为什么 Hibernate 期望我的复合主键有一个单独的表？ (@EmbeddedId)答案

【问题标题】：Why is Hibernate expecting a separate table for my composite primary key? (@EmbeddedId)为什么 Hibernate 期望我的复合主键有一个单独的表？ (@EmbeddedId)
【发布时间】：2021-10-28 00:40:34
【问题描述】：

我正在开发一个排行榜系统作为一个爱好项目。我有三个实体：

Player 代表排行榜上的玩家
StatKey 代表排行榜上某个统计数据的键
PlayerStat 代表给定玩家的特定统计数据，应该有一个由 Player 和 StatKey 组成的复合主键，以及一个将其映射到相关 StatKey 和父 Player 实体的外键。

我对整个 JPA 感到不知所措。在创建复合主键时，我仍然不确定@EmbeddedId 与@IdClass 是否是我最好的选择。根据我在网上看到的，似乎最好的方法是使用@EmbeddedId。

但是，这会导致一些非常奇怪的查询和表结构。我本来期望三个表：

玩家（pk(id)，username，total_level，total_experience，date_created，last_updated）
player_stat (pk(player_id, stat_id), level, experience, date_created, last_updated)
statkey (pk(id), key)

还有两个外键：

player_stat (player_id) 引用 player
player_stat (stat_id) 引用 statkey

所有这些都按预期生成。但是，除了这三个表之外，还会生成第四个表：

player_stats (pk(player_id, stats_player_id, stats_stat_id))

以及以下外键：

player_stats (stats_player_id, stats_stat_id) 引用 player_stat
player_stats (player_id) 引用 player

奇怪的第四张表不仅是 DDL 问题，而且在尝试查询数据库时也会用到。我不知所措。任何帮助将不胜感激！

代码：

@Entity
@Table(name = "player", uniqueConstraints = @UniqueConstraint(name = "player_uk_username", columnNames = "username"))
public class Player {

    @Id
    @Column(nullable = false)
    private UUID id;

    @Column(nullable = false)
    private String username;

    @Column(name = "total_level", nullable = false)
    private int totalLevel;

    @Column(name = "total_experience", nullable = false)
    private long totalExperience;

    @OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    private Set<PlayerStat> stats;

    @Column(name = "date_created", nullable = false, updatable = false)
    private OffsetDateTime dateCreated;

    @Column(name = "last_updated", nullable = false)
    private OffsetDateTime lastUpdated;

    protected Player() {

    }

    public Player(final UUID id, final String username, final Set<PlayerStat> stats) {
        this.id = id;
        this.username = username;
        this.stats = stats;
    }

    @PrePersist
    public void prePersist() {
        this.dateCreated = OffsetDateTime.now();
        this.lastUpdated = this.dateCreated;
    }

    @PreUpdate
    public void preUpdate() {
        this.lastUpdated = OffsetDateTime.now();
    }
}

@Entity
@Table(name = "statkey", uniqueConstraints = @UniqueConstraint(name = "statkey_uk_key", columnNames = "key"))
public class StatKey {

    @Id
    @Column(nullable = false)
    private int id;

    @Column(nullable = false)
    private String key;

    @Column(name = "date_created", nullable = false, updatable = false)
    private OffsetDateTime dateCreated;

    @Column(name = "last_updated", nullable = false)
    private OffsetDateTime lastUpdated;

    protected StatKey() {

    }

    public StatKey(final int id, final String key) {
        this.id = id;
        this.key = key;
    }

    @PrePersist
    public void prePersist() {
        this.dateCreated = OffsetDateTime.now();
        this.lastUpdated = this.dateCreated;
    }

    @PreUpdate
    public void preUpdate() {
        this.lastUpdated = OffsetDateTime.now();
    }
}

@Embeddable
public class PlayerStatKey implements Serializable {

    @JsonUnwrapped(prefix = "player")
    @ManyToOne
    @JoinColumn(name = "player_id", referencedColumnName = "id", foreignKey = @ForeignKey(name = "player_stat_fk_player_id"))
    private Player player;

    @JsonUnwrapped(prefix = "stat")
    @ManyToOne
    @JoinColumn(name = "stat_id", referencedColumnName = "id", foreignKey = @ForeignKey(name = "player_stat_fk_statkey_id"))
    private StatKey statKey;

    protected PlayerStatKey() {

    }

    public PlayerStatKey(final Player player, final StatKey statKey) {
        this.player = player;
        this.statKey = statKey;
    }
}

@Entity
@Table(name = "player_stat")
public class PlayerStat {

    @EmbeddedId
    @JsonUnwrapped
    private PlayerStatKey key;

    @Column(nullable = false)
    private int level;

    @Column(nullable = false)
    private int experience;

    @Column(name = "date_created", nullable = false, updatable = false)
    private OffsetDateTime dateCreated;

    @Column(name = "last_updated", nullable = false)
    private OffsetDateTime lastUpdated;

    protected PlayerStat() {

    }

    public PlayerStat(final PlayerStatKey key, final int level, final int experience) {
        this.key = key;
        this.level = level;
        this.experience = experience;
    }
    

    @PrePersist
    public void prePersist() {
        this.dateCreated = OffsetDateTime.now();
        this.lastUpdated = this.dateCreated;
    }

    @PreUpdate
    public void preUpdate() {
        this.lastUpdated = OffsetDateTime.now();
    }
}

预期：

create table player (id binary not null, date_created timestamp not null, last_updated timestamp not null, total_experience bigint not null, total_level integer not null, username varchar(255) not null, primary key (id))
create table player_stat (date_created timestamp not null, experience integer not null, last_updated timestamp not null, level integer not null, player_id binary not null, stat_id integer not null, primary key (player_id, stat_id))
create table statkey (id integer not null, date_created timestamp not null, key varchar(255) not null, last_updated timestamp not null, primary key (id))
alter table player add constraint player_uk_username unique (username)
alter table statkey add constraint statkey_uk_key unique (key)
alter table player_stat add constraint player_stat_fk_player_id foreign key (player_id) references player
alter table player_stat add constraint player_stat_fk_statkey_id foreign key (stat_id) references statkey

现实：

create table player (id binary not null, date_created timestamp not null, last_updated timestamp not null, total_experience bigint not null, total_level integer not null, username varchar(255) not null, primary key (id))
create table player_stat (date_created timestamp not null, experience integer not null, last_updated timestamp not null, level integer not null, player_id binary not null, stat_id integer not null, primary key (player_id, stat_id))
create table player_stats (player_id binary not null, stats_player_id binary not null, stats_stat_id integer not null, primary key (player_id, stats_player_id, stats_stat_id))
create table statkey (id integer not null, date_created timestamp not null, key varchar(255) not null, last_updated timestamp not null, primary key (id))
alter table player add constraint player_uk_username unique (username)
alter table player_stats add constraint UK_9xeo0h5xwn53uq68knp5llr04 unique (stats_player_id, stats_stat_id)
alter table statkey add constraint statkey_uk_key unique (key)
alter table player_stat add constraint player_stat_fk_player_id foreign key (player_id) references player
alter table player_stat add constraint player_stat_fk_statkey_id foreign key (stat_id) references statkey
alter table player_stats add constraint FKrc4tc1jspyawwida0o2dtcp4j foreign key (stats_player_id, stats_stat_id) references player_stat
alter table player_stats add constraint FKfekdv3tvbrd0b8u6c2fuxk5fw foreign key (player_id) references player

【问题讨论】：

您对 PlayerStat 使用复合主键的计划。我不知道这是否是一个问题，但总的来说，我倾向于在表中使用一维主键。是的，这两个外键一起形成了一个复合键，但是想象一下需要添加第三个外键的情况。如果从那时起前两个可能会重复，那将包括对表的完全重组。此外，如果您的两个外键可能由于任何其他原因而重复，那么您再次需要完全重新配置您的表。
JPA 不知道您的内部项目相关逻辑，因此，从它的行为来看，它似乎在创建表（pk，fk1，fk2）时采取了安全第一的方法。为什么不尝试将 PlayerStat 定义为 PlayerStat(pk, fk1, fk2) 而不是以 (fk1, fk2) 为主键的 PlayerStat(fk1, fk2) ？如果您有一个超出内部数据库相关逻辑范围的数字自动递增主键，那么它将与 JPA 拼命强制执行的表定义相匹配。因此，也许修复就像以 JPA 生成附加表的方式生成表一样简单。

标签： java sql hibernate jpa

【解决方案1】：

除了oa54所说的：

这与 Spring Data JPA 无关。如果您有自动生成的 SQL，我假设您（意外地）使用了 JPA 提供程序的 SQL 生成功能，可能是 Hibernate。我会相应地更新问题标题。
我强烈建议您宁愿使用专用的 SQL 迁移工具（Flyway 或 Liquibase）来编写定义数据结构的 SQL，因为无论如何您将不得不求助于这样的东西。一旦您的类需要新字段，您就需要迁移数据、更改表而不是重新创建它们等。

【讨论】：

欣赏！此外，对于它的价值，虽然我绝对同意＃2，但在这种情况下，奇怪的一代也反映在它所做的查询中（即它期望并需要额外的表才能运行）。跨度>

【解决方案2】：

问题似乎是缺少从父实体到子实体 PlayerStat 的映射。

解决方案是将以下内容添加到StatKey：

@OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL, mappedBy = "primaryKey.statKey")
private Set<PlayerStat> stats;

以及将mappedBy = "primaryKey.player" 添加到Player 中的@OneToMany 注释。

【讨论】：