[1.7.x] Fixed #23452 -- Prevented infinite migrations for empty unique/index_together.

Thanks fwkroon for the report.

Backport of 6d5958c7a3 from master

4 files changed, 62 insertions, 14 deletions

diff --git a/django/db/migrations/autodetector.py b/django/db/migrations/autodetector.py
index a8647243d9..c8b7db7539 100644
--- a/django/db/migrations/autodetector.py
+++ b/django/db/migrations/autodetector.py
@@ -871,23 +871,28 @@ class MigrationAutodetector(object):
             old_model_name = self.renamed_models.get((app_label, model_name), model_name)
             old_model_state = self.from_state.models[app_label, old_model_name]
             new_model_state = self.to_state.models[app_label, model_name]
+
             # We run the old version through the field renames to account for those
-            if old_model_state.options.get(option_name) is None:
-                old_value = None
-            else:
+            old_value = old_model_state.options.get(option_name) or set()
+            if old_value:
                 old_value = set([
                     tuple(
                         self.renamed_fields.get((app_label, model_name, n), n)
                         for n in unique
                     )
-                    for unique in old_model_state.options[option_name]
+                    for unique in old_value
                 ])
-            if old_value != new_model_state.options.get(option_name):
+
+            new_value = new_model_state.options.get(option_name) or set()
+            if new_value:
+                new_value = set(new_value)
+
+            if old_value != new_value:
                 self.add_operation(
                     app_label,
                     operation(
                         name=model_name,
-                        **{option_name: new_model_state.options.get(option_name)}
+                        **{option_name: new_value}
                     )
                 )
 
diff --git a/django/db/migrations/operations/models.py b/django/db/migrations/operations/models.py
index 5b1a23eab7..eade3ee878 100644
--- a/django/db/migrations/operations/models.py
+++ b/django/db/migrations/operations/models.py
@@ -233,9 +233,7 @@ class AlterUniqueTogether(Operation):
     def __init__(self, name, unique_together):
         self.name = name
         unique_together = normalize_together(unique_together)
-        # need None rather than an empty set to prevent infinite migrations
-        # after removing unique_together from a model
-        self.unique_together = set(tuple(cons) for cons in unique_together) or None
+        self.unique_together = set(tuple(cons) for cons in unique_together)
 
     def state_forwards(self, app_label, state):
         model_state = state.models[app_label, self.name.lower()]
@@ -273,9 +271,7 @@ class AlterIndexTogether(Operation):
     def __init__(self, name, index_together):
         self.name = name
         index_together = normalize_together(index_together)
-        # need None rather than an empty set to prevent infinite migrations
-        # after removing unique_together from a model
-        self.index_together = set(tuple(cons) for cons in index_together) or None
+        self.index_together = set(tuple(cons) for cons in index_together)
 
     def state_forwards(self, app_label, state):
         model_state = state.models[app_label, self.name.lower()]
diff --git a/docs/releases/1.7.1.txt b/docs/releases/1.7.1.txt
index 46a19f1f6d..b98ec1b847 100644
--- a/docs/releases/1.7.1.txt
+++ b/docs/releases/1.7.1.txt
@@ -31,3 +31,6 @@ Bugfixes
   ``'auth.User'`` model (:ticket:`11775`). As a side effect, the setting now
   adds a ``get_absolute_url()`` method to any model that appears in
   ``ABSOLUTE_URL_OVERRIDES`` but doesn't define ``get_absolute_url()``.
+
+* Empty ``index_together`` or ``unique_together`` model options no longer
+  results in infinite migrations (:ticket:`23452`).
diff --git a/tests/migrations/test_autodetector.py b/tests/migrations/test_autodetector.py
index 526c018907..0ba05d3495 100644
--- a/tests/migrations/test_autodetector.py
+++ b/tests/migrations/test_autodetector.py
@@ -502,6 +502,50 @@ class AutodetectorTests(TestCase):
         # Right number of migrations?
         self.assertEqual(len(changes), 0)
 
+    def test_empty_foo_together(self):
+        "#23452 - Empty unique/index_togther shouldn't generate a migration."
+        # Explicitly testing for not specified, since this is the case after
+        # a CreateModel operation w/o any definition on the original model
+        model_state_not_secified = ModelState("a", "model",
+            [("id", models.AutoField(primary_key=True))]
+        )
+        # Explicitly testing for None, since this was the issue in #23452 after
+        # a AlterFooTogether operation with e.g. () as value
+        model_state_none = ModelState("a", "model",
+            [("id", models.AutoField(primary_key=True))],
+            {"unique_together": None, "index_together": None}
+        )
+        # Explicitly testing for the empty set, since we now always have sets.
+        # During removal (('col1', 'col2'),) --> () this becomes set([])
+        model_state_empty = ModelState("a", "model",
+            [("id", models.AutoField(primary_key=True))],
+            {"unique_together": set(), "index_together": set()}
+        )
+
+        def test(from_state, to_state, msg):
+            before = self.make_project_state([from_state])
+            after = self.make_project_state([to_state])
+            autodetector = MigrationAutodetector(before, after)
+            changes = autodetector._detect_changes()
+            if len(changes) > 0:
+                ops = ', '.join(o.__class__.__name__ for o in changes['a'][0].operations)
+                self.fail('Created operation(s) %s from %s' % (ops, msg))
+
+        tests = (
+            (model_state_not_secified, model_state_not_secified, '"not specified" to "not specified"'),
+            (model_state_not_secified, model_state_none, '"not specified" to "None"'),
+            (model_state_not_secified, model_state_empty, '"not specified" to "empty"'),
+            (model_state_none, model_state_not_secified, '"None" to "not specified"'),
+            (model_state_none, model_state_none, '"None" to "None"'),
+            (model_state_none, model_state_empty, '"None" to "empty"'),
+            (model_state_empty, model_state_not_secified, '"empty" to "not specified"'),
+            (model_state_empty, model_state_none, '"empty" to "None"'),
+            (model_state_empty, model_state_empty, '"empty" to "empty"'),
+        )
+
+        for t in tests:
+            test(*t)
+
     def test_unique_together_ordering(self):
         "Tests that unique_together also triggers on ordering changes"
         # Make state
@@ -555,7 +599,7 @@ class AutodetectorTests(TestCase):
         # Right actions?
         action = migration.operations[0]
         self.assertEqual(action.__class__.__name__, "AlterIndexTogether")
-        self.assertEqual(action.index_together, None)
+        self.assertEqual(action.index_together, set())
 
     def test_remove_unique_together(self):
         author_unique_together = ModelState("testapp", "Author", [
@@ -574,7 +618,7 @@ class AutodetectorTests(TestCase):
         # Right actions?
         action = migration.operations[0]
         self.assertEqual(action.__class__.__name__, "AlterUniqueTogether")
-        self.assertEqual(action.unique_together, None)
+        self.assertEqual(action.unique_together, set())
 
     def test_proxy(self):
         "Tests that the autodetector correctly deals with proxy models"