title: can I use data transformers to combine fields in forms in symfony2
body: <p>Is it possible to use Data transformers to merge (n) fields in a form into one persistable field?
  If it's possible, how to do it? The cookbook only gives an example to transform one piece of data into another type, but I need to be able to dump N fields into only one for persistence. So if I'm showing 6 fields in the form, only 3 are real fields in DB table, first and second fields are to persisted as is, but the remaining 4 fields are to be store in the third table column.</p>