Fix PostgreSQL implementation of migration

changelogs/unreleased/sh-fix-mysql-migration-10-3.yml

 ---
-title: Fix migration for removing orphaned issues.moved_to_id values in MySQL
+title: Fix migration for removing orphaned issues.moved_to_id values in MySQL and PostgreSQL
 merge_request:
 author:
 type: fixed

db/migrate/20171106151218_issues_moved_to_id_foreign_key.rb

@@ -15,7 +15,9 @@ class IssuesMovedToIdForeignKey < ActiveRecord::Migration
     self.table_name = 'issues'
 
     def self.with_orphaned_moved_to_issues
-      where('NOT EXISTS (SELECT true FROM issues WHERE issues.id = issues.moved_to_id)')
+      # Be careful to use a second table here for comparison otherwise we'll null
+      # out all rows that don't have id == moved_to_id!
+      where('NOT EXISTS (SELECT true FROM issues b WHERE issues.moved_to_id = b.id)')
         .where('moved_to_id IS NOT NULL')
     end
   end

spec/migrations/issues_moved_to_id_foreign_key_spec.rb

@@ -4,7 +4,7 @@ require Rails.root.join('db', 'migrate', '20171106151218_issues_moved_to_id_fore
 # The schema version has to be far enough in advance to have the
 # only_mirror_protected_branches column in the projects table to create a
 # project via FactoryBot.
-describe IssuesMovedToIdForeignKey, :migration, schema: 20171109115718 do
+describe IssuesMovedToIdForeignKey, :migration, schema: 20171114150259 do
   let!(:issue_first) { create(:issue, moved_to_id: issue_second.id) }
   let!(:issue_second) { create(:issue, moved_to_id: issue_third.id) }
   let!(:issue_third) { create(:issue) }