本题不会有代码。想象一下，我有一个应用程序，其中有一个完整的用户注册，注册后，一个帐户被登录。让它成为一个运动鞋商店。所以，这里是所有产品的列表（RecyclerView），用户可以将一些产品添加到他的收藏夹中。问题是如何保证每个注册用户都有自己的OWN收藏列表，这样他自然就看不到别人的了。我对如何通过 Firebase 或 ROOM 数据库完成此操作很感兴趣。
如果缩短问题，听起来可能是这样的：如何根据特定用户显示内容
【问题讨论】：
标签： android firebase kotlin android-recyclerview android-room
这可能是多对多的关系，因此在房间中，您将有一张供用户使用的表格，一张用于收藏可收藏项目的表格，然后是一张映射用户/收藏夹组合的表格。该表通常只存储两个引用/关系/映射/关联...一个给用户，另一个给收藏夹。
因此，许多用户可以将此类项目设为收藏夹，并且用户可以拥有任何收藏夹。您可以使用 SELECT 查询提取用户收藏列表，该查询包含对用户的引用的 WHERE 子句。
作为使用 Room 的工作示例：-
User 表（房间中的实体）：-
@Entity
data class User(
@PrimaryKey
val userId: Long? = null,
val username: String,
val userPassword: String
)
Item 表/实体：-
@Entity
data class Item(
@PrimaryKey
val itemId: Long? = null,
val itemName: String
)
映射/关系/关联 .... 表 UserItemFavourite :-
@Entity(
primaryKeys = ["userIdMap","itemIdMap"],
/* Foreign Keys enforce referential integrity */
foreignKeys = [
ForeignKey(entity = User::class, parentColumns = ["userId"],childColumns = ["userIdMap"], onDelete = CASCADE, onUpdate = CASCADE),
ForeignKey(entity = Item::class, parentColumns = ["itemId"],childColumns = ["itemIdMap"], onDelete = CASCADE, onUpdate = CASCADE)
],
indices = [Index("itemIdMap")]
)
data class UserItemFavourite(
val userIdMap: Long,
val itemIdMap: Long
)
一个 POJO（不是实体），用于获取带有收藏项列表的用户 UserWithFavouriteItems :-
data class UserWithFavouriteItems (
@Embedded
val user: User,
@Relation(
entity = Item::class,
parentColumn = "userId",
entityColumn = "itemId" ,
associateBy = Junction(
value = UserItemFavourite::class,
parentColumn = "userIdMap",
entityColumn = "itemIdMap"
)
)
val itemList: List<Item>
)
另一个获得项目计数的 POJO（奖励）是最喜欢的：-
data class FavouriteItems(
@Embedded
val item: Item,
val favouredcount: Long
)
一些Dao的（数据库交互函数）AllDao "-
@Dao
abstract class AllDao {
@Insert
abstract fun insert(user: User): Long
@Insert
abstract fun insert(item: Item): Long
@Insert
abstract fun insert(userItemFavourite: UserItemFavourite): Long
@Query("SELECT coalesce(userid,-1) FROM user WHERE username=:userName AND userPassword=:password")
abstract fun login(userName: String, password: String): Long
@Query("SELECT * FROM user")
abstract fun getAllusers(): List<User>
@Query("SELECT * from user WHERE userid=:userId")
abstract fun getUserWithFavouriteItems(userId: Long): UserWithFavouriteItems
@Query("SELECT item.*, (SELECT count(*) FROM useritemfavourite WHERE itemIdMap = itemid) AS favouredcount FROM item ORDER BY favouredcount DESC")
abstract fun getFavouriteItemsList(): List<FavouriteItems>
@Query("SELECT (SELECT count(*) FROM user) + (SELECT count(*) FROM item) > 0")
abstract fun isData(): Boolean
}
Database (@Database) 类（告诉数据库它是实体，并允许通过数据库实例获取 dao 的实例）这个还包括获取数据库的实例：-
@Database(entities = [User::class,Item::class,UserItemFavourite::class],version = 1)
abstract class TheDatabase: RoomDatabase() {
abstract fun getAllDao(): AllDao
companion object {
var instance: TheDatabase? = null
fun getInstance(context: Context): TheDatabase {
if (instance == null) {
instance = Room.databaseBuilder(
context,
TheDatabase::class.java,
"thedb.db"
)
.allowMainThreadQueries()
.build()
}
return instance as TheDatabase
}
}
}
最后将它们放在一个 Activity 中：-
class MainActivity : AppCompatActivity() {
lateinit var db: TheDatabase
lateinit var dao: AllDao
val TAG = "USERINFO"
var currentUserid: Long = -1L;
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
db = TheDatabase.getInstance(this)
dao = db.getAllDao()
val passwordAttempts = listOf<String>("notpasswordsarah","passwordsrahnot","passwordsarah")
addSomeData()
for(s: String in passwordAttempts) {
if (login("sarah",s)) break
}
val currentUserWithFavourites = dao.getUserWithFavouriteItems(currentUserid)
Log.d(TAG,"User logged in is ${currentUserWithFavourites.user.username}. Favourites are :-")
for (i: Item in currentUserWithFavourites.itemList) {
Log.d(TAG,"\tFavourite Item is ${i.itemName}")
}
val favouriteItems = dao.getFavouriteItemsList()
for (fi in favouriteItems) {
Log.d(TAG,"Item is ${fi.item.itemName} favourite count is ${fi.favouredcount}")
}
}
private fun addSomeData() {
val test = dao.isData()
if (!dao.isData()) {
val u1 = dao.insert(User(username = "Fred",userPassword = "passwordfred"))
val u2 = dao.insert(User(username = "mary",userPassword = "passwordmary"))
val u3 = dao.insert(User(username = "sarah",userPassword = "passwordsarah"))
val i1 = dao.insert(Item(itemName = "Book"))
val i2 = dao.insert(Item(itemName = "Diary"))
val i3 =dao.insert(Item(itemName = "Pad"))
val i4 = dao.insert(Item(itemName = "Pencil"))
val i5 = dao.insert(Item(itemName = "Pen"))
val i6 = dao.insert(Item(itemName = "Highlighter"))
val i7 = dao.insert(Item(itemName = "Ruler"))
val i8 = dao.insert(Item(itemName = "Compass"))
dao.insert(UserItemFavourite(userIdMap = u1, itemIdMap = i1)) /* Fred has Book */
dao.insert(UserItemFavourite(userIdMap = u1, itemIdMap = i5)) /* Fred has Pen */
dao.insert(UserItemFavourite(userIdMap = u1, itemIdMap = i7)) /* Fred has Ruler */
dao.insert(UserItemFavourite(userIdMap = u3, itemIdMap = i2)) /* Sarah has Diary */
dao.insert(UserItemFavourite(userIdMap = u3,i7)) /* Sarah has Ruler */
}
}
private fun login(username: String, password: String): Boolean {
currentUserid = dao.login(username,password)
if (currentUserid > 0 ) {
Log.d(TAG,"Successfully Logged In.")
} else {
Log.d(TAG,"Login unsuccessful.")
}
return currentUserid > 0
}
}
结果输出到日志：-
2021-07-19 13:56:09.820 D/USERINFO: Login unsuccessful.
2021-07-19 13:56:09.821 D/USERINFO: Login unsuccessful.
2021-07-19 13:56:09.822 D/USERINFO: Successfully Logged In.
2021-07-19 13:56:09.828 D/USERINFO: User logged in is sarah. Favourites are :-
2021-07-19 13:56:09.828 D/USERINFO: Favourite Item is Diary
2021-07-19 13:56:09.828 D/USERINFO: Favourite Item is Ruler
2021-07-19 13:56:09.830 D/USERINFO: Item is Ruler favourite count is 2
2021-07-19 13:56:09.830 D/USERINFO: Item is Book favourite count is 1
2021-07-19 13:56:09.830 D/USERINFO: Item is Diary favourite count is 1
2021-07-19 13:56:09.830 D/USERINFO: Item is Pen favourite count is 1
2021-07-19 13:56:09.830 D/USERINFO: Item is Pad favourite count is 0
2021-07-19 13:56:09.830 D/USERINFO: Item is Pencil favourite count is 0
2021-07-19 13:56:09.830 D/USERINFO: Item is Highlighter favourite count is 0
2021-07-19 13:56:09.831 D/USERINFO: Item is Compass favourite count is 0
重要提示 上述代码仅用于演示，因此简明扼要，缺少某些方面（应该）不适合已发布的应用程序。
【讨论】：