no "group by" clause being added for custom views field handler

I have a custom views field handler which is to add a MAX() field to the query. The code is as follows:

<?php
 
/*
 * @file
 * Field handler to get latest quiz result id.
 */
 
class quiz_views_handler_field_quiz_latest_result_id extends views_handler_field_numeric {
 
  function construct() {
    parent::construct();
    // This will have to change to be set by options if the methods are ever expanded
    $this->group_field = $this->definition['group field'];
  }
 
  function option_definition() {
    $options = parent::option_definition();
    return $options;
  }
 
  function query() {
    $this->ensure_my_table();
    $this->query->add_groupby("$this->table_alias.$this->group_field");
 
    $this->field_alias = $this->query->add_field(
      NULL, "MAX($this->table_alias.$this->real_field)",
      $this->table_alias . '__max_resultid', array('aggregate' => TRUE)
    );
  }
}

and generates this SQL query:

SELECT quiz_node_relationship.child_nid AS quiz_node_relationship_child_nid, node_quiz_node_relationship.title AS node_quiz_node_relationship_title, node_quiz_node_relationship.nid AS node_quiz_node_relationship_nid, quiz_node_results.score AS quiz_node_results_score, quiz_node_results.result_id AS quiz_node_results_result_id, quiz_node_results.time_end AS quiz_node_results_time_end, quiz_node_results.nid AS quiz_node_results_nid, MAX(quiz_node_results.result_id) AS quiz_node_results__max_resultid
FROM
{quiz_node_properties} quiz_node_properties
INNER JOIN {quiz_node_relationship} quiz_node_relationship ON quiz_node_properties.vid = quiz_node_relationship.parent_vid
LEFT JOIN {node} node_quiz_node_relationship ON quiz_node_relationship.child_nid = node_quiz_node_relationship.nid
LEFT JOIN {quiz_node_results} quiz_node_results ON quiz_node_properties.vid = quiz_node_results.vid
LEFT JOIN {users} users_quiz_node_results ON quiz_node_results.uid = users_quiz_node_results.uid
WHERE (( (quiz_node_results.time_end > '0') AND( (users_quiz_node_results.uid = '1') )AND (quiz_node_properties.vid IN  ('5')) ))
ORDER BY quiz_node_results_time_end DESC

The problem is that my code adds in a MAX() field to the query (which works) and the necessary GROUP BY clause but while add_groupby() is executed the clause is never added to the resulting query.

Either there's something in my code that's wrong, or there's something in the query() function of plugins/views_plugin_query_default.inc that's off, particularly the lines:

    $this->has_aggregate = FALSE;
    $non_aggregates = array();

    list($non_aggregates) = $this->compile_fields($fields_array, $query);

    if (count($this->having)) {
      $this->has_aggregate = TRUE;
    }
    if ($this->has_aggregate && (!empty($this->groupby) || !empty($non_aggregates))) {
      $groupby = array_unique(array_merge($this->groupby, $non_aggregates));
      foreach ($groupby as $field) {
        $query->groupBy($field);
      }
      if (!empty($this->having) && $condition = $this->build_condition('having')) {
        $query->havingCondition($condition);
      }
    }

From my debug statements, I can see that my group by clause is in $this->groupby, and $non_aggregates contains the other fields that are selected out, but that $this->has_aggregate is FALSE. As a result of this being FALSE no group by clause is added to the query at all.

I don't know whether this is because of bug in my code or something in views itself.