before_update callback is called when new record is created

All my tables/models have a "revision" field, and I have a
before_update callback to increment the revision field.

Among my tables are:
  events
  rooms
  events_rooms

In my Event model, I have:
  has_many :eventRooms
  has_many(:rooms, :through => :eventRooms)

When I create an event, assign a room or two, and then save, the
"events_rooms" table entry is initially created with a revision of 1,
and then (immediately, automatically, and inexplicably) updated to have
a revision of 2. I do not want every new record to have a revision
number of 2. The revision 2 version is identical to revision 1 (which
it replaces), aside from the revision number.

I can reproduce this in the rails console with the following:
  event = Event.new({'name' => 'Meeting 5'})
  event.rooms << Room.find(['1', '2'])
  event.save

Does anyone have any idea why this could be happening or where I should
look for further troubleshooting?

I'm working trying to strip out all the weird stuff (there's a lot) that
I had in my app, and remove anything proprietary so I can make a simple
test case.
So far, none of my simplifications have made this problem go away.

Oh, yeah, and the standard details:
Rails 4.1.6
MariaDB
SuSE Linux

My workaround is to do my before_update as follows:
    before_update :incr_rev

    private
    def incr_rev
      if changed?
        self.revision = self.revision + 1
      end
    end

With this workaround, it still calls the before_update callback when I
create a new record, but doesn't actually do an update.

I'm baffled that it would be calling the before_update callback if it's
not changed, but it does.

Starting to wonder if I've found a bug.

Just to be clear you are saying that the above code invokes
before_update once where you would not expect it to be called at all?
If so is it invoked by the save or does it get called when the room is
added?

Does it save it twice?

Colin

it does a sql INSERT followed by a sql UPDATE (both) after "event.save".

Strictly it will be during event.save call (before the UPDATE).
Presumably it is adding the room that is triggering the problem. It
does look like a bug to me. Do you know whether it happened with
earlier versions of Rails? If not then almost certainly a bug.

Colin

Dunno... the only versions of Rails that I've ever used are 1.something
and 4.1.6.

Well, I've created an issue in the rails issue tracker. We shall see.

Can you post a link please?

Thanks

Colin

https://github.com/rails/rails/issues/17289