Referencing a field instead of Index

Here's the scenario:
We have a JDBC configuration, lets call it JDBC. We are not outputting an Index for this. Which extracts data every hour to an output file in a target dir. The File JDBC generates, is picked up by an index called Billing_requests.

Now, I have to place a field which indicates the time it extracted. Just to better associate the data's relevance from billing_requests to the time JDBC executes. Now we do not use a document_id for billing_requests as each record should be unique. So, we let Logstash do the Id's so that it will be definitely unique

Is there a way to reference the _path field in billing_requests Index from the JDBC.conf file? Normally you specify the document_id to associate with _id of billing_requests. But is there a way to reference the field?

if (billing_requests.field(_path) == 'FileName.txt') {