如何查询和过滤房间数据库中的关系实体

Question

例如：

用户：

@Entity(tableName = "user")
data class UserEntity(
    @PrimaryKey
    @ColumnInfo(name = "id") val id: String,
    @ColumnInfo(name = "username") val username: String,
    @ColumnInfo(name = "name") val name: String,

发帖：

@Entity(
    tableName = "post",
    foreignKeys = [
        ForeignKey(
            entity = UserEntity::class,
            parentColumns = ["id"],
            childColumns = ["user_id"],
            onDelete = ForeignKey.CASCADE,
            onUpdate = ForeignKey.CASCADE
        )
    ],
    indices = [Index(value = ["user_id"])]
)
data class PostEntity(
    @PrimaryKey
    @ColumnInfo(name = "id") var id: String, 
    @ColumnInfo(name = "user_id") var userId: String,
    @ColumnInfo(name = "body") val body: String,
    @ColumnInfo(name = "like") val like: Int,
    @ColumnType(name = "type") val type: String,
)

数据

data class Data(
    @Embedded
    val user: UserEntity,
    @Relation(parentColumn = "id", entityColumn = "user_id")
    val post: List<PostEntity> = emptyList(),
)

如果我使用SELECT * FROM user，我得到了想要的数据（一个用户和所有帖子），但是我如何过滤特定类型的帖子，比如WHERE post.type = 'sth'，这可能吗？

Answer 1

但是我如何过滤特定类型的帖子，

这完全取决于您要过滤的内容。您可能需要与过滤器匹配但包含所有帖子（与类型无关）的数据对象，在这种情况下您可以使用：-

@Transaction
@Query("SELECT * FROM user JOIN post ON user.id = user_id WHERE post.type = :type")
abstract fun getAllDataFiltered(type: String): List<Data>

• 你会在哪里使用var mylist = yourdao.getAllDataFiltered("sth")之类的东西

但是，由于 post 和 user 的 id 列都被命名为 id，因此歧义会受到干扰（用户 id 变成了 post id，因此没有底层的 post 对象提取）。

如果您将 PostEntity 更改为：-

@Entity(
    tableName = "post",
    foreignKeys = [
        ForeignKey(
            entity = UserEntity::class,
            parentColumns = ["id"],
            childColumns = ["user_id"],
            onDelete = ForeignKey.CASCADE,
            onUpdate = ForeignKey.CASCADE
        )
    ],
    indices = [Index(value = ["user_id"])]
)
data class PostEntity(
    @PrimaryKey
    @ColumnInfo(name = "postid") var id: String, //<<<<<<<<<< CHANGED
    @ColumnInfo(name = "user_id") var userId: String,
    @ColumnInfo(name = "body") val body: String,
    @ColumnInfo(name = "like") val like: Int,
    @ColumnInfo(name = "type") val type: String
)

然后消除歧义，返回的 Data 对象包括帖子类型为 sth 的各个用户的所有帖子。

如果您只想返回只有过滤帖子的 Data 对象，那么您必须绕过 Room 的返回 FULL/COMPLETE 相关对象的技术。

如果您将@Dao 类设为抽象类而不是接口，那么您将使用@Query，例如：-

@Query("SELECT * FROM post WHERE user_id=:userid AND type=:type")
abstract fun getPostsPerUserFiltered(userid: String, type: String): List<PostEntity>

还有一个函数，例如：-

fun getFullyFiltered(type: String): List<Data> {
    var rv: ArrayList<Data> = arrayListOf()
    for(d: Data in getAllDataFiltered(type)) {
        rv.add(Data(d.user,post = getPostsPerUserFiltered(d.user.id,type)))
    }
    return rv
}

这会过滤返回的 Data 对象，但随后会丢弃整个 PostEntity 列表（即每个帖子，无论过滤器如何），然后应用过滤后的帖子。

如果您想要所有用户但只有匹配的帖子（因此可能是一个空的帖子列表），那么您可以使用以下功能：-

fun getAllFilteredData(type: String): List<Data> {
    var rv: ArrayList<Data> = arrayListOf()
    for(u: UserEntity in getUsers()) {
        rv.add(Data(user = u, getPostsPerUserFiltered(u.id,type)))
    }
    return rv
}

即不会对用户应用过滤，而只会对帖子应用过滤。

然后使用上面的内容（注意使用更改的列名（postid 而不是 id））然后考虑以下内容（使用了一个非常标准的 @Database 类）：-

class MainActivity : AppCompatActivity() {
    lateinit var db: TheDatabase
    lateinit var dao: AllDao
    val TAG: String = "DBINFO"
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        db = TheDatabase.getInstance(this)
        dao = db.getAllDao()

        dao.insert(UserEntity(id = "User1","User001","Mary"))
        dao.insert(UserEntity(id = "User2",username = "User002",name = "Sue"))
        dao.insert(UserEntity(id = "User3",username = "User003",name = "Tom"))

        dao.insert(PostEntity(id = "post1",userId = "User1",body = "post1 blah",type = "xxx",like = 0))
        dao.insert(PostEntity(id ="post2", userId = "User2",body ="post2 blah", type = "sth",like = 1))
        dao.insert(PostEntity(id = "post3", userId = "User1", body = "post3 blah", type = "sth", like = 3))

        /*
            No filtering applied
        */
        for(d: Data in dao.getAllData()) {
            logData(d,"ALL")
        }
        /*
            Return FULL Data objects (i.e. with ALL posts) but only those that
            have a post or posts that match the filter
        */
        for (d: Data in dao.getAllDataFiltered("sth")) {
            logData(d,"JOIN")
        }
        /*
            Return partial Data objects, but only for those that
            have a post that matches the type
         */
        for (d: Data in dao.getFullyFiltered("sth")) {
            logData(d,"FULL")
        }

        /*
            Return all partial Data Objects but with partial posts.

         */
        for (d: Data in dao.getAllFilteredData("sth")) {
            logData(d,"POST")
        }
    }

    private fun logData(data: Data,tagSuffix: String) {
        Log.d(TAG + tagSuffix,"Data for user ${data.user.id}, Name is ${data.user.name} etc")
        for (p: PostEntity in data.post) {
            Log.d(TAG + tagSuffix,"\t Post is ${p.id} Type is ${p.type} body is:-\n\t\t${p.body}")
        }
    }
}

输出到日志的结果是：-

根本没有过滤：-

2021-09-17 11:22:00.722 D/DBINFOALL: Data for user User1, Name is Mary etc
2021-09-17 11:22:00.722 D/DBINFOALL:     Post is post1 Type is xxx body is:-
            post1 blah
2021-09-17 11:22:00.723 D/DBINFOALL:     Post is post3 Type is sth body is:-
            post3 blah
2021-09-17 11:22:00.723 D/DBINFOALL: Data for user User2, Name is Sue etc
2021-09-17 11:22:00.723 D/DBINFOALL:     Post is post2 Type is sth body is:-
            post2 blah
2021-09-17 11:22:00.723 D/DBINFOALL: Data for user User3, Name is Tom etc

只有用户被过滤，因为 Room 获取用户的所有帖子 :-

2021-09-17 11:22:00.728 D/DBINFOJOIN: Data for user User2, Name is Sue etc
2021-09-17 11:22:00.728 D/DBINFOJOIN:    Post is post2 Type is sth body is:-
            post2 blah
2021-09-17 11:22:00.728 D/DBINFOJOIN: Data for user User1, Name is Mary etc
2021-09-17 11:22:00.728 D/DBINFOJOIN:    Post is post1 Type is xxx body is:-
            post1 blah
2021-09-17 11:22:00.728 D/DBINFOJOIN:    Post is post3 Type is sth body is:-
            post3 blah

完全过滤：-

2021-09-17 11:22:00.738 D/DBINFOFULL: Data for user User2, Name is Sue etc
2021-09-17 11:22:00.738 D/DBINFOFULL:    Post is post2 Type is sth body is:-
            post2 blah
2021-09-17 11:22:00.738 D/DBINFOFULL: Data for user User1, Name is Mary etc
2021-09-17 11:22:00.738 D/DBINFOFULL:    Post is post3 Type is sth body is:-
            post3 blah

所有用户，但有过滤帖子

2021-09-17 11:22:00.744 D/DBINFOPOST: Data for user User1, Name is Mary etc
2021-09-17 11:22:00.744 D/DBINFOPOST:    Post is post3 Type is sth body is:-
            post3 blah
2021-09-17 11:22:00.744 D/DBINFOPOST: Data for user User2, Name is Sue etc
2021-09-17 11:22:00.744 D/DBINFOPOST:    Post is post2 Type is sth body is:-
            post2 blah
2021-09-17 11:22:00.744 D/DBINFOPOST: Data for user User3, Name is Tom etc

如果 PostEntity 列改回为 id 则结果为

:-

2021-09-17 11:27:00.661 D/DBINFOALL: Data for user User1, Name is Mary etc
2021-09-17 11:27:00.661 D/DBINFOALL:     Post is post1 Type is xxx body is:-
            post1 blah
2021-09-17 11:27:00.661 D/DBINFOALL:     Post is post3 Type is sth body is:-
            post3 blah
2021-09-17 11:27:00.661 D/DBINFOALL: Data for user User2, Name is Sue etc
2021-09-17 11:27:00.662 D/DBINFOALL:     Post is post2 Type is sth body is:-
            post2 blah
2021-09-17 11:27:00.662 D/DBINFOALL: Data for user User3, Name is Tom etc
2021-09-17 11:27:00.664 D/DBINFOJOIN: Data for user post2, Name is Sue etc
2021-09-17 11:27:00.664 D/DBINFOJOIN: Data for user post3, Name is Mary etc
2021-09-17 11:27:00.672 D/DBINFOFULL: Data for user post2, Name is Sue etc
2021-09-17 11:27:00.672 D/DBINFOFULL: Data for user post3, Name is Mary etc
2021-09-17 11:27:00.676 D/DBINFOPOST: Data for user User1, Name is Mary etc
2021-09-17 11:27:00.676 D/DBINFOPOST:    Post is post3 Type is sth body is:-
            post3 blah
2021-09-17 11:27:00.676 D/DBINFOPOST: Data for user User2, Name is Sue etc
2021-09-17 11:27:00.676 D/DBINFOPOST:    Post is post2 Type is sth body is:-
            post2 blah
2021-09-17 11:27:00.677 D/DBINFOPOST: Data for user User3, Name is Tom etc

• 即注意用户是帖子的 ID，因此没有基础帖子。
• 您可以使用@Embedded(prefix = "a_suitable_prefix")。但是，您必须在查询中使用 AS 更改用户（前缀表）的列名，使用明确的列名要简单得多。
• 第 4 次，返回所有用户但仅包含过滤后的帖子不受影响，因为它不使用 Data POJO，这是歧义导致用户 ID 成为帖子 ID 的地方。