Room报错：无法将Cursor转换为方法返回类型

我是Android开发新手，目前正在开发名为water drink reminder的项目，以下是我编写的Dao与数据库相关代码。每次运行应用时都会抛出错误 "Room Not sure how to convert a Cursor to this method's return type"。我检索相关问题后了解到**"不要将LiveData与suspend关键字搭配使用"**，但检查代码后我并未这样使用，却仍然出现该报错，恳请大家帮忙排查解决。

相关代码 #

Dao层 #

@Dao
interface Dao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insertUser(user :User)

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insertDrinkData(drink: Drink)

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insertNotificationInfo(notification : Notification)

    @Update(onConflict = OnConflictStrategy.REPLACE)
    suspend fun updateUser(user: User)

    @Update(onConflict = OnConflictStrategy.REPLACE)
    suspend fun updateNotificationInfo(notification: Notification)

    @Delete
    suspend fun deleteSelectedDrinkData(drink : Drink)

    @Query("SELECT *FROM User_table")
    fun getAllUsers() : LiveData<List<User>>

    @Query("SELECT * FROM $USER_TABLE_NAME LIMIT 1")
    suspend fun readUserData() : User

    @Query("SELECT * FROM $USER_TABLE_NAME LIMIT 1")
    fun readData() : LiveData<User>

    @Query("SELECT * FROM $USER_TABLE_NAME LIMIT 1")
    suspend fun readNotificationData() : Notification

    @Query("SELECT $COLUMN_DATE_DRINK , SUM($COLUMN_AMOUNT_DRINK) as Total GROUP BY $COLUMN_DATE_DRINK")
    suspend fun readDrinkSumData() : MutableList<Sum>

    @Query("SELECT $COLUMN_DATE_DRINK , SUM($COLUMN_AMOUNT_DRINK) as Total GROUP BY $COLUMN_DATE_DRINK")
    fun readDrinkData() : LiveData<MutableList<Sum>>

    @Query("SELECT * FROM $USER_TABLE_NAME WHERE $COLUMN_DATE_DRINK = :date")
    suspend fun readDrinkDataSelectedDay(date:String): MutableList<Drink>
}

数据库定义 #

@Database(entities = [User::class,Drink::class,Notification::class], version = DATABASE_VERSION)
abstract class AppDataBase : RoomDatabase() {
    abstract val daoInstance : Dao
    companion object {
        private var INSTANCE : AppDataBase? = null
        fun getInstance(context : Context) : AppDataBase {
            synchronized(this) {
                var instance : AppDataBase? = INSTANCE
                if (instance == null) {
                    instance = Room.databaseBuilder(
                        context.applicationContext,
                        AppDataBase::class.java,
                        "AppDataBase"
                    ).build()
                }
                return instance
            }
        }
    }
}

Repository层 #

class AppRepository(private val dao: Dao) {

    fun readData() : LiveData<User> = dao.readData()
    fun readDrinkData(): LiveData<MutableList<Sum>> = dao.readDrinkData()
    suspend fun insertUser(user :User): Unit = dao.insertUser(user)
    suspend fun insertDrinkData(drink: Drink): Unit = dao.insertDrinkData(drink)
    suspend fun insertNotificationInfo(notification : Notification): Unit = dao.insertNotificationInfo(notification)
    suspend fun updateUser(user: User) : Unit = dao.updateUser(user)
    suspend fun updateNotificationInfo(notification: Notification): Unit = dao.updateNotificationInfo(notification)
    suspend fun readUserData() : User = dao.readUserData()
    suspend fun readNotificationData() : Notification = dao.readNotificationData()
    suspend fun deleteSelectedDrinkData(drink : Drink): Unit = dao.deleteSelectedDrinkData(drink)
    suspend fun readDrinkSumData(): MutableList<Sum>? = dao.readDrinkSumData()
    suspend fun readDrinkDataSelectedDay(date: String): MutableList<Drink>? {
        return dao.readDrinkDataSelectedDay(date)
    }
}

环境信息 #

• 当前使用Room版本：2.4.2
• 错误截图：

报错核心原因是Room不支持将MutableList作为查询方法的返回类型，无论你是用suspend函数直接返回，还是包裹在LiveData中返回，Room默认只支持自动映射为不可变的List<T>类型，你代码中三处使用MutableList作为查询返回值，直接触发了Cursor无法转换的错误。
另外代码里还有两个隐藏问题会导致后续运行报错：

• SQL语句SELECT *FROM User_table中*和FROM之间缺少空格，存在语法风险
• readNotificationData()方法查询的是用户表$USER_TABLE_NAME，但返回类型是Notification实体，表和返回类不匹配

具体修复步骤 #

1. 修改Dao层所有返回MutableList的方法，将返回类型替换为List：

   // 替换对应三个查询方法
   @Query("SELECT $COLUMN_DATE_DRINK , SUM($COLUMN_AMOUNT_DRINK) as Total GROUP BY $COLUMN_DATE_DRINK")
   suspend fun readDrinkSumData() : List<Sum>
   
   @Query("SELECT $COLUMN_DATE_DRINK , SUM($COLUMN_AMOUNT_DRINK) as Total GROUP BY $COLUMN_DATE_DRINK")
   fun readDrinkData() : LiveData<List<Sum>>
   
   @Query("SELECT * FROM $USER_TABLE_NAME WHERE $COLUMN_DATE_DRINK = :date")
   suspend fun readDrinkDataSelectedDay(date:String): List<Drink>
   

2. 修正SQL语法问题，将SELECT *FROM User_table改为SELECT * FROM User_table
3. 修正readNotificationData()的查询表名，替换为Notification实体对应的表名常量
4. 同步修改Repository层对应方法的返回类型，去掉多余的可空声明，和Dao层返回类型保持一致：

   fun readDrinkData(): LiveData<List<Sum>> = dao.readDrinkData()
   suspend fun readDrinkSumData(): List<Sum> = dao.readDrinkSumData()
   suspend fun readDrinkDataSelectedDay(date: String): List<Drink> {
       return dao.readDrinkDataSelectedDay(date)
   }
   

5. 额外检查：自定义统计结果类Sum要保证字段名和查询返回的列名完全匹配，比如查询中聚合结果别名为Total，Sum类中对应字段要么命名为total/Total，要么用@ColumnInfo(name = "Total")注解标注，避免字段映射失败。

修改完成后Clean工程重新编译，该Cursor转换报错就会消失。

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