VersionConflictEngineException after upgrading to Rails 4.2.5.1

Question

I get this exception from time to time after updating Rails to 4.2.5.1 and updating the user document

Elasticsearch :: Transport :: Transport :: Errors :: Conflict: [409] {"error": "VersionConflictEngineException [[users] [0] [user] [1]: version conflict, current [7], provided [6 ]] "," status ": 409}

However, I am not passing any explicit version in the update request. I believe that two different processes are trying to update the same document at the same time, but have not changed anything except the Rails version.

I am using elasticsearch-model (0.1.8) elasticsearch-rails (0.1.8) elasticsearch (1.0.15) elasticsearch-api (1.0.15) and elasticsearch-transport (1.0.15)

Elasticsearch version Version: 1.2.2, Build: 9902f08 / 2014-07-09T12: 02: 32Z, JVM: 1.7.0_65