QueryDSL/JPQL:如何建立联接查询? [英] QueryDSL / JPQL : how to build a join query?

问题描述

我试图阅读QueryDSL文档，但我仍然很困惑.我习惯于编写很多SQL，但这是我第一次真正使用QueryDSL w/JPQL(JPA2)时遇到的困难.

I've tried to read through the QueryDSL docs but I am still very confused. I'm accustomed to writing a lot of SQL, but this is my first real crack at using QueryDSL w/ JPQL (JPA2).

我有以下实体:

@Entity
public class Provider implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private Long id;

    @Version
    @Column(name = "version")
    private Integer version;


    private String name;

    @ManyToMany(cascade=CascadeType.ALL)
    @JoinTable(name = "provider_contact", joinColumns = @JoinColumn(name = "contact_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "provider_id", referencedColumnName = "id"))
    @OrderColumn
    private Collection<Contact> contact;
}

其中Contact是一个简单实体，其中pk的id.

where Contact is a simple entity with an id for a pk.

@Entity
public class Contact {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private Long id;

    /**
     * User first name
     */
    @NotNull
    private String firstName;

    /**
     * User last name
     */
    @NotNull
    private String lastName;
}

我正在尝试编写一个查询，该查询在给定特定的Contact.id和Provider.id的情况下返回Contact对象.如果Contact对象不是提供者的Contact集合的一部分，那么我正在寻找一个空值.

I'm trying to write a query which returns a Contact object given a specific Contact.id and Provider.id. If the Contact object is not a part of the Provider's Contact collection, I'm looking for a null value.

我尝试了以下操作:

public Contact getContact( long providerId, long contactId ){
    Predicate p = QProvider.provider.id.eq(providerId).and(QContact.contact.id.eq(contactId));
    JPQLQuery query = new JPAQuery(em);
    return query.from(QProvider.provider).innerJoin(QProvider.provider.contact).where(p).singleResult(QContact.contact);
}

但出现以下错误:

Caused by: java.lang.IllegalArgumentException: Undeclared path 'contact'. Add this path as a source to the query to be able to reference it.
    at com.mysema.query.types.ValidatingVisitor.visit(ValidatingVisitor.java:78)
    at com.mysema.query.types.ValidatingVisitor.visit(ValidatingVisitor.java:30)
    at com.mysema.query.types.PathImpl.accept(PathImpl.java:94)

我认为这与我的谓词引用QContact.contact方向而不是QProvider.provider.contact对象的一部分有关，但是我真的很茫然，无法确定应该如何处理完成.

I'm presuming it has something to do with the fact that my predicate references QContact.contact direction and not part of the QProvider.provider.contact object, but I'm really at a loss as to figure out how this should be done.

我甚至在正确的轨道上吗?我什至也不确定我的加入是否正确.

Am I even on the right track? I'm not even sure my join is correct either.

推荐答案

这应该有效

public Contact getContact(long providerId, long contactId) {
    QProvider provider = QProvider.provider;
    QContact contact = QContact.contact;
    return new JPAQuery(em).from(provider)
        .innerJoin(provider.contact, contact)
        .where(provider.id.eq(providerId), contact.id.eq(contactId))
        .singleResult(contact);
}

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