos):
             # Increment `sort_order` on objects with:
             #     sort_order >= new_pos and sort_order < old_pos
             to_shift = to_shift.filter(sort_order__gte=new_pos, sort_order__lt=old_pos)
-            to_shift.update(sort_order=models.F('sort_order') + 1)
+            self._update(to_shift)
             self.sort_order = new_pos
 
         # self.sort_order increased.

orderable/tests/models.py

@@ -6,10 +6,16 @@
 class Task(Orderable):
     """A basic orderable model for tests."""
 
+    def __str__(self):
+        return 'Task {}'.format(self.pk)
+
 
 class SubTask(Orderable):
     """An orderable model with unique_together."""
     task = models.ForeignKey('Task')
 
     class Meta:
         unique_together = ('task', 'sort_order')
+
+    def __str__(self):
+        return 'SubTask {}'.format(self.pk)

orderable/tests/test_models.py

@@ -1,4 +1,6 @@
-from django.test import TestCase
+from hypothesis import example, given
+from hypothesis.extra.django import TestCase
+from hypothesis.strategies import integers, lists
 
 from .models import SubTask, Task
 
@@ -50,12 +52,14 @@ def test_insert_on_create(self):
         old_3 = Task.objects.create(sort_order=3)
 
         # Insert between old_1 and old_2
-        with self.assertNumQueries(4):
+        with self.assertNumQueries(6):
             # Queries:
             #     Savepoint
+            #     Savepoint
             #     Bump old_2 to position 3
+            #     Release savepoint
+            #     Release savepoint
             #     Save new in position 2
-            #     Commit
             new = Task.objects.create(sort_order=old_2.sort_order)
 
         tasks = Task.objects.all()
@@ -114,14 +118,16 @@ def test_decrease_order(self):
         item5 = Task.objects.create(sort_order=5)
 
         # Move item4 to position 2
-        with self.assertNumQueries(6):
+        with self.assertNumQueries(8):
             # Queries:
             #     Savepoint
             #     Find end of list
             #     Move item4 to end of list
+            #     Savepoint
             #     Bump item2 and item3 on by one
+            #     Release savepoint
+            #     Release savepoint
             #     Save item4 to new desired position
-            #     Commit
             item4.sort_order = item2.sort_order
             item4.save()
 
@@ -207,3 +213,16 @@ def test_changing_parent(self):
         subtask_2.task = task
         subtask_2.save()
         self.assertSequenceEqual(task.subtask_set.all(), [subtask, subtask_2])
+
+    @given(lists(integers(min_value=1), min_size=1, unique=True))
+    @example([2, 3, 1])
+    @example([2, 3, 4])
+    def test_save_subtask_no_errors(self, sort_orders):
+        """Ensure Orderable.save does not raise IntegrityError."""
+        task = Task.objects.create()
+
+        for order in sort_orders:
+            subtask = SubTask.objects.create(task=task, sort_order=order)
+
+        subtask.sort_order = 2
+        subtask.save()

requirements.txt

@@ -3,4 +3,5 @@ django-discover-runner==1.0
 flake8==2.2.5
 flake8-docstrings==0.2.1
 flake8-import-order==0.5.3
-psycopg2==2.5.1
+hypothesis==2.0.0
+psycopg2==2.6.1

setup.cfg

@@ -2,7 +2,7 @@
 universal = 1
 [flake8]
 application-import-names = orderable
-ignore = D100,D101,D102,D203,D204
+ignore = D100,D101,D102,D104,D105,D203,D204
 import-order-style = google
 max-complexity = 10
 max-line-length = 90