我创建了两个类Musician和Band，它们与类Album有很多关系。我还使用PostgreSQL数据库来反映关系数据库中的类。我使用Postman创建了几个Musician，Band和Album实例。我还手动在许多表（album_band和album_musician）中的postgres命令行关系中创建了代码。但是，当我在Band，Musician或Album上执行GET请求时，我没有从对应关系中获取元素列表，只有类本身的组件（例如，Musician包含字段name，，surname和dateOfBirth（与List<Album> albums类有很多关系），我只能检索Album，name和surname（dateOfBirth保持为空）。下面是类和表。List<Album> albums类：
@Entity
@Table(name="album")
public class Album {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
@Column(name="title")
private String title;
@ManyToMany(targetEntity = Band.class, mappedBy = "albums")
private List<Band> bands;
@ManyToMany(targetEntity = Musician.class, mappedBy = "albums")
private List<Musician> musicians;
@Embedded
@Column(name="duration")
private Duration duration;
@Column(name="dateofrelease")
@JsonFormat(shape=JsonFormat.Shape.STRING, pattern="dd/MM/yyyy", timezone="CET")
private Date dateOfRelease;
@Column(name="coverpath")
private String coverPath;
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public Duration getDuration() {
return duration;
}
public void setDuration(Duration duration) {
this.duration = duration;
}
public Date getDateOfRelease() {
return dateOfRelease;
}
public void setDateOfRelease(Date dateOfRelease) {
this.dateOfRelease = dateOfRelease;
}
public String getCoverPath() {
return coverPath;
}
public void setCoverPath(String coverPath) {
this.coverPath = coverPath;
}
public List<Band> getBands() {
return bands;
}
public void setBands(List<Band> bands) {
this.bands = bands;
}
public List<Musician> getMusicians() {
return musicians;
}
public void setMusicians(List<Musician> musicians) {
this.musicians = musicians;
}
}
Album类：@Entity
@Table(name="musician")
public class Musician {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
@Column(name="name")
private String name;
@Column(name="surname")
private String surname;
@Column(name="dateofbirth")
@JsonFormat(shape=JsonFormat.Shape.STRING, pattern="dd/MM/yyyy", timezone="CET")
private Date dateOfBirth;
@ManyToMany(cascade = CascadeType.ALL)
@JoinTable(name = "album_musician",
joinColumns = @JoinColumn(name = "album_id", referencedColumnName = "id"),
inverseJoinColumns = @JoinColumn(name = "musician_id",
referencedColumnName = "id"))
private List<Album> albums;
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSurname() {
return surname;
}
public void setSurname(String surname) {
this.surname = surname;
}
public Date getDateOfBirth() {
return dateOfBirth;
}
public void setDateOfBirth(Date dateOfBirth) {
this.dateOfBirth = dateOfBirth;
}
public List<Album> getAlbums() {
return albums;
}
public void setAlbums(List<Album> albums) {
this.albums = albums;
}
}
Musician类：@Entity
@Table(name="band")
public class Band {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
@Column(name="name")
private String name;
@ManyToMany(cascade = CascadeType.ALL)
@JoinTable(name = "album_band",
joinColumns = @JoinColumn(name = "album_id", referencedColumnName = "id"),
inverseJoinColumns = @JoinColumn(name = "band_id",
referencedColumnName = "id"))
private List<Album> albums;
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Album> getAlbums() {
return albums;
}
public void setAlbums(List<Album> albums) {
this.albums = albums;
}
}
Band表： Table "public.album"
Column | Type | Collation | Nullable | Default
---------------+---------+-----------+----------+---------
id | bigint | | not null |
title | text | | |
dateofrelease | date | | |
coverpath | text | | |
hours | integer | | |
minutes | integer | | |
seconds | integer | | |
Indexes:
"album_pkey" PRIMARY KEY, btree (id)
Referenced by:
TABLE "album_band" CONSTRAINT "album_band_album_id_fkey" FOREIGN KEY (album_id) REFERENCES album(id)
TABLE "album_musician" CONSTRAINT "album_musician_album_id_fkey" FOREIGN KEY (album_id) REFERENCES album(id)
album表： Table "public.musician"
Column | Type | Collation | Nullable | Default
-------------+--------+-----------+----------+---------
id | bigint | | not null |
name | text | | |
surname | text | | |
dateofbirth | date | | |
Indexes:
"musician_pkey" PRIMARY KEY, btree (id)
Referenced by:
TABLE "album_musician" CONSTRAINT "album_musician_musician_id_fkey" FOREIGN KEY (musician_id) REFERENCES musician(id)
musician表： Table "public.band"
Column | Type | Collation | Nullable | Default
--------+--------+-----------+----------+---------
id | bigint | | not null |
name | text | | |
Indexes:
"band_pkey" PRIMARY KEY, btree (id)
Referenced by:
TABLE "album_band" CONSTRAINT "album_band_band_id_fkey" FOREIGN KEY (band_id) REFERENCES band(id)
band表： Table "public.album_musician"
Column | Type | Collation | Nullable | Default
-------------+--------+-----------+----------+---------
album_id | bigint | | not null |
musician_id | bigint | | not null |
Indexes:
"album_musician_pkey" PRIMARY KEY, btree (album_id, musician_id)
Foreign-key constraints:
"album_musician_album_id_fkey" FOREIGN KEY (album_id) REFERENCES album(id)
"album_musician_musician_id_fkey" FOREIGN KEY (musician_id) REFERENCES musician(id)
album_musician表： Table "public.album_band"
Column | Type | Collation | Nullable | Default
----------+--------+-----------+----------+---------
album_id | bigint | | not null |
band_id | bigint | | not null |
Indexes:
"album_band_pkey" PRIMARY KEY, btree (album_id, band_id)
Foreign-key constraints:
"album_band_album_id_fkey" FOREIGN KEY (album_id) REFERENCES album(id)
"album_band_band_id_fkey" FOREIGN KEY (band_id) REFERENCES band(id)
最佳答案
我假设您是手动创建联接表album_band的，但这不是必需的。让Spring JPA自动为您创建表，从而提高兼容性。
关于java - 恢复多对多关系元素的问题，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/57458928/