Question

现在，在一个简短的例子中，如果我有两个实体Person和Applicant并且我想编写一个查询来获取我的数据库中的所有申请人，我最终会获得引用查询以下设置。我正在使用Single_Table_Inheritance策略（fyi）

public class Person {

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

带有对另一个实体的引用的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>();
}

跳转并尝试在Person中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;  
    }

如果我尝试更改我的查询以使用嵌套加入，我会收到一条错误，指出关系无法识别。我试着像这样查询：

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

已更新

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();

我同意的唯一一个问题是它首先创建的是

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的选择：

[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]

Answer 1

您无法在JPA中嵌套联接，但在查询申请人时不需要 JPQL“从申请人中选择一个LEFT JOIN FETCH a.schools LEFT JOIN FETCH a.school，其中a.active =：active”应该有效，因为Appliant是一个人，所以学校关系也是如此。只有当申请人与人有关系并且您需要获取人和学校的加入时，才能进行嵌套。

如果是这种情况，EclipseLink允许批量读取或通过此处描述的查询提示进行连接： 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/提取连接： 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

批量读取将导致2个查询，而在1中获取连接。

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();

EclipseLink继承查询：避免加入

1 个答案: