How to create a play-morphia model which contains multiple gridfs blobs

Album.java

package models;

import com.google.code.morphia.annotations.Entity;
import play.modules.morphia.Model;

import java.util.List;

/**
 * An album could store multiple photos persistent in the gridfs
 */
@Entity(value = "album", noClassnameStored = true)
public class Album extends Model {
    ...
    @Reference
    public List<Asset> photos;

    @Reference
    public Asset frontImage;

    @Reference
    public Asset thumbnail;
    ...
}

Resource.java

package models;

import com.google.code.morphia.annotations.Entity;
import play.data.validation.MaxSize;
import play.modules.morphia.Blob;
import play.modules.morphia.Model;

import javax.activation.MimetypesFileTypeMap;
import java.io.File;

/**
 * Store blob type assets, e.g. images, videos etc
 */
@Entity(value = "asset", noClassnameStored = true)
public class Asset extends Model {

    public Blob content;

    @MaxSize(10000)
    public String description;

    public Asset(File file) {
        content = new Blob(file);
    }
    ...
}