Using the following config + the latest code as of 9/19/14, this error is thrown when the output receives an event:
(eval):22:in `block in initialize': undefined method `flat_map' for nil:NilClass (NoMethodError)
from /media/truecrypt1/git/danielredoak/logstash/lib/logstash/pipeline.rb:272:in `call'
from /media/truecrypt1/git/danielredoak/logstash/lib/logstash/pipeline.rb:272:in `output'
from /media/truecrypt1/git/danielredoak/logstash/lib/logstash/pipeline.rb:231:in `outputworker'
from /media/truecrypt1/git/danielredoak/logstash/lib/logstash/pipeline.rb:160:in `block in start_outputs'
Config Ex:
Related file, code starts at 328:
https://github.com/elasticsearch/logstash/blob/4211522de1cf996891c6b93f93ba1ab4e408d587/lib/logstash/config/config_ast.rb#L328