@Entity
public class Tag implements Serializable {
...
    private Set<Document> taggedDocuments;
...
    @ManyToMany(mappedBy="tags")
    public Set<Document> getTaggedDocuments() {
       return taggedDocuments;
    }
...
}

@Proxy(lazy=true)
@Entity
@Inheritance(strategy=InheritanceType.JOINED)
@Table(uniqueConstraints = {@UniqueConstraint(columnNames = {"hashkey","source_id"})})
@Indexed(index=Indexable.DOCUMENT_INDEX_DIR)
@NamedQueries(value={@NamedQuery(name="document.byIds", query="select d from Document d where d.id in (:ids)")})
public class Document implements Serializable {
...
    protected Set<Tag> tags;
...
    @ManyToMany(fetch=FetchType.LAZY, cascade={CascadeType.MERGE, CascadeType.PERSIST, CascadeType.REFRESH})
    public Set<Tag> getTags() {
       return tags;
    }
...
}

public class TagPersistenceTest extends TestCase {
...
    public void testSaveTagAndTaggedDocument() {

      this.tag = new Tag();
      //save Tag
      this.persistenceManager.saveOrUpdate(this.tag);
            
                //create Document
      this.document = new Document();

                //set Document's tags
      Set<Tag> documentTags = new HashSet<Tag>();
      documentTags.add(this.tag);
      this.document.setTags(documentTags);

      //save document reference into Tag
      Set<Document> taggedDocuments = new HashSet<Document>();
      taggedDocuments.add(this.document);
      this.tag.setTaggedDocuments(taggedDocuments);
      //update tag
      this.persistenceManager.saveOrUpdate(this.tag);
      
      //save Document
      this.persistenceManager.saveOrUpdate(this.document);

                //some tests: this are OK
      List<Document> list = persistenceManager.findAll(Document.class);
      Document docFromDB=list.get(0);
      Set<Tag> tagsOfDocFromDB = docFromDB.getTags();
      Iterator iter = tagsOfDocFromDB.iterator();
      Tag documentTag=(Tag) iter.next();
      assertEquals("Tag returned should be equal to the Tag passed",this.tag,documentTag);
      assertEquals("Tag returned should have the same id of the Tag passed",this.tag.getId(),documentTag.getId());
      assertEquals("Tag returned should store only one document",1,documentTag.getTaggedDocuments().size());

                //trying to remove Document and some tests: also this tests are OK
      this.persistenceManager.remove(docFromDB);
      assertEquals("No one Document should be present in database", 0, this.persistenceManager.findAll(Document.class).size());
      assertEquals("After removing the document, the tag should still be present in database", 1, this.persistenceManager.findAll(Tag.class).size());

                //PROBLEMATIC TEST: this is NOT OK
      assertTrue("Tag should have no tagged documents",taggedDocumentsOfReturnedTag.isEmpty());

    }
...
}