Make unique_together work more seamlessly.

Fixes #7

Add Orderable.validate_unique to exclude `sort_order` when it is
unique_together with something..

CHANGELOG.rst

@@ -1,3 +1,8 @@
+UPCOIMING
+======
+
+* Add Orderable.validate_unique to exclude `sort_order` when it is unique_together with something.
+
 v6.0.1
 ======
 

orderable/models.py

@@ -52,6 +52,23 @@ def prev(self):
 
         return self.get_filtered_manager().before(self)
 
+    def validate_unique(self, exclude=None):
+        if self._is_sort_order_unique_together_with_something():
+            exclude = exclude or []
+            if 'sort_order' not in exclude:
+                exclude.append('sort_order')
+        return super().validate_unique(exclude=exclude)
+
+    def _is_sort_order_unique_together_with_something(self):
+        """
+        Is the sort_order field unique_together with something
+        """
+        unique_together = self._meta.unique_together
+        for fields in unique_together:
+            if 'sort_order' in fields and len(fields) > 1:
+                return True
+        return False
+
     @staticmethod
     def _update(qs):
         """

orderable/tests/test_models.py

@@ -1,3 +1,4 @@
+from django.core.exceptions import ValidationError
 from hypothesis import example, given
 from hypothesis.extra.django import TestCase
 from hypothesis.strategies import integers, lists
@@ -237,3 +238,40 @@ def test_save_subtask_no_errors(self, sort_orders):
 
         subtask.sort_order = 2
         subtask.save()
+
+    def test_validate_unique(self):
+        """validate_unique should validate when sort_order conflicts."""
+        task = Task.objects.create()
+        subtask = SubTask.objects.create(task=task, sort_order=1)
+        SubTask.objects.create(task=task, sort_order=2)
+
+        subtask.sort_order = 2
+        try:
+            subtask.validate_unique()
+        except ValidationError:
+            self.fail("SubTask.clean() raised ValidationError unexpectedly!")
+
+    def test_validate_unique_exclude_sort_order(self):
+        """validate_unique should validate when sort_order excluded."""
+        task = Task.objects.create()
+        subtask = SubTask.objects.create(task=task, sort_order=1)
+        SubTask.objects.create(task=task, sort_order=2)
+
+        subtask.sort_order = 2
+        try:
+            subtask.validate_unique(exclude=('sort_order',))
+        except ValidationError:
+            self.fail("SubTask.clean() raised ValidationError unexpectedly!")
+
+    def test_validate_unique_not_unique_sort_order(self):
+        """
+        validate_unique should validate when sort_order not in unique_together.
+        """
+        task = Task.objects.create(sort_order=1)
+        Task.objects.create(sort_order=2)
+
+        task.sort_order = 1
+        try:
+            task.validate_unique()
+        except ValidationError:
+            self.fail("Task.clean() raised ValidationError unexpectedly!")