Kotlin中Room Entity与Data Class互转及报错解决

问题说明 #

从网络获取了Exchange数据类，尝试存入Room数据库时出现报错，同时在Repository的getAll()方法中无法将ExchangeEntity正确转换为Exchange对象，另外编写的类型转换器也不确定是否正确。

相关代码 #

Room实体类 #

@Entity(tableName = "ExchangeValues")
data class ExchangeEntity(
    @ColumnInfo(name = "base_code") val base_code: String,
    @ColumnInfo(name = "conversion_rates") val conversion_rates: ConversionRates,
    @ColumnInfo(name = "result") val result: String,
    @PrimaryKey(autoGenerate = true) val uid: Int? = null
)

Dao接口 #

@Dao
interface ExchangeDao {
    @Query("SELECT * FROM ExchangeValues")
    suspend fun getAll(): List<ExchangeEntity>

    @Query("UPDATE ExchangeValues SET base_code=:base_code, conversion_rates=:conversion_rates, result=:result")
    suspend fun update(base_code: String, conversion_rates: ConversionRates, result: String)
}

Exchange数据类 #

@Serializable
data class Exchange(
    val base_code: String,
    val conversion_rates: ConversionRates,
    val documentation: String,
    val result: String,
    val terms_of_use: String,
    val time_last_update_unix: Int,
    val time_last_update_utc: String,
    val time_next_update_unix: Int,
    val time_next_update_utc: String
) {
    fun toEntity() = ExchangeEntity(
        base_code = base_code,
        conversion_rates = conversion_rates,
        result = result
    )
}

@Serializable
data class ConversionRates(
    val conversionRates: Map<String, Double>
)

ExchangeRepositoryImpl实现 #

class ExchangeRepositoryImpl @Inject constructor(
    private val dao: ExchangeDao
) : ExchangeRepository {
    override suspend fun getAll(): Flow<List<Exchange>> {
        return flow {
            emit(dao.getAll())
        }
    }

    override suspend fun update(exchange: Exchange) {
        dao.update(exchange.base_code, exchange.result, exchange.conversion_rates)
    }
}

类型转换器 #

class ExchangeConverter {
    @TypeConverter
    fun fromSource(conversionRates: ConversionRates): String {
        val gson = Gson()
        return gson.toJson(conversionRates)
    }

    @TypeConverter
    fun toSource(json: String): ConversionRates {
        val gson = Gson()
        val typeToken = object : TypeToken<List<ConversionRates>>() {}.type
        return Gson().fromJson(json, typeToken)
    }
}

解决方案 #

1. 修复类型转换器的错误 #

当前toSource方法错误使用了List<ConversionRates>的TypeToken，但实际存储的是单个ConversionRates对象，需要修正TypeToken类型，同时要让Room识别该转换器：

// 修正后的转换器
class ExchangeConverter {
    @TypeConverter
    fun fromSource(conversionRates: ConversionRates): String {
        return Gson().toJson(conversionRates)
    }

    @TypeConverter
    fun toSource(json: String): ConversionRates {
        val typeToken = object : TypeToken<ConversionRates>() {}.type
        return Gson().fromJson(json, typeToken)
    }
}

在Room Database类上添加注解，注册转换器：

@Database(entities = [ExchangeEntity::class], version = 1)
@TypeConverters(ExchangeConverter::class)
abstract class AppDatabase : RoomDatabase() {
    abstract fun exchangeDao(): ExchangeDao
}

2. 修复Repository的getAll()转换逻辑 #

dao.getAll()返回List<ExchangeEntity>，需要将其转换为List<Exchange>。由于原ExchangeEntity缺少Exchange的部分字段，有两种处理方式：

• 方式一：扩展ExchangeEntity字段（推荐）
  修改ExchangeEntity补全所有Exchange字段，确保数据完整：

  @Entity(tableName = "ExchangeValues")
  data class ExchangeEntity(
      @ColumnInfo(name = "base_code") val base_code: String,
      @ColumnInfo(name = "conversion_rates") val conversion_rates: ConversionRates,
      @ColumnInfo(name = "documentation") val documentation: String,
      @ColumnInfo(name = "result") val result: String,
      @ColumnInfo(name = "terms_of_use") val terms_of_use: String,
      @ColumnInfo(name = "time_last_update_unix") val time_last_update_unix: Int,
      @ColumnInfo(name = "time_last_update_utc") val time_last_update_utc: String,
      @ColumnInfo(name = "time_next_update_unix") val time_next_update_unix: Int,
      @ColumnInfo(name = "time_next_update_utc") val time_next_update_utc: String,
      @PrimaryKey(autoGenerate = true) val uid: Int? = null
  )
  

  更新Exchange的toEntity()方法：

  fun toEntity() = ExchangeEntity(
      base_code = base_code,
      conversion_rates = conversion_rates,
      documentation = documentation,
      result = result,
      terms_of_use = terms_of_use,
      time_last_update_unix = time_last_update_unix,
      time_last_update_utc = time_last_update_utc,
      time_next_update_unix = time_next_update_unix,
      time_next_update_utc = time_next_update_utc
  )
  

  给ExchangeEntity添加反向转换方法：

  fun toExchange() = Exchange(
      base_code = base_code,
      conversion_rates = conversion_rates,
      documentation = documentation,
      result = result,
      terms_of_use = terms_of_use,
      time_last_update_unix = time_last_update_unix,
      time_last_update_utc = time_last_update_utc,
      time_next_update_unix = time_next_update_unix,
      time_next_update_utc = time_next_update_utc
  )
  

  最后修改Repository的getAll()：

  override suspend fun getAll(): Flow<List<Exchange>> {
      return flow {
          emit(dao.getAll().map { it.toExchange() })
      }
  }
  

• 方式二：给缺失字段设默认值
  如果不需要完整Exchange对象，转换时给缺失字段设默认值：

  override suspend fun getAll(): Flow<List<Exchange>> {
      return flow {
          emit(dao.getAll().map { entity ->
              Exchange(
                  base_code = entity.base_code,
                  conversion_rates = entity.conversion_rates,
                  documentation = "",
                  result = entity.result,
                  terms_of_use = "",
                  time_last_update_unix = 0,
                  time_last_update_utc = "",
                  time_next_update_unix = 0,
                  time_next_update_utc = ""
              )
          })
      }
  }
  

3. 修复Dao的update方法（添加WHERE条件） #

原update语句无WHERE条件，会更新表中所有行，需添加条件指定更新的记录：

@Query("UPDATE ExchangeValues SET base_code=:base_code, conversion_rates=:conversion_rates, result=:result WHERE uid=:uid")
suspend fun update(uid: Int, base_code: String, conversion_rates: ConversionRates, result: String)

修改Repository的update方法，传入对应uid：

override suspend fun update(exchange: Exchange, uid: Int) {
    dao.update(uid, exchange.base_code, exchange.conversion_rates, exchange.result)
}

4. 修复Repository的update方法参数顺序 #

原调用dao.update时参数顺序错误，导致字段赋值混乱，修正顺序：

override suspend fun update(exchange: Exchange) {
    dao.update(exchange.base_code, exchange.conversion_rates, exchange.result)
}

内容的提问来源于stack exchange，提问作者NewPartizal