I have a Page that displays a list of Members in a group, but I need to change the sort of the group members. I though the esiest way would be to add a relation editor to the pagetype with the sort order added via a Data Extension

Here is my extension on Group

class MyGroup extends DataExtension {
    static $many_many_extraFields = array(
        'Members' => array(
            'SortOrder' => "Int"
        )
    );
}

On the Page I have the following:

if($this->GroupID != 0 && Permission::check("APPLY_ROLES")) {
    $group = Group::get()->byID($this->GroupID);
    $fields->addFieldsToTab("Root.Members", array(
         GridField::create(
            "Members",
            "Members",
            $group->DirectMembers(),
            GridFieldConfig_RelationEditor::create()->addComponents(
                new GridFieldSortableRows('SortOrder')
            )
        )
    ));
}

When I try to sort the Members I get an error

Uncaught SS_DatabaseException: Couldn't run query:
UPDATE "" SET "SortOrder" = 1 WHERE "" = 12 AND "" = 18

I'm not sure why GridField isn't getting the Columns