与 Room 的多对多关系导致错误：实体和 Pojos 必须具有可用的公共构造函数答案

【问题标题】：Many to many relationship with Room resulting in Error: Entities and Pojos must have a usable public constructor与 Room 的多对多关系导致错误：实体和 Pojos 必须具有可用的公共构造函数
【发布时间】：2017-10-13 13:18:14
【问题描述】：

我遇到了标题中所述的问题。我有以下实体：

@Entity(tableName = "Person")
data class Person(@PrimaryKey var id: Int,
              var firstName: String,
              var surname: String,
              var age: Int,
              var numberOfHobbies: Int) {
    @Ignore
    constructor() : this(0, "", "", 0, 0)
}

@Entity(tableName = "Skill")
data class Skill(@PrimaryKey var id: Int,
             var skillName: String) {
    @Ignore
    constructor() : this(0, "")
}

@Entity(tableName = "PersonSkill")
data class PersonSkill(var personId: Int,
                   var skillId: Int) {
    @Ignore
    constructor() : this(0, 0)

    @field:PrimaryKey(autoGenerate = true)
    var id: Int = 0
}

还有以下关系：

data class SkillWithPersons(
    @Embedded var skill: Skill = Skill(0, "UNKNOWN"),
    @Relation(
            parentColumn = "id",
            entityColumn = "skillId",
            entity = PersonSkill::class,
            projection = arrayOf("personId")
    ) var personIds: List<Int> = emptyList()
) {
      constructor() : this(Skill(0, "UNKNOWN"), emptyList())
 }

data class PersonWithSkills(
    @Embedded var person: Person = Person(0, "UNKNOWN", "UNKNOWN", 0, 0),
    @Relation(
            parentColumn = "id",
            entityColumn = "personId",
            entity = PersonSkill::class,
            projection = arrayOf("skillId")
    ) var skillIds: List<Int> = emptyList()
) {
      constructor(): this(Person(0, "UNKNOWN", "UNKNOWN", 0, 0), emptyList())
}

我已经尝试了所有方法，但它不起作用。我在使用 kotlin-kapt 时不断收到以下错误：

e: error: Entities and Pojos must have a usable public constructor. You can have an empty constructor or a constructor whose parameters match the fields (by name and type).
e: 

e:   Tried the following constructors but they failed to match:
e:   Integer(int) : [value : null]
e:   Integer(java.lang.String) : [s : null]
e: error: Entities and Pojos must have a usable public constructor. You can have an empty constructor or a constructor whose parameters match the fields (by name and type).
e: 

e: java.lang.IllegalStateException:

我正在使用以下版本：

带有 Gradle 4 的 Android Studio 3.0，房间：1.0.0-alpha9-1, 构建工具：26.0.2，科特林：1.1.51

使用@Relation 似乎有一个错误，因为 kotin-kapt 似乎无法处理它。有没有人遇到过这种情况？我什至尝试从@Relation 中删除projection，但即使这样似乎也没有什么不同。

【问题讨论】：

您是否在 Java 和注释处理器中尝试过相同的 Room 架构而不是 kapt？
从默认构造函数中删除 @Ignore 为我消除了错误。
@Virusman 对我不起作用

标签： java android kotlin android-room

【解决方案1】：

Room 中的@Relation 标签目前仅支持“Florina Muntenescu”在她的博客 post 中提到的一对多关系。

同样对于上述问题，如下所述在您的数据类中进行更改

@Entity(tableName = "Person")
data class Person(@PrimaryKey 
                  var id: Int = 0,
                  var firstName: String = "",
                  var surname: String = "",
                  var age: Int = 0,
                  var numberOfHobbies: Int = 0)

@Entity(tableName = "Skill")
data class Skill(@PrimaryKey 
                 var id: Int = 0,
                 var skillName: String = "")

@Entity(tableName = "PersonSkill")
data class PersonSkill(@PrimaryKey 
                       var id: Int = 0
                       var personId: Int = 0,
                       var skillId: Int = 0)



data class SkillWithPersons(
    @Embedded 
    var skill: Skill? = null 
    @Relation(
            parentColumn = "id",
            entityColumn = "skillId",
            entity = PersonSkill::class,
            projection = arrayOf("personId")) 
            var personIds: List<Int> = ArrayList()
            )

data class PersonWithSkills(
    @Embedded 
    var person: Person? = null
    @Relation(
            parentColumn = "id",
            entityColumn = "personId",
            entity = PersonSkill::class,
            projection = arrayOf("skillId")) 
            var skillIds: List<Int> = ArrayList()
)

这样您就可以摆脱错误。

【讨论】：