Hi Hakan,
There is no need to write any logic for field mapping, until there is a particular logic involved.
What you need to do is in addition to steps mentioned in your query section is, do mapping with context tab-field so that data can automatically flow between tables and WDA.
Please refer last secbelow thread:
http://scn.sap.com/thread/3473863
Warm Regards,
Shyam Agrawal