Add an option to explicitly omit the creation of hidden field with object id

Some NoSQL "ORMs" allows to create inline associations, in this case
associated objects does not have its own IDs. An option to omit the
creation of hidden field with associated object id would be useful.
This is 3 lines patch with ~10 months history. Could anyone finish it
please?

https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/4551

I think you should ask Santiago to look at it.