It is possible to use GROUP_CONCAT for multiple multi-value fields

Just thought this may help someone out.

migrate_example wine.inc around line 626 states:

// We can only handle a single multi-value source field using GROUP_CONCAT
// as we did above - insert others with a query against the related table
// with multiple values here, so the values can run through the mapping process

You can use GROUP_CONCAT for multiple multi-value fields like so:

$query->addExpression('GROUP_CONCAT(DISTINCT tn.tid)', 'list_tags');
$query->addExpression('GROUP_CONCAT(DISTINCT f.filepath)', 'list_images');

Full example of use: (I used this to update a d6 site to d7 to bypass cck update issues)

<?php
// The Node migrations are very similar - implement the commonalities here
abstract class NodeMigration extends BasicMigration {
	public $print = FALSE;
	public function __construct($type, $description, $newType = NULL, $dump = FALSE) {
		$this->print = $dump;
		parent::__construct();
		$this->description = t($description);
		$this->dependencies = array('Tags', 'Users');
		
		$this->map = new MigrateSQLMap($this->machineName,
			array(
				'nid' => array('type' => 'int', 'unsigned' => TRUE, 'not null' => TRUE, 'description' => 'node ID', 'alias' => 'id',)
			),
			MigrateDestinationNode::getKeySchema()
		);
		
		$query = db_select('node',			'n', array('target' => 'old_lgb'));
		$query->leftJoin('node_revisions',		'nr',	'nr.vid = n.vid');
		$query->leftJoin('content_field_image',	'cfi',	'cfi.vid=n.vid');
		$query->leftJoin('files',				'f',		'f.fid = cfi.field_image_fid');
		$query->leftJoin('term_node',			'tn',	'tn.vid =n.vid');
		$query->leftJoin('upload',				'u',		'u.vid = n.vid');
		$query->leftJoin('files',				'f2',	'f2.fid = u.fid');
		
		$query->fields('n',array('nid', 'title', 'uid', 'status', 'created', 'changed', 'promote', 'sticky'));
		$query->fields('nr',array('body', 'format'));
		$query->condition('type', $type);
		$query->addExpression('GROUP_CONCAT(DISTINCT tn.tid)',			'list_tags');
		$query->addExpression('GROUP_CONCAT(DISTINCT f.filepath)',		'list_images');
		//$query->addExpression('GROUP_CONCAT(DISTINCT u.description)',	'list_filedescriptions');
		$query->addExpression('GROUP_CONCAT(DISTINCT f2.filepath)',		'list_files');
		
		$query ->groupBy('n.nid');
		
		$this->highwaterField = array(
			'name' => 'changed', // Column to be used as highwater mark
			'alias' => 'n',           // Table alias containing that column
		);
		// Note that it is important to process rows in the order of the highwater mark
		$query->orderBy('changed');
		
		$this->source = new MigrateSourceSQL($query, array(), NULL, array('map_joinable' => FALSE));
		$this->destination = new MigrateDestinationNode($newType ? $newType : $type);
		
		// Mapped fields
		$this->addFieldMapping('nid', 'nid');
		$this->addFieldMapping('is_new')->defaultValue(TRUE);
		$this->addFieldMapping('uid','uid');
		$this->addFieldMapping('created','created');
		$this->addFieldMapping('changed','changed');
		$this->addFieldMapping('status','status');
		$this->addFieldMapping('promote','promote');
		$this->addFieldMapping('sticky','sticky');
		$this->addFieldMapping('title','title');
		$this->addFieldMapping('body', 'body')->arguments(array('format' => 'filtered_html'));
		
		$arguments = MigrateFileFieldHandler::arguments(drupal_get_path('module', 'migrate_lgb') . '/import_files', 'file_copy', FILE_EXISTS_RENAME);
		$this->addFieldMapping('field_image', 'list_images')
			->arguments($arguments)
			->separator(',');
		
		$arguments = MigrateFileFieldHandler::arguments(drupal_get_path('module', 'migrate_lgb') . '/import_files', 'file_copy', FILE_EXISTS_RENAME);
		$this->addFieldMapping('field_file', 'list_files')
			->arguments($arguments)
			->separator(',');

		$this->addFieldMapping('field_tags', 'list_tags')
		->separator(',')
		->sourceMigration('Tags')
		->arguments(array('source_type' => 'tid'));
		
		$this->addFieldMapping('field_photocredit')->defaultValue('');
		$this->addFieldMapping('language')->defaultValue('und');
		// Unmapped
		$this->addFieldMapping('revision_uid')->issueGroup(t('DNM'));
		$this->addFieldMapping('revision')->issueGroup(t('DNM'));
		$this->addFieldMapping('field_amazon')->issueGroup(t('DNM'));
		$this->addFieldMapping('meta_keywords')->issueGroup(t('DNM'));
		$this->addFieldMapping('meta_description')->issueGroup(t('DNM'));
		$this->addFieldMapping('path')->issueGroup(t('DNM'));
		$this->addFieldMapping('comment')->issueGroup(t('DNM'));
		$this->addFieldMapping('pathauto_perform_alias')->issueGroup(t('DNM'));
		$this->addFieldMapping(NULL, 'format')->issueGroup(t('DNM'));
		//$this->addFieldMapping(NULL, 'list_filedescriptions')->issueGroup(t('DNM'));
	}

	public function complete(stdClass $destination, stdClass $source) { 
		if($this->print) {
			print var_dump($destination) . "\r\n";
		}
	}
}

class BlogMigration extends NodeMigration {
	public function __construct() {
		parent::__construct('blog', 'We are importing blogs from lgb');
	}
	
}

class PageMigration extends NodeMigration {
	public function __construct() {
		parent::__construct('page', 'We are importing pages from lgb');
		$this->removeFieldMapping('field_amazon');
		$this->addFieldMapping('field_sidebar');
	}
	
}

class PlcNotesMigration extends NodeMigration {
	public function __construct() {
		parent::__construct('plc_notes', 'We are importing pages from lgb');
		$this->removeFieldMapping('field_amazon');
		$this->removeFieldMapping('meta_description');
	}
	
}

class BookPageMigration extends NodeMigration {
	public function __construct() {
		parent::__construct('book', 'We are importing pages from lgb', 'page');
		$this->removeFieldMapping('field_amazon');
		$this->addFieldMapping('field_sidebar')->defaultValue('1');
	}
	
}
?>