Fix Originality report creation fk violations

Fixes PLAT-3257

Test Plan:
- Verify you can successfully create an originality report
  with an attachment on a different shard than the report.

- Run through a simple workflow of the plagiarism detection platform
  to verify all works as before when attachment is on the same shard
  or a second shard.

Change-Id: I0c42d3ec5634e269c896b46bc8111f95bbaca903
Reviewed-on: https://gerrit.instructure.com/146144
Reviewed-by: Rob Orton <rob@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins
Product-Review: Weston Dransfield <wdransfield@instructure.com>

db/migrate/20180406151719_remove_attachment_foreign_key_from_originality_reports.rb

@@ -0,0 +1,31 @@
+#
+# 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 RemoveAttachmentForeignKeyFromOriginalityReports < ActiveRecord::Migration[5.1]
+  tag :predeploy
+
+  def up
+    remove_foreign_key :originality_reports, column: :attachment_id
+    remove_foreign_key :originality_reports, column: :originality_report_attachment_id
+  end
+
+  def down
+    add_foreign_key :originality_reports, :attachments, column: :attachment_id
+    add_foreign_key :originality_reports, :attachments, column: :originality_report_attachment_id
+  end
+end

spec/models/originality_report_spec.rb

@@ -285,6 +285,32 @@ describe OriginalityReport do
       end.not_to change(OriginalityReport, :count)
     end
 
+    context 'with sharding' do
+      specs_require_sharding
+
+      let(:new_shard) { Shard.create! }
+      let(:new_shard_attachment) { new_shard.activate { attachment_model } }
+      let(:submission) { submission_model }
+
+      it 'allows cross-shard attachment associations' do
+        report = OriginalityReport.create!(
+          submission: submission,
+          originality_score: 50,
+          attachment: new_shard_attachment
+        )
+        expect(report.attachment).to eq new_shard_attachment
+      end
+
+      it 'allows cross-shard report attachment associations' do
+        report = OriginalityReport.create!(
+          submission: submission,
+          originality_score: 50,
+          originality_report_attachment: new_shard_attachment
+        )
+        expect(report.originality_report_attachment).to eq new_shard_attachment
+      end
+    end
+
     context 'when lti link is present' do
       let!(:link) do
         Lti::Link.create!(