@Id
    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumns({
            @JoinColumn(name = "column1", referencedColumnName = "column1"), // shared
            @JoinColumn(name = "column2", referencedColumnName = "column2"), // shared
            @JoinColumn(name = "column3", referencedColumnName = "column3"), // shared
            @JoinColumn(name = "column4", referencedColumnName = "column4"),
            @JoinColumn(name = "column5", referencedColumnName = "column5")
    })
    private A a;

    @Id
    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumns({
            @JoinColumn(name = "column6", referencedColumnName = "column1"), // shared
            @JoinColumn(name = "column7", referencedColumnName = "column2"), // shared
            @JoinColumn(name = "column8", referencedColumnName = "column3"), // shared
            @JoinColumn(name = "column9", referencedColumnName = "column6")
    })
    private M m;