"detached entity passed to persist error" with JPA/EJB code

The error occurs because the object's ID is set. Hibernate distinguishes between transient and detached objects and persist works only with transient objects. If persist concludes the object is detached (which it will because the ID is set), it will return the "detached object passed to persist" error. You can find more details here and here.

However, this only applies if you have specified the primary key to be auto-generated: if the field is configured to always be set manually, then your code works.

Let's say you have two entities Album and Photo. Album contains many photos, so it's a one to many relationship.

Album class

@Entity
public class Album {
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    Integer albumId;

    String albumName;

    @OneToMany(targetEntity=Photo.class,mappedBy="album",cascade={CascadeType.ALL},orphanRemoval=true)
    Set<Photo> photos = new HashSet<Photo>();
}

Photo class

@Entity
public class Photo{
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    Integer photo_id;

    String photoName;

    @ManyToOne(targetEntity=Album.class)
    @JoinColumn(name="album_id")
    Album album;

}

What you have to do before persist or merge is to set the Album reference in each photos.

        Album myAlbum = new Album();
        Photo photo1 = new Photo();
        Photo photo2 = new Photo();

        photo1.setAlbum(myAlbum);
        photo2.setAlbum(myAlbum);       

That is how to attach the related entity before you persist or merge.