Spring Data JPA JDBC 语句 MADNESS [重复]

Question

我有一个简单的实体：

@Entity
@Table(name = "dish_type")
class DishType : Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    var id: Long = -1
    var name: String? = null
    var description: String? = null
    @OneToMany(mappedBy = "dishType")
    var dishTypeLocales: List<DishTypeLocale>? = null
}

@Entity
@Table(name = "dish_type_locale")
class DishTypeLocale : Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    var id: Long = -1

    @Enumerated(EnumType.STRING)
    var locale: LocaleString? = null
    var value: String? = null

    @ManyToOne
    @JoinColumn(name = "dish_type_id")
    var dishType: DishType? = null
}

道：

interface DishTypeService {
    fun findAll(withLocale: Boolean): List<DishTypeDto>
}

@Service
@Transactional
open class DishTypeServiceImpl(private val dishTypeRepository: DishTypeRepository) : DishTypeService {

    override fun findAll(withLocale: Boolean): List<DishTypeDto> {
        return this.dishTypeRepository.findAll().map { DishTypeDto(it, withLocale) }
    }

}

@Repository
interface DishTypeRepository : JpaRepository<DishType, Long>

DTO：

class DishTypeDto {
    var id: Long = -1
    var description: String? = null
    var locale: List<DefaultLocalizationDto>? = null

    constructor()

    constructor(dishType: DishType, withLocale: Boolean) {
        this.id = dishType.id
        this.description = dishType.description
        if (withLocale) {
            this.locale = dishType.dishTypeLocales?.map { DefaultLocalizationDto(it) }
        }
    }

    override fun toString(): String {
        return "DishTypeDto{" +
                "id=" + id +
                ", description='" + description + '\'' +
                '}'
    }
}


class DefaultLocalizationDto {
    var locale: Int? = null
    var name: String? = null
    var description: String? = null

    constructor()

    constructor(locale: DishTypeLocale) {
        this.locale = locale.locale?.code
        this.name = locale.value
    }

    override fun toString(): String = "DefaultLocalizationDto(locale=$locale, name=$name, description=$description)"
}

如果是 DishTypeService.findAll(false)，我们有 1 条语句发送到 DB：

Session Metrics {
    6789040 nanoseconds spent acquiring 1 JDBC connections;
    0 nanoseconds spent releasing 0 JDBC connections;
    146499 nanoseconds spent preparing 1 JDBC statements;
    3929488 nanoseconds spent executing 1 JDBC statements;
    0 nanoseconds spent executing 0 JDBC batches;
    0 nanoseconds spent performing 0 L2C puts;
    0 nanoseconds spent performing 0 L2C hits;
    0 nanoseconds spent performing 0 L2C misses;
    0 nanoseconds spent executing 0 flushes (flushing a total of 0 entities and 0 collections);
    43774 nanoseconds spent executing 1 partial-flushes (flushing a total of 0 entities and 0 collections)
}

如果是 DishTypeService.findAll(true) 语句 == table.size（在我的情况下为 282）：

Session Metrics {
    11570010 nanoseconds spent acquiring 1 JDBC connections;
    0 nanoseconds spent releasing 0 JDBC connections;
    4531164 nanoseconds spent preparing 282 JDBC statements;
    60280410 nanoseconds spent executing 282 JDBC statements;
    0 nanoseconds spent executing 0 JDBC batches;
    0 nanoseconds spent performing 0 L2C puts;
    0 nanoseconds spent performing 0 L2C hits;
    0 nanoseconds spent performing 0 L2C misses;
    0 nanoseconds spent executing 0 flushes (flushing a total of 0 entities and 0 collections);
    60464 nanoseconds spent executing 1 partial-flushes (flushing a total of 0 entities and 0 collections)
}

如何告诉 Spring，获取 1 中的所有数据（ok mb 2-3 statements）？

@OneToMany(mappedBy = "dishType", fetch = FetchType.EAGER)
var dishTypeLocales: List<DishTypeLocale>? = null

渴望没有帮助

我知道，我可以从 DishType 中删除 dishTypeLocales，然后在 2 个单独的方法中获取所有 dishTypes，然后获取所有 dishTypeLocales，然后在代码中映射它们，但是还有更好的方法吗？

我正在使用：

    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.0.0.RELEASE</version>

数据库：Postgresql 9.6

Answer 1

您遇到了关系的延迟加载，这是OneToMany 关系的默认行为。

这里有 4 个选项：

1. 为关系设置 EAGER 加载（我知道您写道，您已经尝试过但没有成功，但相信我，它有效。您可能只是遇到了不同的问题）
2. 使用FETCH JOIN 加载实体和关系。为此，您必须使用以下查询（草稿）在存储库中编写自定义方法：SELECT dt FROM DishType dt JOIN FETCH dt.dishTypeLocales
3. 使用 Spring Data JPA 的功能，Fetch/LoadGraph
4. 使用projections，因为这是一个只读操作。最好在读取数据时避免使用实体，仅在修改数据时使用实体。