Determine if this field is click sortable.
Overrides FieldPluginBase::click_sortable
function click_sortable() {
// Not click sortable in any case.
if (empty($this->definition['click sortable'])) {
return FALSE;
}
// A field is not click sortable if it's a multiple field with
// "group multiple values" checked, since a click sort in that case would
// add a join to the field table, which would produce unwanted duplicates.
if ($this->multiple && $this->options['group_rows']) {
return FALSE;
}
return TRUE;
}