You need to map the existent table and set migrate=False, also you will need to specify which field is the 'id' or primary keys, and for all fields you need to tell the datatype.
msdb.define_table( 'test', FIeld('cod', 'id'), Field('field1', "datatype"), Field('field2', "datatype"), migrate=False ) look here: http://web2py.com/books/default/chapter/29/6#Legacy-databases-and-keyed-tables -- Bruno Rocha [http://rochacbruno.com.br]