Make wrong migrations idempotent

db/migrate/20170707183807_add_group_id_to_milestones.rb

@@ -2,6 +2,8 @@ class AddGroupIdToMilestones < ActiveRecord::Migration
   DOWNTIME = false
 
   def up
+    return if column_exists? :milestones, :group_id
+
     change_column_null :milestones, :project_id, true
 
     add_column :milestones, :group_id, :integer

db/migrate/20170707184243_add_group_milestone_id_indexes.rb

@@ -6,6 +6,8 @@ class AddGroupMilestoneIdIndexes < ActiveRecord::Migration
   DOWNTIME = false
 
   def up
+    return if index_exists?(:milestones, :group_id)
+
     add_concurrent_foreign_key :milestones, :namespaces, column: :group_id, on_delete: :cascade
 
     add_concurrent_index :milestones, :group_id

db/migrate/20170707184244_remove_wrong_versions_from_schema_versions.rb

 class RemoveWrongVersionsFromSchemaVersions < ActiveRecord::Migration
-  def change
-    execute "UPDATE schema_migrations SET version = '20170707183807' WHERE version = '20170723183807'"
-    execute "UPDATE schema_migrations SET version = '20170707184243' WHERE version = '20170724184243'"
+  DOWNTIME = false
+
+  def up
+    execute("DELETE FROM schema_migrations WHERE version IN ('20170723183807', '20170724184243')")
+  end
+
+  def down
   end
 end