Message Title

This looks accurate, in our code, the following will trigger the exception being thrown because @ManyToMany's mappedBy is not defined:

    @ManyToMany(cascade = { CascadeType.ALL }, fetch = FetchType.LAZY)
    @JoinTable(name = "collection_resource", joinColumns = { @JoinColumn(nullable = false, name = "resource_id") }, inverseJoinColumns = { @JoinColumn(
            nullable = false, name = "collection_id") })
    @XmlTransient
    @IndexedEmbedded(depth = 2)
    @Cache(usage = CacheConcurrencyStrategy.TRANSACTIONAL, region = "org.tdar.core.bean.resource.Resource.resourceCollections")
    private Set<ResourceCollection> resourceCollections = new LinkedHashSet<ResourceCollection>();

...
    String mappedBy() default "";