Rack::Mongoid::Middleware defines call() to be:

        def call(env)
          ::Mongoid.unit_of_work { @app.call(env) }
        end

But consider what happens in the case of Rails streaming. call() returns before the page is finished rendering. Any Mongoid queries that happen after call() returns are added to a cleared-out IdentityMap, but the values from these queries are not cleared out until call() returns from the next request on the same thread.

I found evidence for this when taking a heap dump of my jruby on rails app. The heap dump was taken with no requests in flight (I had taken down the front-end apache server).

I am not sure if Rails or Rack provides a after-response hook for streaming. Without such a hook, I'm not sure if this can be fixed easily.