检索对象列表时，Android room/TypeConverter问题 [英] Android room/TypeConverter issue while retriving list of objects

问题描述

我正在使用Room实现本地缓存.我创建了typeconverter来将对象列表转换为json并返回.但是我从错误中检索json数据时收到映射问题:

I am implementing a local cache using Room. I have created typeconverter to convert list of objects to json and back. But I am receiving mapping issue while retrieving data from json with error:

The columns returned by the query does not have the fields [title,media] in 
com.example.theApp.data.FlickrImage even though they are annotated as non-null or 
primitive. Columns returned by the query: [items]

另一个这样的人:

error: Cannot figure out how to read this field from a cursor.
private final com.example.theApp.data.Media media = null;

我在这里尝试了其他答案，但与该问题没有直接关联.

I tried other answers here but its not associated directly with this issue.

这是我的 typeconverter :

class FlickrImageConverters {

@TypeConverter
fun fromImageListToJson(stat: List<FlickrImage>): String {
    return Gson().toJson(stat)
}

/**
 * Convert a json to a list of Images
 */
@TypeConverter
fun fromJsonToImagesList(jsonImages: String): List<FlickrImage> {
    val type = object : TypeToken<List<FlickrImage>>() {}.type
    return Gson().fromJson<List<FlickrImage>>(jsonImages, type)
    }

}

这是我的实体课程:

@Entity
data class DatabaseImagesEntity(
    @PrimaryKey
    @TypeConverters(FlickrImageConverters::class)
    @SerializedName("item")
    val items: List<FlickrImage>)   

道课 @道 界面ImagesDao {

Dao class @Dao interface ImagesDao {

@Query("select * from DatabaseImagesEntity")
fun getImages(): List<FlickrImage>


@Insert(onConflict = OnConflictStrategy.REPLACE)
fun insertAll(images: List<FlickrImage>)

}

FlickrImage类

data class FlickrImage(val title: String, val media: Media)

媒体类

data class Media(val m: String)

LatestImage类

data class LatestImages(val items: List<FlickrImage>)

如果您遇到此问题，并且知道解决方法，请告诉我.

Please let me know if you faced this issue and if you know the solution for this.

房间数据库实施

@Database(entities = [DatabaseImagesEntity::class], version = 1, 
exportSchema = false)
@TypeConverters(FlickrImageConverters::class)
abstract class FlickrDatabase: RoomDatabase() {
abstract val imagesDao: ImagesDao
}


private lateinit var INSTANCE: FlickrDatabase


fun getDatabase(context: Context): FlickrDatabase{

synchronized(FlickrDatabase::class.java){
    if(!::INSTANCE.isInitialized){
        INSTANCE = Room.databaseBuilder(context.applicationContext,
            FlickrDatabase::class.java,
            "flickerImages").build()
    }
}
return INSTANCE
}

推荐答案

问题是我将数据保存在错误的实体，错误的TypeConverters中，结果是在创建数据库时使用了错误的Entity类.

The issue was I was saving data in the wrong entity, wrong TypeConverters and as a result, I was using the wrong Entity class at the time of database creation.

以下是我必须进行的一些更改以存储对象列表:

Here are the necessary changes I had to make to store the list of objects:

Flickr数据类

@Entity(tableName = "FlickerImage")
data class FlickrImage(
@PrimaryKey(autoGenerate = true)
val id: Int,
val title: String,
@TypeConverters(MediaConverter::class)
val media: Media)

媒体类的TypeConvertors

class MediaConverter {

@TypeConverter
fun fromMediaToJson(stat: Media): String {
    return Gson().toJson(stat)
    }

/**
 * Convert a json to a list of Images
 */
@TypeConverter
fun fromJsonToMedia(jsonImages: String): Media {
    val type = object : TypeToken<Media>() {}.type
    return Gson().fromJson<Media>(jsonImages, type)
    }
}

DAO类

@Dao
interface ImagesDao {

@Query("select * from FlickerImage")
fun getImages(): LiveData<List<FlickrImage>>

数据库类

@Database(entities = [FlickrImage::class], version = 1, exportSchema = false)
@TypeConverters(MediaConverter::class)
abstract class FlickrDatabase: RoomDatabase() {
    abstract val imagesDao: ImagesDao
}


private lateinit var INSTANCE: FlickrDatabase


fun getDatabase(context: Context): FlickrDatabase{

synchronized(FlickrDatabase::class.java){
    if(!::INSTANCE.isInitialized){
        INSTANCE = Room.databaseBuilder(context,
            FlickrDatabase::class.java,
            "flickerImages").build()
        }
    }
    return INSTANCE
}


@Insert(onConflict = OnConflictStrategy.REPLACE)
fun insertAll(images: List<FlickrImage>)

}

这篇关于检索对象列表时，Android room/TypeConverter问题的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！