I'm trying to implement a model for private messages between two or more users. That means I've got two Entities: User PrivateMessage The User model shouldn't be edited, so I'm trying to set up an unidirectional relationship: @Entity (name = "User") @Table (name = "user") public class User implements Serializable { @Id String username; String password; // something like this ... } The PrivateMessage Model addresses multiple receivers and has exactly one sender. So I need something like this: @Entity (name = "PrivateMessage") @Table (name = "privateMessage") @XmlRootElement @XmlType (propOrder = {"id", "sender", "receivers", "title", "text", "date", "read"}) public class PrivateMessage implements Serializable { private static final long serialVersionUID = -9126766942868177246L; @Id @GeneratedValue private Long id; @NotNull private String title; @NotNull private String text; @NotNull @Temporal(TemporalType.TIMESTAMP) private Date date; @NotNull private boolean read; @NotNull @ElementCollection(fetch = FetchType.EAGER, targetClass = User.class) private Set<User> receivers; @NotNull @OneToOne private User sender; // and so on } The according 'privateMessage' table won't be generated and just the relationship between the PM and the many receivers is satisfied. I'm confused about this. Everytime I try to set a 'mappedBy' attribute, my IDE marks it as an error. It seems to be a problem that the User-entity isn't aware of the private message which maps it. What am I doing wrong here? I've solved some situation similar to this one, but none of those solutions will work here. Thanks in advance!