您用作基础的答案的核心原则是，如果提取了超过 1 个列，则列名匹配（重复的列名不明确）：-
Room 会自动将方法的参数绑定到绑定参数中。这是通过将参数的名称与绑定参数的名称相匹配来完成的。
此外，TypeConverter 只能转换单个值：-
每个转换器方法应接收 1 个参数并具有非 void 返回类型
示例
在不考虑您可能已经序列化 URI 和 URL 的情况下，这里有一个示例可以帮助您提取数据，假设 URI 和 URL 是纯字符串。即不需要 TypeConverters。如果使用序列化的 URI 和 URL 和 BLOBS，显然会有所不同。然后你需要一个 TypeConverters 为每个
该示例基于您的问题中包含的内容，但所解释的内容可能会有所不同。
MyDataPair（MyDataEntity的内部类）相当于元组/对，其成员名称为first和second 这些很重要，因为提取时列名必须匹配这些名称（上面提到的第一点）。
首先是我称为 MyDataEntity 的实体（这可能会有所不同，但基本上是结构 bar String V Blob）：-
@Entity(tableName = MyDataEntity.MYTABLE_TABLE_NAME)
class MyDataEntity{
/*
CONSTANTS for COLUMN AND TABLE NAMES
*/
companion object {
const val ID_COLUMN_NAME: String = "_id"
const val URI_COLUMN_NAME: String = "uri"
const val IMAGEURL_COLUMN_NAME: String = "urlimage"
const val COLOR_COLUMN_NAME: String = "color"
const val MYTABLE_TABLE_NAME: String = "mydata"
}
@PrimaryKey
@ColumnInfo(name = ID_COLUMN_NAME)
var id: Long? = null
@ColumnInfo(name = URI_COLUMN_NAME)
var uri: String? = null
@ColumnInfo(name = IMAGEURL_COLUMN_NAME)
var imageUrl: String? = null
@ColumnInfo(name = COLOR_COLUMN_NAME)
var color: Long? = null
constructor()
@Ignore
constructor(id: Long?,uri: String?,imageurl: String,color: Long?) {
this.id = id
this.uri = uri
this.imageUrl = imageurl
this.color = color
}
class MyDataPair(first: String?, second: String?) {
var first: String? = first
var second: String? = second
@Ignore
/* NOT USED/COMPLETE */
fun getFirstAsUri(): URI {
/* code here to build URI */
return URI(first)
}
@Ignore
/* NOT USED/COMPLETE */
fun getSecondAsUrl(): URL {
/* code here to build URL */
return URL(second)
}
}
}
The Dao MyDataDao（包括您的查询和特定的输出列名，以适应 MyDataEntity 中的内部 MyDataPair 类（第一和第二））：-
@Dao
interface MyDataDao {
/* For inserting test data */
@Insert
fun insertMyDataRow(myDataEntity: MyDataEntity): Long
/* Efectively your query BUT WITH specific output/result column names to suit the MyDataPair class */
@Query("SELECT ${MyDataEntity.URI_COLUMN_NAME} AS first,${MyDataEntity.IMAGEURL_COLUMN_NAME} AS second FROM ${MyDataEntity.MYTABLE_TABLE_NAME} WHERE ${MyDataEntity.COLOR_COLUMN_NAME} IS NULL")
fun findWithMissingColor(): List<MyDataEntity.MyDataPair>
/* Clear the table for renability */
@Query("DELETE FROM ${MyDataEntity.MYTABLE_TABLE_NAME}")
fun clearMyDataEntityTable(): Int
}
数据库我的数据库
@Database(entities = [MyDataEntity::class],version = 1)
abstract class MyDatabase: RoomDatabase() {
abstract fun getMyDataDao(): MyDataDao
companion object {
const val DATABASE_NAME = "mydatabase"
}
}
最后是一个 Activity MainActivity 将它们放在一起：-
class MainActivity : AppCompatActivity() {
lateinit var db: MyDatabase
lateinit var dao: MyDataDao
val TAG = "MYDBINFO"
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
db = Room.databaseBuilder(this,MyDatabase::class.java,MyDatabase.DATABASE_NAME)
.allowMainThreadQueries()
.build()
dao = db.getMyDataDao()
var deleted_rows = dao.clearMyDataEntityTable()
/* CLEAR ANY EXIST DATALOAD AND LOAD SOME DATA */
Log.d(TAG, "$deleted_rows were deleted from the table ${MyDataEntity.MYTABLE_TABLE_NAME}")
dao.insertMyDataRow(MyDataEntity(null,"uri1","imageurl1", 0x00ffffff))
dao.insertMyDataRow(MyDataEntity(null,"uri2","imageurl2",null))
dao.insertMyDataRow(MyDataEntity(null,"uri3","imageurl3",0xAAAAAA))
dao.insertMyDataRow(MyDataEntity(null,"uri4","imageurl4",null))
/* EXTRACT ROWS WHERE COLOR IS NULL */
var results = dao.findWithMissingColor()
/* WRITE SOME INFO ABOUT THE EXTRACTED DATA TO THE LOG */
for(mdp: MyDataEntity.MyDataPair in results) {
Log.d(TAG,"STRING_URI = ${mdp.first} STRING_URL = ${mdp.second}")
}
}
}
- 为了方便和简洁运行主线程
- 插入 4 行 2 颜色为 NULL，2 行颜色为 NULL
结果来自运行
运行后的数据库是（根据 Database Inspector）：-
日志包括：-
2021-04-12 12:29:18.085 D/MYDBINFO: 4 were deleted from the table mydata
2021-04-12 12:29:18.099 D/MYDBINFO: STRING_URI = uri2 STRING_URL = imageurl2
2021-04-12 12:29:18.099 D/MYDBINFO: STRING_URI = uri4 STRING_URL = imageurl4