Prevent caller from specifying id during Migration.create()

The patch is to remove id when it was set before creating a migration
in objects, and check if the migration exists before it is created.

Related to blueprint icehouse-objects.

Change-Id: I37ff279f5f74d17ce9c173458ff35826fb10f143

nova/objects/migration.py

@@ -13,6 +13,7 @@
 #    under the License.
 
 from nova import db
+from nova import exception
 from nova.objects import base
 from nova.objects import fields
 from nova.objects import instance as instance_obj
@@ -57,6 +58,9 @@ class Migration(base.NovaPersistentObject, base.NovaObject):
 
     @base.remotable
     def create(self, context):
+        if self.obj_attr_is_set('id'):
+            raise exception.ObjectActionError(action='create',
+                                              reason='already created')
         updates = self.obj_get_changes()
         updates.pop('id', None)
         db_migration = db.migration_create(context, updates)

nova/tests/objects/test_migration.py

@@ -14,6 +14,7 @@
 
 from nova import context
 from nova import db
+from nova import exception
 from nova.objects import migration
 from nova.openstack.common import timeutils
 from nova.tests import fake_instance
@@ -81,6 +82,19 @@ class _TestMigrationObject(object):
         mig.create(ctxt)
         self.assertEqual(fake_migration['dest_compute'], mig.dest_compute)
 
+    def test_recreate_fails(self):
+        ctxt = context.get_admin_context()
+        fake_migration = fake_db_migration()
+        self.mox.StubOutWithMock(db, 'migration_create')
+        db.migration_create(ctxt, {'source_compute': 'foo'}).AndReturn(
+            fake_migration)
+        self.mox.ReplayAll()
+        mig = migration.Migration()
+        mig.source_compute = 'foo'
+        mig.create(ctxt)
+        self.assertRaises(exception.ObjectActionError, mig.create,
+                          self.context)
+
     def test_save(self):
         ctxt = context.get_admin_context()
         fake_migration = fake_db_migration()