title: updating records from controller in web2py
body: <p>I want to update some field in a record of database form controller given i know which record that is and which field that is.How do i do that? </p>
  
  <p>Eg:In the table 'students' in the database there is field called gender which is either 'male' or 'female'.After adding many  records,i add a new field called 'gender_description'.It should be 'girl' when gender is female and 'boy' if gender is male.I want to populate this new field for all the records automatically.How do i achieve this?</p>