如何在 Kotlin 的委托属性上放置 JPA 注释？

Question

假设我有一个 JPA 实体，例如

@Entity
@Table(name = "plasmid_reference")
class PlasmidReference {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    var id: Int? = null

    @Column(name = "project_id")
    var projectId: Int? = null

    @Column(name = "plasmid_id")
    var plasmidId: Int? = null
}

现在，我知道对于plasmid_reference中的每个条目加入tablenone 这些属性可以为空。（事实上，数据库强制执行此操作。）

如何告诉 Kotlin？

一种天真的方法是尝试 lateinit...

@Entity
@Table(name = "plasmid_reference")
class PlasmidReference {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    lateinit var id: Int

    @Column(name = "project_id")
    lateinit var projectId: Int

    @Column(name = "plasmid_id")
    lateinit var plasmidId: Int
}

这甚至无法编译。

'lateinit' modifier is not allowed on properties of primitive types

好的，让我们使用建议的解决方案并转换为使用委托。

@Entity
@Table(name = "plasmid_reference")
class PlasmidReference {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    var id by notNull<Int>()

    @Column(name = "project_id")
    var projectId by notNull<Int>()

    @Column(name = "plasmid_id")
    var plasmidId by notNull<Int>()
}

这仍然无法编译，因为

This annotation is not applicable to target 'member property with delegate'

好的……这听起来像是科特林的东西，在 Java 中，我会把注释扔到一个字段上，所以也许……

@Entity
@Table(name = "plasmid_reference")
class PlasmidReference {

    @field:Id
    @field:GeneratedValue(strategy = GenerationType.IDENTITY)
    var id by notNull<Int>()

    @field:Column(name = "project_id")
    var projectId by notNull<Int>()

    @field:Column(name = "plasmid_id")
    var plasmidId by notNull<Int>()
}

不，事实证明，并非如此。仍然无法编译，因为

'@field:' annotations could be applied only to properties with backing fields

好吧，让我们注释一下“存储委托属性的委托实例的字段”（参见 https://kotlinlang.org/docs/reference/annotations.html#annotation-use-site-targets）

@Entity
@Table(name = "plasmid_reference")
class PlasmidReference {

    @delegate:Id
    @delegate:GeneratedValue(strategy = GenerationType.IDENTITY)
    var id by notNull<Int>()

    @delegate:Column(name = "project_id")
    var projectId by notNull<Int>()

    @delegate:Column(name = "plasmid_id")
    var plasmidId by notNull<Int>()
}

这确实编译但会抛出运行时异常，因为

The attribute [plasmidId] is not present in the managed type [EntityTypeImpl@960932033:PlasmidReference 
[ javaType: class com.[...].PlasmidReference descriptor: RelationalDescriptor(com.[...].PlasmidReference --> [DatabaseTable(plasmid_reference)]), mappings: 3]].
    at org.eclipse.persistence.internal.jpa.metamodel.ManagedTypeImpl.getAttribute(ManagedTypeImpl.java:151)
    at org.springframework.data.jpa.repository.query.QueryUtils.toExpressionRecursively(QueryUtils.java:635)

啊啊啊，我没主意了。

我该如何解决？

Answer 1

将其移至构造函数定义应该可以解决问题

@Entity
@Table(name = "plasmid_reference")
class PlasmidReference(
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    var id: Int? = null,
    @Column(name = "project_id")
    var projectId: Int,
    @Column(name = "plasmid_id")
    var plasmidId: Int
)

如何在 Kotlin 的委托属性上放置 JPA 注释？

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