Spring Data JPA Projection 从数据库中选择的字段

Question

我正在按照 Spring Data Docs http://docs.spring.io/spring-data/jpa/docs/current/reference/html/#projections 中的示例测试 Spring Data 1.10.4.RELEASE

我注意到一些问题，我有 2 个问题。

首先让我们假设我有这两个实体：

@Entity
public class Person {

  @Id @GeneratedValue
  private Long id;
  private String firstName, lastName;

  @OneToOne
  private Address address;
}

@Entity
public class Address {

  @Id @GeneratedValue
  private Long id;
  private String street, state, country;
}

• 问题 1：

对于以下预测：

interface PersonLimited {  

  String getFirstName(); 

  AddressLimited getAddress();
}

interface AddressLimited {  

  String getCountry(); 
}

当我运行 findPersonByFirstNameProjectedForLimitedData

interface PersonRepository extends CrudRepository<Person, Long> {

  @Query("select p from Person p where p.firstName = ?1")
  PersonLimited findPersonByFirstNameProjectedForLimitedData(String firstName);
}

它返回的正是预期的：

{
    firstName: 'Homer',
    address: {
        country: 'USA'
    }
}

现在，如果我查看生成的 SQL，这就是我所拥有的：

SELECT person0_.firstName      AS col_0_0_, 
       address1_.id            AS id1_13_, 
       address1_.street        AS street2_13_, 
       address1_.state         AS state3_13_, 
       address1_.country       AS country4_13_
FROM   person person0_ 
       LEFT OUTER JOIN address address1_ 
                    ON person0_.addressId = address1_.id 
WHERE  person0_.firstName = ?  

“Person”实体的投影仅选择“fistName”，这是 100% 正确的，因为在 PersonLimited 接口中我只定义了“getFirstName”。

但是对于“Address”实体，它选择了所有字段，这是错误的，因为在AddressLimited接口中我只定义了“getCountry”，它应该只选择“country”。

生成的查询应该是这样的：

SELECT person0_.firstName      AS col_0_0_, 
       address1_.country       AS country4_13_
FROM   person person0_ 
       LEFT OUTER JOIN address address1_ 
                    ON person0_.addressId = address1_.id 
WHERE  person0_.firstName = ?  

所以问题是，为什么它不只为地址“实体”选择“国家”字段？为什么需要选择所有字段？这是 Spring 中的错误吗？

• 问题 2：

对于与上面相同的投影，

当我运行 findAllPersonsProjectedForLimitedData

interface PersonRepository extends CrudRepository<Person, Long> {

  @Query("select p from Person p")
  List<PersonLimited> findAllPersonsProjectedForLimitedData();
}

它返回的正是预期的：

[
     {
        firstName: 'Homer',
        address: {
            country: 'USA'
        }
     },
     {
        firstName: 'Maggie',
        address: {
            country: 'USA'
        }
     }
]

现在，如果我查看生成的 SQL，这就是我所拥有的：

SELECT person0_.id                 AS id1_18_, 
       person0_.firstName          AS firstName2_18_, 
       person0_.lastName           AS lastName3_18_, 
       person0_.addressid          AS company4_18_
FROM   person person0_ 

SELECT address0_.id         AS id1_13_0_, 
       address0_.street     AS street2_13_0_, 
       address0_.state      AS state3_13_0_, 
       address0_.country    AS country4_13_0_
FROM   address address0_ 
WHERE  address0_.id = ? 

这里，Person 和 Address 实体的投影都选择了所有错误的字段，它应该只选择“firstName”和“country”。

生成的查询应该是这样的：

SELECT person0_.firstName        AS firstName2_18_
FROM   person person0_ 

SELECT address0_.country    AS country4_13_0_
FROM   address address0_ 
WHERE  address0_.id = ? 

这是正常行为吗，不应该只选择我们需要的字段吗？

谢谢，

Answer 1

如果您想将注释 @Query 与 Spring Data Projections 一起使用，则必须使用字段别名，并且需要确保为与投影字段匹配的项目设置别名。以下代码适用于问题 1：

interface PersonRepository extends CrudRepository<Person, Long> {

  @Query("select p.firstName as firstname, p.address as address from Person p where p.firstName = ?1")
  PersonLimited findPersonByFirstNameProjectedForLimitedData(String firstName);
}

您可以使用的另一种替代方法是使用Property Expressions 定义您的查询。只要有可能：

interface PersonRepository extends CrudRepository<Person, Long> {

  List<PersonLimited> findByFirstName(String firstName);
}

Answer 2

我遇到了同样的问题，当我将投影从接口更改为 pojo 类时，它会正确生成 SQL。