Added another migration-executor test to avoid regressions.

tests/migrations/test_executor.py

@@ -325,3 +325,38 @@ class ExecutorUnitTests(TestCase):
         should_be_rolled_back = [b2_impl, a4_impl, a2_impl, a3_impl]
         exp = [(m, True) for m in should_be_rolled_back]
         self.assertEqual(plan, exp)
+
+    def test_backwards_nothing_to_do(self):
+        """
+        If the current state satisfies the given target, do nothing.
+
+        a: 1 <--- 2
+        b:    \- 1
+        c:     \- 1
+
+        If a1 is applied already and a2 is not, and we're asked to migrate to
+        a1, don't apply or unapply b1 or c1, regardless of their current state.
+        """
+        a1_impl = FakeMigration('a1')
+        a1 = ('a', '1')
+        a2_impl = FakeMigration('a2')
+        a2 = ('a', '2')
+        b1_impl = FakeMigration('b1')
+        b1 = ('b', '1')
+        c1_impl = FakeMigration('c1')
+        c1 = ('c', '1')
+        graph = MigrationGraph()
+        graph.add_node(a1, a1_impl)
+        graph.add_node(a2, a2_impl)
+        graph.add_node(b1, b1_impl)
+        graph.add_node(c1, c1_impl)
+        graph.add_dependency(None, a2, a1)
+        graph.add_dependency(None, b1, a1)
+        graph.add_dependency(None, c1, a1)
+
+        executor = MigrationExecutor(None)
+        executor.loader = FakeLoader(graph, {a1, b1})
+
+        plan = executor.migration_plan({a1})
+
+        self.assertEqual(plan, [])