NoMethodError exception raised if search hits lack source document

[magnusbaeck]

As noted in https://discuss.elastic.co/t/error-undefined-method-for-nil-nilclass-in-logstash/54234, the elasticsearch input assumes that the hits in the search results contain the source document via the _source key, but this isn't true with a query like this:

{"fields": ["clientIP"], "query": { "match_all": {} } }

Such a query results in an "Error: undefined method `[]' for nil:NilClass" exception. I suppose the same might happen if the index's mappings are configured to not store the original document.

How should we deal with this situation? Use the available fields in lieu of the complete document?