Question

我面临着n + 1个选择查询的奇怪问题。我的映射看起来像这样：

@Entity
@IdClass(MyTablePK.class)
@Table(name = "my_table", schema = "schema")
public class MyTable {

    @Id
    @Column(name = "name", nullable = false, length = 12)
    private String name="";

    @Id
    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "myStringValue", referencedColumnName = "myStringValue")
    private AdditionalData data;

    ... (other fields, getters, setters)
}

public class MyTablePK implements Serializable {

    private String name;

    private AdditionalData data;

    (getters,setters)

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        MyTablePK that = (MyTablePK) o;

        if (name!= null ? !name.equals(that.name) : that.name!= null) return false;
        return !(data!= null ? !data.equals(that.data) : that.data!= null);

    }

    @Override
    public int hashCode() {
        int result = name!= null ? name.hashCode() : 0;
        result = 31 * result + (data!= null ? data.hashCode() : 0);
        return result;
    }
}

@Entity
@Table(name = "info", schema = "schema")
public class AdditionalData implements Serializable {

    @Id
    @Column(name = "recno")
    private Long recno;

    @Column(name = "info1", length = 3)
    private String info1;

    @Column(name = "info2", length = 3)
    private String info2;

    ... (getters, setters)

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        AdditionalData data = (AdditionalData) o;

        return recno.equals(data.recno);

    }

    @Override
    public int hashCode() {
        return recno.hashCode();
    }
}

现在，我从MyTable中选择所有值。正如预期的那样，我得到n + 1个选择，对于每个MyTable行，一个新的AdditionalData查询到达。为了打架，我写了一个连接获取查询：

FROM MyTable mytab join fetch mytab.data

然而......并没有改变任何事情。

现在，有趣的是，如果我暂时忽略业务需求，并删除@IdClass使name成为唯一的@Id - 一切正常，所有数据都通过单个查询获得。这是为什么？我不能用复合id的一部分来对抗n + 1选择吗？

如果它是相关的 - 我使用Hibernate 4.3.5.Final和Oracle数据库

Answer 1

这可能与此处的已知问题有关：https://hibernate.atlassian.net/browse/HHH-10292

尝试将myStringValue列映射两次。在加入列中加入String，另一次加入AdditionalData时加入insertable = false, updatable = false。

Hibernate ManyToOne n + 1选择id

1 个答案: