有人可以帮助我更好地理解 Room 持久性库吗？
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【中文标题】有人可以帮助我更好地理解 Room 持久性库吗？【英文标题】：Could someone help me understand Room persistence library better? 【发布时间】：2021-10-11 10:04:33 【问题描述】：我已经开始为 android 制作应用程序，并决定了解更多关于数据库的信息。我认为对于一个简单的应用程序，我对 SQL 的理解已经足够好了。我遇到的麻烦是房间。我已经阅读了很多并观看了教程，但我仍然不能完全理解实现它的正确方法。这是我收集的。我认为我应该拥有实体、DAO、数据库和存储库。
我的应用背后的想法是这样的。我会进行锻炼、锻炼等。
锻炼应该有锻炼。
现在我想象它是 2 个表：锻炼表、锻炼表
每个锻炼和锻炼都应该有一个唯一的 id/c 主键
锻炼实体将具有 2 个属性：锻炼 ID、锻炼名称
Exercise 实体有 2 个属性：exerciseId、exerciseName
我计划用多对多或一对多关系连接这两个表，但这不是我现在遇到的问题。
现在，在为这两个实体创建数据类之后，我为每个实体创建了一个 Dao 类。 所以，WorkoutDao 和Exercise Dao（因为我读到每个实体有一个dao 是一种很好的做法）。现在我不太明白的是，很多人在网上说我应该每个实体有一个数据库，这对我来说似乎很奇怪。我不应该拥有一个包含所有相互关联的表的数据库。如果每个表都有一个 DAO，我该如何连接这些表。另外，我不知道是每个实体/表有一个存储库还是每个 database.entity 有一个存储库。
还有一个关于 ViewModel 与 Room 结合的问题。我收集到每个 Activity 应该有一个 ViewModel，基本上在 Fragment 之间共享。假设我有一个具有 recyclerView 和锻炼列表的活动，当我单击列表中的任何锻炼时，我会转到另一个具有回收器视图的活动，该视图显示单击的锻炼所具有的每个锻炼。两个视图模型会实例化同一个数据库吗？因为这似乎不是最好的解决方案。因为锻炼视图模型应该只有一个锻炼列表，而锻炼视图模型应该有一个锻炼列表（嗯，点击锻炼的锻炼）。我将如何解决这个问题？
无论如何，对不起，如果我写的内容令人困惑，如果没有理解，我会尝试更好地解释它，我对 kotlin 和 android studio 还是很陌生
【问题讨论】：
【参考方案1】：这里有几个要点可以消除您的困惑：
-
您可以为所有实体使用一个数据库。
每个实体都应该有单独的存储库。
DAO（Data Access Object）类主要用于数据库操作（INSERT、UPDATE、DELETE）。
您可以通过
@ForeignKey 注释到相应字段来创建一个表与另一个表的关系。
您不应在 Activity/viewModel 中初始化数据库。您应该创建一个单例类，它将在 Application 类中启动数据库。这就是您仅在应用启动时才初始化数据库的方式。
【讨论】：
感谢您的回答。我想我明白了，但我仍然感到困惑的一件事是我是否应该每个数据库有一个 DAO 或每个实体一个 DAO。如果是后者，我如何查询关系并获取数据。例如，如果我有一个 WorkoutWithExercises 类，我是在 WorkoutDao 还是 ExerciseDao 中创建一个查询，因为我正在关注的教程只使用一个 DAO，我有点迷失了 或者我只是为那个关系创建另一个 Dao？ 一切都是为了整理你的东西。您可以在任何 DAO 中查询任何实体。如果您在 WorkOutDAO 文件中编写与锻炼相关的查询，它将增加您的代码可读性。除此之外，在单个 DAO 中编写所有查询并不是犯罪【参考方案2】：我不应该拥有一个包含所有相互关联的表的数据库吗？
是的。
我计划用多对多或一对多关系连接这两个表，但这不是我现在遇到的问题。
您可能想要多对多，即锻炼可以有很多锻炼，而一个锻炼可以有很多作为父母的锻炼。
因此，您可以为关系使用第三个表，其中包含锻炼 ID 的列和练习 ID 的列。这样的表有很多名称，例如关联表、引用表....
如果每个表都有一个 DAO，我该如何连接这些表。 可以定义和访问多个 Dao。
参见下面的演示（@Database 类TheDatabase），简而言之，您只需让@Database 类知道它们并允许检索 dao，Room 会为您完成所有底层工作。
演示
这是一个基于您的描述的基本工作示例，这还包括每个实体的 dao 以及所有 dao 的组合：-
锻炼实体（表格）：-
@Entity
data class Workout(
@PrimaryKey
val workoutId: Long? = null,
val workoutName: String
)
WorkoutDao :-
@Dao
abstract class WorkoutDao
@Insert
abstract fun insert(workout: Workout): Long
@Query("SELECT * FROM workout")
abstract fun getAllWorkouts(): List<Workout>
@Query("SELECT * FROM exercise WHERE exerciseId=:exerciseId")
abstract fun getWorkoutById(exerciseId: Long): Exercise
练习实体（表格）
@Entity(
indices = [Index(value = ["exerciseName"],unique = true)] // enforce unique exercise name.
)
data class Exercise(
@PrimaryKey
val exerciseId: Long? = null,
val exerciseName: String
)
ExerciseDao
@Dao
abstract class ExerciseDao
@Insert
abstract fun insert(exercise: Exercise): Long
@Query("SELECT * FROM exercise")
abstract fun getAllExercises(): List<Exercise>
@Query("SELECT * FROM exercise WHERE exerciseId=:exerciseId")
abstract fun getExerciseById(exerciseId: Long): Exercise
由于您可能想要映射表 WorkoutExerciseMap 实体（表）的多对多关系：-
@Entity(
primaryKeys = ["workoutIdMap","exerciseIdMap"], // a combination of workout/exercise is primary key
indices = [Index("exerciseIdMap")], // Room issues warning if not indexed
foreignKeys = [
// Foreign keys, each defines a constraint (rule) saying value to be store MUST exist in the parent table
// i.e. the value to be stored in the workoutIdMap MUST be the id of an existing Workout
ForeignKey(
entity = Workout::class, // the entity/table that the FK points to
parentColumns = ["workoutId"], // the column in the parent table
childColumns = ["workoutIdMap"], // column in this table where
onDelete = ForeignKey.CASCADE, // if a Workout is deleted then delete the children
onUpdate = ForeignKey.CASCADE // if a workoutId is changed then change the children
),
ForeignKey(entity = Exercise::class,parentColumns = ["exerciseId"],childColumns = ["exerciseIdMap"])
]
)
data class WorkoutExerciseMap(
val workoutIdMap: Long,
val exerciseIdMap: Long
)
WorkoutExerciseMapDao :-
@Dao
abstract class WorkoutExerciseMapDao
@Insert
abstract fun insert(workoutExerciseMapDao: WorkoutExerciseMap): Long
@Query("SELECT * FROM workoutexercisemap")
abstract fun getAllWorkoutExerciseMaps(): List<WorkoutExerciseMap>
@Transaction
@Query("SELECT * FROM workout")
abstract fun getAllWorkoutsWithExercises(): List<WorkoutWithExercises>
为了能够通过相关练习获得锻炼需要 POJO（不是实体），所以 WorkoutWithExercises :-
data class WorkoutWithExercises(
@Embedded
val workout: Workout,
@Relation(
entity = Exercise::class,
parentColumn = "workoutId",
entityColumn = "exerciseId",
associateBy = Junction(WorkoutExerciseMap::class,parentColumn = "workoutIdMap",entityColumn = "exerciseIdMap")
)
val exercises: List<Exercise>
)
关于将单个 Dao 放在 AllDao 是否是坏/好，顾名思义，ALL 的 dao 在一个类中：-
@Dao
abstract class AllDao
/*
As exercise has a unique index on exercisename skip if same exercise name is used
otherwise duplicating name will result in an exception
*/
@Insert(onConflict = OnConflictStrategy.IGNORE)
abstract fun insert(exercise: Exercise): Long
@Insert
abstract fun insert(workout: Workout): Long
@Insert
/*
not much use (if any) of returning Long as value will be the
rowid (hidden column).
*/
abstract fun insert(workoutExerciseMap: WorkoutExerciseMap): Long
@Query("SELECT * FROM workout")
abstract fun getAllWorkouts(): List<Workout>
@Query("SELECT * FROM workout WHERE workout.workoutId=:workoutId")
abstract fun getWorkoutById(workoutId: Long): Workout
@Query("SELECT * FROM Exercise")
abstract fun getAllExercises(): List<Exercise>
@Query("SELECT * FROM exercise WHERE exercise.exerciseId=:exerciseId")
abstract fun getExerciseById(exerciseId: Long): Exercise
@Query("SELECT * FROM workout")
@Transaction
abstract fun getAllWorkoutsWithExercises(): List<WorkoutWithExercises>
绑定所有组件的是@Database 类TheDatabase：-
@Database(entities = [Workout::class,Exercise::class,WorkoutExerciseMap::class],version = 1)
abstract class TheDatabase: RoomDatabase()
abstract fun getWorkoutDao(): WorkoutDao
abstract fun getExerciseDao(): ExerciseDao
abstract fun getWorkoutExerciseMapDao(): WorkoutExerciseMapDao
/* Versus all in one Dao ???? */
abstract fun getAllDao(): AllDao
companion object
@Volatile
private var instance: TheDatabase? = null
fun getDatabaseInstance(context: Context): TheDatabase
if (instance == null)
instance = Room.databaseBuilder(context,TheDatabase::class.java,"workoutexercise.db")
.allowMainThreadQueries()
.build()
return instance as TheDatabase
以上内容的实际使用通过 MainActivity 活动进行演示：-
class MainActivity : AppCompatActivity()
lateinit var db: TheDatabase
lateinit var workoutDao: WorkoutDao
lateinit var exerciseDao: ExerciseDao
lateinit var workoutExerciseMapDao: WorkoutExerciseMapDao
lateinit var allDao: AllDao
private final var TAG = "WOEINFO"
override fun onCreate(savedInstanceState: Bundle?)
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
db = TheDatabase.getDatabaseInstance(this)
workoutDao = db.getWorkoutDao()
exerciseDao = db.getExerciseDao()
workoutExerciseMapDao = db.getWorkoutExerciseMapDao()
var ex1 = exerciseDao.insert(Exercise(exerciseName = "Exercise1"))
var ex2 = exerciseDao.insert(Exercise(exerciseName = "Exercise2"))
var ex3 = exerciseDao.insert(Exercise(exerciseName = "Exercise3"))
var ex4 = exerciseDao.insert(Exercise(exerciseName = "Exercise4"))
var ex5 = exerciseDao.insert(Exercise(exerciseName = "Exercise5"))
var wo1 = workoutDao.insert(Workout(workoutName = "Workout1"))
var wo2 = workoutDao.insert(Workout(workoutName = "Workout2"))
allDao = db.getAllDao()
var ex6 = allDao.insert(Exercise(exerciseName = "Exercise6"))
var ex7 = allDao.insert(Exercise(exerciseName = "Exercise7"))
var wo3 = allDao.insert(Workout(workoutName = "Workout3"))
var wo4 = allDao.insert(Workout(workoutName = " Workout4"))
var wo5 = allDao.insert(Workout(workoutName = "Workout5"))
// Add 4 exercises to Workout1
workoutExerciseMapDao.insert(WorkoutExerciseMap(wo1,ex7))
allDao.insert(WorkoutExerciseMap(wo1,ex5))
workoutExerciseMapDao.insert(WorkoutExerciseMap(wo1,ex3))
workoutExerciseMapDao.insert(WorkoutExerciseMap(wo1,ex1))
// Add 3 Exercises to Workout2
allDao.insert(WorkoutExerciseMap(wo2,ex2))
allDao.insert(WorkoutExerciseMap(wo2,ex4))
allDao.insert(WorkoutExerciseMap(wo2,ex6))
// Add 2 Exercises to Workout3
workoutExerciseMapDao.insert(WorkoutExerciseMap(wo3,ex3))
workoutExerciseMapDao.insert(WorkoutExerciseMap(wo3,ex4))
// Add 1 Exercise to Workout 4
allDao.insert(WorkoutExerciseMap(wo4,ex5))
// Don't add anything to Workout 5
for(wwe: WorkoutWithExercises in allDao.getAllWorkoutsWithExercises())
Log.d(TAG,"Workout is $wwe.workout.workoutName")
for(ex: Exercise in wwe.exercises)
Log.d(TAG,"\tExercise is $ex.exerciseName")
/* Show effect of onConflictStrategy.IGNORE */
allDao.insert(Exercise(exerciseName = "Exercise1"))
for(ex: Exercise in allDao.getAllExercises())
Log.d(TAG,"Exercise is $ex.exerciseName")
/* effect without onConflictStrategy.IGNORE i.e. exception
i.e. exerciseDao does not have onConflictStrategy.IGNORE coded
*/
exerciseDao.insert(Exercise(exerciseName = "Exercise1"))
for(ex: Exercise in exerciseDao.getAllExercises())
Log.d(TAG,"Exercise is $ex.exerciseName")
结果
当上述运行时，日志包括：-
2021-08-07 10:17:07.368 D/WOEINFO: Workout is Workout1
2021-08-07 10:17:07.369 D/WOEINFO: Exercise is Exercise1
2021-08-07 10:17:07.369 D/WOEINFO: Exercise is Exercise3
2021-08-07 10:17:07.369 D/WOEINFO: Exercise is Exercise5
2021-08-07 10:17:07.369 D/WOEINFO: Exercise is Exercise7
2021-08-07 10:17:07.369 D/WOEINFO: Workout is Workout2
2021-08-07 10:17:07.369 D/WOEINFO: Exercise is Exercise2
2021-08-07 10:17:07.369 D/WOEINFO: Exercise is Exercise4
2021-08-07 10:17:07.369 D/WOEINFO: Exercise is Exercise6
2021-08-07 10:17:07.369 D/WOEINFO: Workout is Workout3
2021-08-07 10:17:07.369 D/WOEINFO: Exercise is Exercise3
2021-08-07 10:17:07.369 D/WOEINFO: Exercise is Exercise4
2021-08-07 10:17:07.369 D/WOEINFO: Workout is Workout4
2021-08-07 10:17:07.369 D/WOEINFO: Exercise is Exercise5
2021-08-07 10:17:07.369 D/WOEINFO: Workout is Workout5
在第一次尝试插入名为“Exercise1”的重复练习之后：-
2021-08-07 10:17:07.376 D/WOEINFO: Exercise is Exercise1
2021-08-07 10:17:07.376 D/WOEINFO: Exercise is Exercise2
2021-08-07 10:17:07.376 D/WOEINFO: Exercise is Exercise3
2021-08-07 10:17:07.376 D/WOEINFO: Exercise is Exercise4
2021-08-07 10:17:07.376 D/WOEINFO: Exercise is Exercise5
2021-08-07 10:17:07.376 D/WOEINFO: Exercise is Exercise6
2021-08-07 10:17:07.377 D/WOEINFO: Exercise is Exercise7
当第二次尝试插入名为“Exercise1”的练习时，但使用来自没有 onConflictStrategy.IGNORE 的 ExperimentDao 的插入，则：-
2021-08-07 10:17:07.380 32394-32394/a.a.so68682797exerciseappexample E/AndroidRuntime: FATAL EXCEPTION: main
Process: a.a.so68682797exerciseappexample, PID: 32394
java.lang.RuntimeException: Unable to start activity ComponentInfoa.a.so68682797exerciseappexample/a.a.so68682797exerciseappexample.MainActivity: android.database.sqlite.SQLiteConstraintException: UNIQUE constraint failed: Exercise.exerciseName (code 2067 SQLITE_CONSTRAINT_UNIQUE)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
....
【讨论】：
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