@Entity
static class Loan {
    @Id
    @Column(name = "LOAN_ID")
    private Long id;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "loan")
    private List<Extension> extensions = new ArrayList<>();
}

@Entity
static class Extension {
    @Id
    @Column(name = "EX_LOAN_ID")
    private Long exLoanId;

    @Id
    @Column(name = "EX_NO")
    private Long exNo;

    @Column(name = "EX_EXTENSION_DAYS")
    private Long exExtensionDays;

    @ManyToOne @MapsId
    @JoinColumn(name = "EX_LOAN_ID")
    private Loan loan;
}

@Entity
static class Loan {
    @Id
    @Column(name = "LOAN_ID")
    private Long id;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "loan")
    private List<Extension> extensions = new ArrayList<>();
}

@Entity
static class Extension {
    @Id
    @Column(name = "EX_LOAN_ID")
    private Long exLoanId;

    @Id
    @Column(name = "EX_NO")
    private Long exNo;

    @Column(name = "EX_EXTENSION_DAYS")
    private Long exExtensionDays;

    @ManyToOne
    @JoinColumn(name = "EX_LOAN_ID", 
                insertable = false, updatable = false)
    private Loan loan;
}