@Entity
@Table(name = "user_properties")
@AssociationOverrides({
   @AssociationOverride(name="id.user", joinColumns = @JoinColumn(name="user_id", nullable=false)),
   @AssociationOverride(name="id.project", joinColumns = @JoinColumn(name="project_id", nullable=false))
})
@AttributeOverrides({
   @AttributeOverride(name = "id.property_name", column = @Column(name = "property_name", nullable = false))
})
public class WebAppUserProperty implements Serializable {
   private static final long serialVersionUID = -1325251396121465392L;

   @EmbeddedId
   private UserPropertyPK id = new UserPropertyPK();

   @Column(name = "property_value", nullable = false)
   private String property_value; // value

   // Getter, Setter, ...
}
@Embeddable
class UserPropertyPK implements Serializable {
   private static final long serialVersionUID = 572566244363842292L;

   @ManyToOne
   private WebAppUser user; // user

   @ManyToOne
   private WebAppProject project; // project

   @Column(name = "property_name", nullable = false)
   private String property_name; // name

   @Override
   public boolean equals(Object obj) {
      if (this == obj) {
         return true;
      }
      if (!(obj instanceof UserPropertyPK)) {
         return false;
      }

      final UserPropertyPK castOther = (UserPropertyPK) obj;

      return new EqualsBuilder().append(user, castOther.user)
            .append(project, castOther.project).append(property_name, castOther.property_name).isEquals();
   }

   @Override
   public int hashCode() {
      return new HashCodeBuilder().append(user).append(project).append(property_name).toHashCode();
   }

   // Getter, Setter, ...
}

@Override
public void setProperty(WebAppUserProperty webAppUserProperty) {
   sessionFactory.getCurrentSession().saveOrUpdate(webAppUserProperty);
}