@Entity
@IdClass(AssigneeId.class)
public class Assignee {
        @Id
        private String pid;

        @Id
        private String tid;

        @OneToOne(cascade = CascadeType.ALL)
        @JoinColumns({
                @JoinColumn(name = "pid", referencedColumnName="project", insertable = false, updatable = false),
                @JoinColumn(name = "tid", referencedColumnName="tid", insertable = false, updatable = false)
        })
        private Task task;
        ...
}

@Entity
public class Task {
        @Id
        private String name;

        private String project;
        private String tid;
        ...
}