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现在，在一个简短的示例中，如果我有两个实体Person并且Applicant我想编写一个查询来获取我的数据库中的所有申请人，我最终会通过以下设置获得引用的查询。我正在使用 Single_Table_Inheritance 策略（仅供参考）

public class Person {

    @ManyToOne(fetch = FetchType.LAZY) 
    @JoinColumn(name = "schoolID")
    private School school;
}

引用另一个实体的 Teacher 对象。

public class Applicant extends Person {

  @OneToMany(targetEntity = ApplicantSchool.class, cascade = { javax.persistence.CascadeType.PERSIST, javax.persistence.CascadeType.REMOVE }, mappedBy = "applicant")
    private Set<ApplicantSchool> schools = new HashSet<ApplicantSchool>();
}

在Person中跳转并尝试查询学校关系的查询尝试

 public List<Applicant> getAllApplicants(){

        EntityManager entityManager = factory.createEntityManager();
        @SuppressWarnings("unchecked")
        List<Applicant> applicants = entityManager.createQuery("Select a from Applicant a LEFT JOIN FETCH a.schools WHERE a.active = :active ")
        .setParameter("active", true).setHint("eclipselink.refresh", "true")
        .getResultList();
        //List<Applicant> applicants =  Applicant.findAllApplicants();
        System.out.println("The long query getting applicants");
        entityManager.close();
        return applicants;  
    }

如果我尝试将查询更改为使用嵌套连接，则会收到一条错误消息，提示无法识别关系。我试图这样查询：

entityManager.createQuery("Select a from Applicant a LEFT JOIN FETCH a.schools WHERE LEFT JOIN FETCH a.person.school a.active = :active ")

更新

删除关键字“left join fetch”后，我的新查询：

List<Applicant> applicants = entityManager.createQuery("Select a from Applicant a WHERE a.active = :active ")
        .setParameter("active", true)
        .getResultList();

我同意它首先创建的唯一查询是这个

SELECT personID, TYPE, DATEADDED, FIRSTNAME, LASTNAME, MIDDLENAME, ACTIVE, BIRTHDAY, EMAILADDRESS, ETHNICITY, GENDER, HISPANIC, IMAGEPATH, MARITAL, NATIVELANGUAGE, PRIMARYTELEPHONE, RELIGIOUSAFFILIATION, SECONDARYTELEPHONE, version, addressID, schoolID, MAJOR FROM PERSON WHERE ((ACTIVE = ?) AND (TYPE = ?))
    bind => [true, AP]

返回此堆栈跟踪并创建以下 sql 选择：

[EL Fine]: sql: 2012-07-13 17:25:52.297--ServerSession(1529073996)--Connection(726700617)--Thread(Thread["http-bio-8080"-exec-18,5,main])--SELECT schoolID, ACTIVE, ADMISSIONSEMAILADDRESS, ADMISSIONSPHONE, CODE, description, HELPGENERALEMAILADDRESS, NAME, PRIMARYPHONE, version, addressID FROM SCHOOL WHERE (schoolID = ?)
    bind => [1]
[EL Fine]: sql: 2012-07-13 17:25:52.305--ServerSession(1529073996)--Connection(1615948530)--Thread(Thread["http-bio-8080"-exec-18,5,main])--SELECT id, ACTIVE, CODE, DESCRIPTION, NAME, version, SCHOOLDEPARTMENT_schoolID FROM DEPARTMENT WHERE (SCHOOLDEPARTMENT_schoolID = ?)
    bind => [1]
[EL Fine]: sql: 2012-07-13 17:25:52.308--ServerSession(1529073996)--Connection(893810654)--Thread(Thread["http-bio-8080"-exec-18,5,main])--SELECT programID, ACTIVE, CODE, description, NAME, PROGRAMTYPE, REQUIREDCREDITS, version, SCHOOL_schoolID FROM PROGRAM WHERE (SCHOOL_schoolID = ?)
    bind => [1]
[EL Fine]: sql: 2012-07-13 17:25:52.31--ServerSession(1529073996)--Connection(399107363)--Thread(Thread["http-bio-8080"-exec-18,5,main])--SELECT personID, TYPE, DATEADDED, FIRSTNAME, LASTNAME, MIDDLENAME, ACTIVE, BIRTHDAY, EMAILADDRESS, ETHNICITY, GENDER, HISPANIC, IMAGEPATH, MARITAL, NATIVELANGUAGE, PRIMARYTELEPHONE, RELIGIOUSAFFILIATION, SECONDARYTELEPHONE, version, addressID, schoolID, MAJOR, studentId FROM PERSON WHERE (schoolID = ?)
    bind => [1]
[EL Fine]: sql: 2012-07-13 17:25:52.314--ServerSession(1529073996)--Connection(464642021)--Thread(Thread["http-bio-8080"-exec-18,5,main])--SELECT registrationId, SEMESTERTYPE, version, YEAR, semesterCourseId, personID FROM REGISTRATION WHERE (personID = ?)
    bind => [501]
[EL Fine]: sql: 2012-07-13 17:25:52.322--ServerSession(1529073996)--Connection(395757277)--Thread(Thread["http-bio-8080"-exec-18,5,main])--SELECT registrationId, SEMESTERTYPE, version, YEAR, semesterCourseId, personID FROM REGISTRATION WHERE (personID = ?)
    bind => [552]
[EL Fine]: sql: 2012-07-13 17:25:52.327--ServerSession(1529073996)--Connection(328677993)--Thread(Thread["http-bio-8080"-exec-18,5,main])--SELECT registrationId, SEMESTERTYPE, version, YEAR, semesterCourseId, personID FROM REGISTRATION WHERE (personID = ?)
    bind => [602]
[EL Fine]: sql: 2012-07-13 17:25:52.358--ServerSession(1529073996)--Connection(873312397)--Thread(Thread["http-bio-8080"-exec-18,5,main])--SELECT addressID, CITY, COUNTRY, STATE_US, STREETADDRESS, STREETADDRESS2, version, ZIPCODE FROM ADDRESS WHERE (addressID = ?)
    bind => [2]
[EL Fine]: sql: 2012-07-13 17:25:52.365--ServerSession(1529073996)--Connection(509650638)--Thread(Thread["http-bio-8080"-exec-18,5,main])--SELECT applicantSchoolID, CREDITSCOMPLETED, FROMMONTH, FROMYEAR, GPA, NAME, SCHOOLTYPE, TOMONTH, TOYEAR, version, APPLICANT_personID FROM APPLICANTSCHOOL WHERE (APPLICANT_personID = ?)
    bind => [151]
[EL Fine]: sql: 2012-07-13 17:25:52.385--ServerSession(1529073996)--Connection(17334753)--Thread(Thread["http-bio-8080"-exec-18,5,main])--SELECT applicantSchoolID, CREDITSCOMPLETED, FROMMONTH, FROMYEAR, GPA, NAME, SCHOOLTYPE, TOMONTH, TOYEAR, version, APPLICANT_personID FROM APPLICANTSCHOOL WHERE (APPLICANT_personID = ?)
    bind => [51]
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1 回答 1

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您不能在 JPA 中嵌套连接，但在查询申请人时不需要这样做。
JPQL“从申请人中选择一个 LEFT JOIN FETCH a.schools LEFT JOIN FETCH a.school where a.active = :active ”应该可以工作，因为申请人是一个人，因此有学校关系。仅当申请人与人有关系并且您需要获取加入人和学校时才可以嵌套。

如果是这种情况，EclipseLink 允许通过此处描述的查询提示进行批量读取或连接：http: //wiki.eclipse.org/EclipseLink/Examples/JPA/QueryOptimization

您还可以使用此处描述的 EclipseLink 注释在映射级别指定批处理/获取连接：http: //www.eclipse.org/eclipselink/documentation/2.4/jpa/extensions/a_batchfetch.htm#CHDCCIDA 和 http://www。 eclipse.org/eclipselink/documentation/2.4/jpa/extensions/a_joinfetch.htm#CEGFFHHA

批量读取将产生 2 个查询，而 fetch joins 产生 1 个。

对于上述查询，您还可以使用：

em.createQuery("Select a from Applicant a WHERE a.active = :active ")
         .setParameter("active", true)
         .setQueryHint("eclipselink.left-join-fetch", "a.schools");
         .setQueryHint("eclipselink.left-join-fetch", "a.school");
         .getResultList();
于 2012-07-16T17:10:43.810 回答