Spring Boot实体、关系和存储库混淆

Question

我对关系如何在实体中工作以及这对我的JPA存储库意味着什么感到困惑。

我有一个名为Loan的类，它为每个贷款存储一个专辑列表。

我有一个贷款库和一个专辑库。当我启动应用程序时，相册存储库中充满了相册。该专辑是自动生成的。

@Entity
public class Album implements Serializable {

private static final long serialVersionUID = 0x63A6DA99AA12AAA8L;

@Column @GeneratedValue(strategy = GenerationType.AUTO) @Id private Integer albumId;
@Column (unique=true) private String barcode;
@Column private String band;
@Column private String title;
@Column private String genre;
@Column private Integer year;
@Column private String artworkFilename;
@Column private Boolean enabled;
@Column private Boolean isLoanable;
@Column private Integer numberOfCopies;

@ManyToOne
private Loan loan;

public class Loan implements Serializable {

private static final long serialVersionUID = 0x62B6DA99AA12AAA8L;

public void setLoanId(Integer loanId) {
    this.loanId = loanId;
}

@Column @GeneratedValue(strategy = GenerationType.AUTO) @Id private Integer loanId;

@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
private List<Album> albums= new ArrayList<>();

@Column private Integer customerId;
@Column private Date dateLoaned;
@Column private Date dateToReturn;
@Column private Boolean expired;

我也很困惑为什么专辑必须引用带有ManyToOne注释的贷款。为什么相册必须提到贷款？

我大多习惯于关系数据库，所以也许我在以错误的方式思考事情。如果我只能添加新的专辑到贷款，那么它失败的目的，我正在努力做。

Answer 1

    Loan and Album tables have a one-to-many relationship.
    So in Album your mapping should be like below:
    @ManyToOne
    @JoinColumn(name = "loan_id")
    private Loan loan;
Considering loan_id is the primary key of Loan.
And in Loan your mapping should be like below:
@OneToMany(mappedBy = "loan", cascade = CascadeType.ALL)
    private List<Album> albums;

@OneToMany and @ManyToOne defines a one-to-many and many-to-one relationship between 2 entities. @JoinColumn indicates the entity is the owner of the relationship: the corresponding table has a column with a foreign key to the referenced table. mappedBy indicates the entity is the inverse of the relationship