Merge branch 'zj-drop-fk-if-exists' into 'master'

Only remove FK if it exists

See merge request !11828

changelogs/unreleased/zj-drop-fk-if-exists.yml

+---
+title: Remove foreigh key on ci_trigger_schedules only if it exists
+merge_request:
+author:

db/migrate/20170425112628_remove_foreigh_key_ci_trigger_schedules.rb

@@ -4,10 +4,20 @@ class RemoveForeighKeyCiTriggerSchedules < ActiveRecord::Migration
   DOWNTIME = false
 
   def up
-    remove_foreign_key :ci_trigger_schedules, column: :trigger_id
+    if fk_on_trigger_schedules?
+      remove_foreign_key :ci_trigger_schedules, column: :trigger_id
+    end
   end
 
   def down
     # no op, the foreign key should not have been here
   end
+
+  private
+
+  # Not made more generic and lifted to the helpers as Rails 5 will provide
+  # such an API
+  def fk_on_trigger_schedules?
+    connection.foreign_keys(:ci_trigger_schedules).include?("ci_triggers")
+  end
 end