Transfer old images into Sonata Media Bundle. This command is used for adding sonata media gallery to new structure from old database.

MigrateImagesCommand.php

<?php

namespace YourBundle\Command;

use Symfony\Bundle\FrameworkBundle\Command\ContainerAwareCommand;
use Symfony\Component\Console\Input\InputInterface;
use Symfony\Component\Console\Output\OutputInterface;
use Application\Sonata\MediaBundle\Entity\Media;
use Application\Sonata\MediaBundle\Entity\Gallery;
use Application\Sonata\MediaBundle\Entity\GalleryHasMedia;
use Sonata\MediaBundle\Entity\MediaManager;
use Sonata\MediaBundle\Entity\GalleryManager;
use Doctrine\ORM\EntityManager;

class MigrateImagesCommand extends ContainerAwareCommand
{
    private $output;
    private $em;
    /*
    * Sonata Media Manager
    */
    private $mediaManager;
    /*
    * Sonata Gallery Manager
    */
    private $galleryManager;
    /*
    * Directory of old images
    */
    private $uploadDirectory;
    
    public function __construct(EntityManager $em, MediaManager $mediaManager, GalleryManager $galleryManager, $uploadDirectory, $name = null) {
        $this->em = $em;
        $this->mediaManager = $mediaManager;
        $this->galleryManager = $galleryManager;
        $this->uploadDirectory = $uploadDirectory;
        parent::__construct($name);
    }
    protected function configure()
    {
        $this
            ->setName('migrate:images')
            ->setDescription('Migrates old images to sonata media bundle.')
        ;
    }
    protected function execute(InputInterface $input, OutputInterface $output)
    {
        $output->writeln('<comment>Running Image Migration...</comment>');
        $this->output = $output;
        /*
         * Decreases memory use when dealing with a lot of data
         */
        $this->em->getConnection()->getConfiguration()->setSQLLogger(null);
        
        $this->migrateImages();
        $output->writeln('<comment>Done!</comment>');
    }  
    
    private function migrateImages(){
        $this->output->writeln('<comment>Starting</comment>');
        
        $items = $this->em->getRepository('YourBundle:Items')->findAll();
        $count = count($items);
        $iteration = 0;
        gc_enable();
        try{
            foreach($items as $item){
                $gallery = $this->createGallery($item);    
                
                if(!$gallery)
                    continue;
                
                $item->setGallery($gallery);    
                $this->em->persist($item);
                
                $tempObjects[] = $item;
                if (++$iteration % 10 == 0) {
                  $this->em->flush();
                  $count -=10;
                  $this->output->writeln("<comment>Flushed, $count items left</comment>");
                  
                  // IMPORTANT - clean entities
                  foreach($tempObjects as $tempObject) {
                    $this->em->detach($tempObject);
                  }
                  $tempObjects = null;
                  gc_collect_cycles();
                }
            }
            $this->em->flush();
            $this->output->writeln('<info>SUCCESS</info>');
        } catch (\Exception $e) {
            $this->output->writeln('<error>'.$e->getMessage().'</error>');
        }
    }
    private function createGallery($item){
        
         /*
         * This is array of images, write code for grabing images depending on your needs
         * Check out multiple db connections for symfony and use that logic
         */
        $images;
        
        if(count($images) === 0)
            return false;
        
        $gallery = $this->createSonataGallery($item->getName());  
        
        foreach($images as $image){
            $media = $this->createSonataMedia($image);
            $galleryHasMedia = $this->createSonataGalleryHasMedia($media, $image->getPosition());
            $gallery->addGalleryHasMedias($galleryHasMedia);
        }
        $this->galleryManager->save($gallery);
        
        return $gallery;
    }
    private function createSonataGallery($name){
        
        $gallery = new Gallery();
        $gallery->setName($name);
        $gallery->setContext('default'); //your context
        $gallery->setDefaultFormat('default_small'); //your format
        $gallery->setEnabled(0); //required
        
        return $gallery;
    }
    private function createSonataMedia($image){
        
        $media = new Media();
        $media->setBinaryContent($this->uploadDirectory.$image->getName());
        $media->setContext('default'); //your context
        $media->setProviderName('sonata.media.provider.image');//sonata image provider
        $media->setName($image->getName());
        $this->mediaManager->save($media); 
        
        return $media;
    }
    private function createSonataGalleryHasMedia($media,$position){
        
        $galleryHasMedia = new GalleryHasMedia();
        $galleryHasMedia->setMedia($media);
        $galleryHasMedia->setPosition($position);
        
        return $galleryHasMedia;
    }
    
}

services.yml

parameters:
    upload_directory: '%kernel.root_dir%/../web/uploads/'
services:
    app.migrate.images.command:
      class: YourBundle\Command\MigrateImagesCommand
      arguments: ["@doctrine.orm.entity_manager","@sonata.media.manager.media","@sonata.media.manager.gallery","%upload_directory%"]
      tags:
          - { name: console.command }