$unitfield = new HasManyComplexTableField(
   $this,
   'Units',
   'Unit',
   array(
    'Sessions' => '# of Sessions',
    'Date1' => 'Starting Date'
   ),
   'getCMSFields_forPopup'//,
   //"`Unit.MyWorkshopID` = '$this->ID' OR `Unit.MyWorkshopID` = ''"
);
$unitfield->setParentClass('Workshop');

$fields->addFieldToTab( 'Root.Content.Dates', $unitfield );