'Copy all the values from old_field into new_field
@param db : database instance'

@staticmethod
def _copy_field(db, tablename, fieldname_old, fieldname_new):

    dict_update = {}
    field_old = db[tablename][fieldname_old]
    for row in db().select(field_old):
        dict_update[fieldname_new] = row[fieldname_old]
        query = (field_old == row[fieldname_old])
        db(query).update(**dict_update)
