Retrive a field from an Idoc

Hi eveybody

I´ve got the next scenario:

JDBC---- >PI -(Idoc) ----->SAP

SAP -(Aleaud) -- ->PI --

-

>JDBC.

How could I retrive a field from reference Idoc when the Aleaud Idoc is processed??

Im going to try to explain it better.

In the Outbound Idoc travel a field called FIELD_A.

Before the Aleaud Idoc is sent to PI I need to retrive this FIELD_A that travelled in the reference Idoc and send this FIELD_A in the Aleaud Idoc because I need it in the query used in the JDBC adapter.

How can I do this????

Thanks everybody.

Regards