Pipeline not working with logstash-output-elasticsearch plugin when using doc_as_upsert => true

[Dineshs91 (Dinesh S)]

Pipeline has no effect while using action => update and doc_as_upsert => true.
When I change the action to index, it works.

Here is my logstash configuration (output section)

output {
  elasticsearch {
     action => update
     doc_as_upsert => true
     pipeline => "log_pipeline"
     hosts => ["localhost:9200"]
 }
}

Does action => index and doc_as_upsert => true work together ?
From documentation I understand that upsert is a special case when using action => index

Thanks