add foreign key index to parallel importers

refs CORE-929

test plan
 - migrations should run

Change-Id: Ibd2966a5b43bec1e1ff9086d69d13193335a0767
Reviewed-on: https://gerrit.instructure.com/146267
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>

db/migrate/20180408212014_add_missing_foreign_key_index_to_parallel_importer.rb

@@ -0,0 +1,25 @@
+#
+# Copyright (C) 2018 - present Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+
+class AddMissingForeignKeyIndexToParallelImporter < ActiveRecord::Migration[5.1]
+  tag :predeploy
+
+  def change
+    add_index :parallel_importers, :attachment_id
+  end
+end