That would be wonderful. Unfortunately, IdentityMap was disabled by default in 3.1 because it was incomplete, and it was pulled from 4.0 for a while due to lack of development:

https://github.com/rails/rails/pull/5261

Though there is now a company volunteering to fund completion of the feature, starting Real Soon Now:

https://github.com/rails/rails/issues/5442

IdentityMap would, obviously, be great. Though I didn't realize it could serve as a replacement for model caching; didn't IdentityMap, like the query cache, only last for the lifetime of a request? Or could it be persisted in the Rails.cache of your choice for an arbitrary lifespan?