Jakarta ee 获取集合大小的标准API

我有两个实体： 第一人称表人称

@Entity
public class Person implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", nullable = false)
    private Integer id;
    @Column(name = "name", nullable = false, length = 2147483647)
    private String name;
    @Column(name = "first_name", nullable = false, length = 2147483647)
    private String firstName;
    @OneToMany(cascade = CascadeType.ALL, mappedBy = "peopleId")
    private List<PeopleEmail> peopleEmailList;

    //... constuctors
    //... getters setters
}

正如您所看到的，这两个实体都处于一对多关系中。 我想创建另一个类：

public class PersonAndCompany{
   private String personName;
   private String companyName;
   private int emailCount;

    //... constuctors
    //... getters setters
}

我想写一个typequery，用人名和公司名填充PersonAndCompany.class字段，另一个类和电子邮件数，其中个人电子邮件数超过2。我想使用标准api。我写了一些代码，但我不知道如何在PersonAndCompany.class中添加条件where和填充emailcount

CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<PersonAndCompany> cq = cb.createQuery(PersonAndCompany.class);
Root<Person> person = cq.from(Person.class);
Join<Person, Company> company = person.join(Person_.companyId);
cq.where(cb.greaterThan(cb.size(person.get(Person_.peopleEmailList)), 2));

Selection<PersonAndCompany> select = cb.construct(PersonAndCompany.class,
                        person.get(Person_.firstName),
                        company.get(Company_.name));
cq.select(select);
TypedQuery<PersonAndCompany> query = em.createQuery(cq);
return query.getResultList();

---

我认为在select子句中为一些JPA提供和一些数据库包含一个子查询是可能的

Select p.firstName, c.name, (select count(e) from Email e where e.person = p) from Peron p join p.company c where size(p.emails) > 2

否则，您将需要使用计数和某种分组方式。where子句中的大小检查可以移动到having子句中

在JPQL中，它类似于，标准是等价的

Select p.id, p.firstName, c.name, count(e) from Peron p join p.company c join p.peopleEmaiList e group by p.id, p.firstName, c.name having count(e) > 2

---

您也可以读取对象，然后用Java获取集合的大小。您可以对电子邮件和公司使用联接或批取来避免n+1查询。

当我想将派生值或计算值转换到不同的结构时，我更喜欢TupleQueries，如下所示：

CriteriaBuilder cb = getEntityManager().getCriteriaBuilder();
CriteriaQuery<Tuple> cq = cb.createTupleQuery(); 
// add cq.from and joins 
// add cq.where predicates 
cq.select( cb.tuple(   
  person.get(Person_.firstName).alias("person"),  
  company.get(Company_.name).alias("company"),  
  // and maybe ...
  cb.size(person.get(Person_.peopleEmailList)).alias("emailcount") );
// ...
...
TypedQuery<Tuple> typedQuery = getEntityManager().createQuery(cq);
for(Tuple t : typedQuery.getResultList()) {
  PersonAndCompany ret = new PersonAndCompany();
  ret.setPersonName( t.get("person", String.class) );
  ret.setCompanyName( t.get("company", String.class) );
  ret.setEmailCount( t.get("emailcount", Integer.class) );
}
return ret;

希望有帮助。祝你好运

CriteriaBuilder cb = getEntityManager().getCriteriaBuilder();
CriteriaQuery<Tuple> cq = cb.createTupleQuery(); 
// add cq.from and joins 
// add cq.where predicates 
cq.select( cb.tuple(   
  person.get(Person_.firstName).alias("person"),  
  company.get(Company_.name).alias("company"),  
  // and maybe ...
  cb.size(person.get(Person_.peopleEmailList)).alias("emailcount") );
// ...
...
TypedQuery<Tuple> typedQuery = getEntityManager().createQuery(cq);
for(Tuple t : typedQuery.getResultList()) {
  PersonAndCompany ret = new PersonAndCompany();
  ret.setPersonName( t.get("person", String.class) );
  ret.setCompanyName( t.get("company", String.class) );
  ret.setEmailCount( t.get("emailcount", Integer.class) );
}
return ret;