code: public List collect(String attributeName) {
          List results = new ArrayList();
          collect(results, attributeName);
          return results;
      }
docs: Collects values from a result set that correspond to a attribute name.
  For example, if a list contains collection of <code>Person</code> models, then
  you can collect first names like this:
  <pre>
  List firstNames = Person.findAll().collect("first_name");
  </pre>
  provided that the corresponding table has a column <code>first_name</code>.
  <p><p/>
  Keep in mind, that if all you need is a one column data, this method of getting it is not
  the most efficient (because since you are using a model, you will query all columns from a table,
  but will use only one). In these cases, you might want to consider {@link Base#firstColumn(String, Object...)} and
  {@link DB#firstColumn(String, Object...)}.
  
  @param attributeName name of attribute to collect.
  @return list of collected values for a column.