ManyToMany / ManyToOne implemented with SonataAdminBundle and SonataMediaBundle.

Project.php

<?php

namespace Acme\Bundle\ProjectBundle\Entity;

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\ORM\Mapping as ORM;
use Acme\Bundle\ProjectBundle\Entity\ProjectHasMedia;

/**
 * Project
 *
 * @ORM\Table()
 * @ORM\Entity(repositoryClass="Acme\Bundle\ProjectBundle\Entity\ProjectRepository")
 */
class Project implements Taggable
{
    /**
     * @var integer
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var ArrayCollection
     *
     * @ORM\OneToMany(targetEntity="Acme\Bundle\ProjectBundle\Entity\ProjectHasMedia", mappedBy="project", cascade={"all"})
     * @ORM\JoinTable(name="project_galleries")
     */
    private $projectMedia;

    /**
     * Constructor
     */
    public function __construct()
    {
        $this->projectMedia = new ArrayCollection();
    }

    /**
     * Get id
     *
     * @return integer 
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * Add projectMedia
     *
     * @param Acme\Bundle\ProjectBundle\Entity\ProjectHasMedia
     * @return Project
     */
    public function addProjectMedia(ProjectHasMedia $projectMedia)
    {
        $this->projectMedia[] = $projectMedia;

        return $this;
    }

    /**
     * Remove projectMedia
     *
     * @param Acme\Bundle\ProjectBundle\Entity\ProjectHasMedia
     */
    public function removeProjectMedia(ProjectHasMedia $projectMedia)
    {
        $this->projectMedia->remove($projectMedia);
    }

    /**
     * Get projectMedia
     *
     * @return Acme\Bundle\ProjectBundle\Entity\ProjectHasMedia
     */
    public function getProjectMedia()
    {
        return $this->projectMedia;
    }

    /**
     * Set projectMedia
     *
     * @param array
     * @return Project
     */
    public function setProjectMedia($media)
    {
        $this->projectMedia = new ArrayCollection();

        foreach ($media as $m) {
            $m->setProject($this);
            $this->addProjectMedia($m);
        }

        return $this;
    }
}

ProjectAdmin.php

<?php

namespace Acme\Bundle\ProjectBundle\Admin;

use Sonata\AdminBundle\Admin\Admin;
use Sonata\AdminBundle\Datagrid\ListMapper;
use Sonata\AdminBundle\Datagrid\DatagridMapper;
use Sonata\AdminBundle\Form\FormMapper;

class ProjectAdmin extends Admin
{
    protected $context = 'default';

    /**
     * Create form for creating and updating
     *
     * @param \Sonata\AdminBundle\Form\FormMapper $formMapper
     */
    protected function configureFormFields(FormMapper $formMapper)
    {
        $formMapper
            ->add('projectMedia', 'sonata_type_collection', array(
                'cascade_validation' => true,
            ), array(

                'edit'              => 'inline',
                'inline'            => 'table',
                'sortable'          => 'position',
                'link_parameters'   => array(
                    'context'       => $this->context,
                ),
            ))
        ;
    }

    /**
     * Kinda Hackish methods to fix potential bug with SonataAdminBundle. I have not 
     * confirmed this is necessary but I've seen this implemented more than once.
     */
    public function prePersist($project)
    {
        $project->setProjectMedia($project->getProjectMedia());
    }

    public function preUpdate($project)
    {
        $project->setProjectMedia($project->getProjectMedia());
    }
}

ProjectHasMedia.php

<?php

namespace Acme\Bundle\ProjectBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

use Application\Sonata\MediaBundle\Entity\Gallery;
use Acme\Bundle\ProjectBundle\Entity\Project;

/**
 * @ORM\Table()
 * @ORM\Entity
 */
class ProjectHasMedia {
    /**
     * @var integer
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;

    /**
     * @ORM\ManytoOne(targetEntity="Acme\Bundle\ProjectBundle\Entity\Project", inversedBy="projectMedia", cascade={"all"})
     */
    protected $project;

    /**
     * @ORM\ManyToOne(targetEntity="Application\Sonata\MediaBundle\Entity\Gallery")
     * @ORM\JoinTable(name="project_gallery")
     */
    protected $gallery;


    public function __construct()
    {

    }

    /**
     * Get id
     *
     * @return integer
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * Set project
     *
     * @param \Acme\Bundle\ProjectBundle\Entity\Project $project
     * @return ProjectHasMedia
     */
    public function setProject(Project $project = null)
    {
        $this->project = $project;

        return $this;
    }

    /**
     * Get project
     *
     * @return \Acme\Bundle\ProjectBundle\Entity\Project
     */
    public function getProject()
    {
        return $this->project;
    }

    /**
     * Get gallery
     *
     * @return Gallery
     */
    public function getGallery()
    {
        return $this->gallery;
    }

    /**
    * Set gallery
    *
    * @param Gallery
    * @return ProjectHasMedia
    */
    public function setGallery(Gallery $gallery)
    {

        $this->gallery = $gallery;

        return $this;
    }
}

ProjectHasMediaAdmin.php

<?php

namespace Acme\Bundle\ProjectBundle\Admin;

use Sonata\AdminBundle\Admin\Admin;
use Sonata\AdminBundle\Datagrid\ListMapper;
use Sonata\AdminBundle\Datagrid\DatagridMapper;
use Sonata\AdminBundle\Form\FormMapper;

class ProjectHasMediaAdmin extends Admin
{
    protected $context = 'default';

    protected function configureFormFields(FormMapper $formMapper)
    {
        $formMapper
            ->add('gallery', 'sonata_type_model_list', array(
                'help'              => 'Each individual gallery will be displayed as a carousel.',
                'required'          => false,
            ), array(
                'admin_code' => 'sonata.media.admin.gallery',
            ))
        ;
    }
}