如何插入“一对一”房间数据库

Question

我正在尝试在 android 中插入一个具有一对一关系的对象。 更准确地说，我正在尝试遵循以下示例 googles official documentation

我正在使用两个类用户和库。 我遇到的问题是如何在数据库中插入元素。 我的代码是这样写的

  @Entity
data class Library(
    @PrimaryKey(autoGenerate = true) val libraryId: Long = 0,
    val userOwnerId: Long
)


@Entity
data class User(
    @PrimaryKey(autoGenerate = true) val userId: Long =0,
    val name: String,
    val age: Int
) 

data class UserAndLibrary(
    @Embedded val user: User,
    @Relation(
        parentColumn = "userId",
        entityColumn = "userOwnerId"
    )
    val library: Library
)

插入的逻辑是这样的

private val repository = UserRepository(application)

    fun insertUser() {
        val user1 = User(name = "User1", age = 31)
        val user2 = User(name = "User2", age = 32)
        val library1 = Library(userOwnerId = user1.userId)
        val library2 = Library(userOwnerId = user2.userId)

        viewModelScope.launch {
            repository.insertUser(user1)
            
            repository.insertUser(user2)
            
            repository.insertLibrary(library1)
            
            repository.insertLibrary(library2)
         
        }

repository 和 dao 类的代码如下所示

//repository
suspend fun insertUser(user: User) = appDataBase.userDao().insertUser(user)

suspend fun insertLibrary(library: Library)=appDataBase.userDao().insertLibrary(library)

//dao
@Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insertUser(user: User)

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insertLibrary(library: Library)

问题是库表中的外键总是0。

Answer 1

我认为您的问题是由于您插入库数据的方式所致。更具体地说，当您插入库时，您是否设置了库userOwnerId 的值？ （我不相信或将其设置为 0）。

考虑以下使用您的类（用户、库和 userAndLibrary）的问题，根据问题使用以下@Dao 和一个非常标准的@Database：-

@Dao
abstract class AllDao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    abstract fun insert(library: Library): Long
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    abstract fun insert(user: User): Long
    @Query("SELECT * FROM user")
    abstract fun getAllUsers(): List<User>
    @Query("SELECT * FROM library")
    abstract fun getAllLibraries(): List<Library>
    @Query("SELECT * FROM user")
    abstract fun getUsersAndThierLibrary(): List<UserAndLibrary>
}

• 插入返回 Long 的注意事项（即插入行的 id）

结合上述活动中的代码（为了方便和简洁，在主线程上运行）：-

    db = TheDatabase.getInstance(this)
    dao = db.getAllDao()

    /* Explanatory Insert */
    var firstUser = User(name = "Susan",age = 20)
    var firstUserId = dao.insert(firstUser)
    dao.insert(Library(userOwnerId = firstUserId))

    /* More Concise Inserts */
    dao.insert(Library(userOwnerId = dao.insert(User(name ="Fred",age = 21))))
    dao.insert(Library(userOwnerId = dao.insert(User(name ="Mary",age= 19))))

    for (u: User in dao.getAllUsers()) {
        Log.d(TAG," User is ${u.name} ID is ${u.userId} age is ${u.age}")
    }
    for (l: Library in dao.getAllLibraries()) {
        Log.d(TAG,"Library is ${l.libraryId} owned by ${l.userOwnerId}")
    }
    for(ual: UserAndLibrary in dao.getUsersAndThierLibrary()) {
        Log.d(TAG,"User = ${ual.user.name} Library ID is ${ual.library.libraryId}")
    }

• 再次注意在插入用户时使用返回的值（userId）。

可以看出，上面插入了 3 个用户和库。然后它提取所有用户、所有库和 UserAndLibrary。结果是：-

2021-07-31 12:29:41.888 D/MYINFO:  User is Susan ID is 1 age is 20
2021-07-31 12:29:41.889 D/MYINFO:  User is Fred ID is 2 age is 21
2021-07-31 12:29:41.889 D/MYINFO:  User is Mary ID is 3 age is 19
2021-07-31 12:29:41.890 D/MYINFO: Library is 1 owned by 1
2021-07-31 12:29:41.890 D/MYINFO: Library is 2 owned by 2
2021-07-31 12:29:41.890 D/MYINFO: Library is 3 owned by 3
2021-07-31 12:29:41.894 D/MYINFO: User = Susan Library ID is 1
2021-07-31 12:29:41.895 D/MYINFO: User = Fred Library ID is 2
2021-07-31 12:29:41.895 D/MYINFO: User = Mary Library ID is 3

• 即一切都如预期的那样。

Answer 2

这工作得很好，但我必须承认这不是那么用户友好，也没有从谷歌的任何地方记录下来。批评者是针对谷歌而不是你。 是否可以做这样的事情（c#和实体框架示例）

var dept = new Department()
{
    Name = "Admin"
};
 
var emp = new Employee()
{
    Name = "Matt",
    Designation = "Head",
    Department = dept
};
 
using (var context = new CompanyContext())
{
    context.Add(emp);
    await context.SaveChangesAsync();
}

这将是使用框架库的一种非常直观的方式