java – Spring Data JPA Projection从DB中选择字段
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我按照Spring Data Docs http://docs.spring.io/spring-data/jpa/docs/current/reference/html/#projections中的示例测试了Spring Data 1.10.4.RELEASE
我注意到一些问题,我有两个问题.
首先让我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时
interface PersonRepository extends CrudRepository<Person, Long> {
@Query("select p from Person p where p.firstName = ?1")
PersonLimited findPersonByFirstNameProjectedForLimitedData(String firstName);
}
它完全返回预期的结果:
{
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”.
但对于“地址”实体,它选择所有字段,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”.
生成的查询应该类似于:
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,为什么不选择地址“实体”的“国家/地区”字段?为什么需要选择所有字段?这是春天的错误吗?
>问题2:
对于与上述相同的投影,
当我运行findAllPersonsProjectedForLimitedData时
interface PersonRepository extends CrudRepository<Person, Long> {
@Query("select p from Person p")
List<PersonLimited> findAllPersonsProjectedForLimitedData();
}
它完全返回预期的结果:
[
{
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 = ?
在这里,Person和Address实体的投影是选择所有错误的字段,它应该只选择“firstName”和“country”.
生成的查询应该类似于:
SELECT person0_.firstName AS firstName2_18_
FROM person person0_
SELECT address0_.country AS country4_13_0_
FROM address address0_
WHERE address0_.id = ?
这是正常的行为,不应该只选择我们需要的字段吗?
谢谢,
解决方法:
如果要使用带有Spring Data Projections的注释@Query,则必须使用字段别名,并且需要确保对与投影字段匹配的项目进行别名.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方法是使用Property Expressions定义查询.只要可能:
interface PersonRepository extends CrudRepository<Person, Long> {
List<PersonLimited> findByFirstName(String firstName);
}
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