问题描述
@Entity(tableName = "servers")
data class Server(
@ColumnInfo(name = "server_id")
var serverId: Long,@ColumnInfo(name = "account_id")
var accountId: Int,@ColumnInfo(name = "device_id")
var deviceid: Int,@ColumnInfo(name = "company_id")
var companyId: Int
.......
@Entity(tableName = "sites")
data class Site(
@ColumnInfo(name = "server_id")
var serverId: Long,@ColumnInfo(name = "site_id")
val siteId: Int,@ColumnInfo(name = "description",defaultValue = "")
val description: String
)
@Entity(tableName = "groups")
data class Group(
@ColumnInfo(name = "server_id")
val serverId: Long,@ColumnInfo(name = "group_id")
var groupId: Int,defaultValue = "")
val description: String
......
所以我们可以看到，对于每台服务器，我们都有站点，并且每个站点都有几个组。 现在我制作了一个名为 ServerSiteWithGroup 的 POJO，其中包含一个 Server 和另一个 POJO SiteWithGroup
data class ServerSiteWithGroup(
@Embedded
val server: Server,@Relation(parentColumn = "server_id",entityColumn = "server_id")
val siteWithGroup: SiteWithGroup
)
data class SiteWithGroup(
@Embedded
val group: Group,@Relation(parentColumn = "site_id",entityColumn = "site_id")
val site: Site
)
因此，鉴于我想对 Room 数据库进行一次查询并获取给定 serverId,siteId,groupId 的服务器、站点、组对象
我试过了，但没有用
@Transaction
@Query("Select * from groups
inner join servers on groups.server_id = servers.server_id
where groups.server_id = :serverId
and groups.site_id = :siteId
and groups.group_id = :groupId")
fun getSiteWithGroup(serverId: Long,siteId: Int,groupId: Int): LiveData<ServerSiteWithGroup>
我该如何解决这个问题？
解决方法
首先，您似乎没有使用 @PrimaryKey 注释定义任何 PRIMARY 键，这会导致编译错误。
其次，@Query 不应返回 LiveData<ServerSiteWithGroup>，但应返回一组 LiveData，这些 LiveData 本身是（我相信）ServerSiteWithGroup 的数组，因此我认为应该是 LiveData<List<ServerSiteWithGroup>>。
- Perahps 首先尝试这个，但请考虑在答案末尾添加的甚至更多部分。 .
我该如何解决这个问题？
- 您没有提及/说明实际问题是什么。但是，以下是基于您提供的信息的解决方案。 （但为了方便，不使用
LiveData）
在您拥有 ServerId 的组实体中，这是不必要的，因为作为组父级的站点将服务器作为其父级。这不是错误，而是不必要的。
虽然理论上不是必需的，id 可以是 Long，因此我总是建议在关系中使用 Long 而不是 Int。
示例 这是一个基于您的代码的工作示例。
服务器
@Entity(tableName = "servers")
data class Server(
@PrimaryKey(autoGenerate = true)
@ColumnInfo(name = "server_id")
var serverId: Long?,@ColumnInfo(name = "account_id")
var accountId: Int,@ColumnInfo(name = "device_id")
var deviceId: Int,@ColumnInfo(name = "company_id")
var companyId: Int
)
-
Primarykey已定义。 -
autogenerate和Long?允许生成 ID。
网站
@Entity(tableName = "sites")
data class Site(
@PrimaryKey(autoGenerate = true)
@ColumnInfo(name = "site_id")
val siteId: Long?,@ColumnInfo(name = "server_id")
var serverId: Long,@ColumnInfo(name = "description",defaultValue = "")
val description: String
)
群组
@Entity(tableName = "groups")
data class Group(
@PrimaryKey(autoGenerate = true)
@ColumnInfo(name = "group_id")
var groupId: Long?,@ColumnInfo(name = "server_id") //?
val serverId: Long,@ColumnInfo(name = "site_id")
val siteId: Long,defaultValue = "")
val description: String
)
Alldao Dao 允许在各个级别插入和提取，特别是使用 @relation 和 JOIN。
interface AllDao {
@Insert
fun insertServer(server: Server): Long
@Insert
fun insertSite(site: Site): Long
@Insert
fun insertGroup(group: Group): Long
@Query("SELECT * FROM servers")
fun getAllServers(): List<Server>
@Query("SELECT * FROM sites")
fun getAllSites(): List<Site>
@Query("SELECT * FROM groups")
fun getAllGroups(): List<Group>
@Transaction
@Query("SELECT * FROM sites")
fun getAllSitesWithGroups(): List<SiteWithGroup>
@Transaction
@Query("SELECT * FROM servers")
fun getAllServersWithSitesWithGroups(): List<ServerWithSiteWithGroup>
@Transaction
@Query("SELECT * FROM servers INNER JOIN sites ON servers.server_id = sites.server_id INNER JOIN groups on sites.site_id = groups.site_id")
fun getAllServersWithSitesWithGroupsUsingJoin(): List<ServerWithSiteWithGroup>
@Transaction
@Query("Select * from groups " +
"inner join servers on groups.server_id = servers.server_id " +
"where groups.server_id = :serverId " +
"and groups.site_id = :siteId " +
"and groups.group_id = :groupId")
fun getSiteWithGroup(serverId: Long,siteId: Int,groupId: Int): List<ServerWithSiteWithGroup>
}
- 注意原始查询之前的最后两个
@Query相似但有细微差别（查看结果） - 请注意，您的查询只返回 List 而不是 LiveData
SiteWithGroup 组与其父站点的 POJO 关系（一个站点可以有多个组）
data class SiteWithGroup(
@Embedded
var site: Site,@Relation(entity = Group::class,entityColumn = "site_id",parentColumn = "site_id")
var groups: List<Group>
)
- 虽然在这种情况下没有必要，但我更喜欢对实体（类）进行编码
ServerWithSiteWithGroup
data class ServerWithSiteWithGroup(
@Embedded
var server: Server,@Relation(entity = Site::class,entityColumn = "server_id",parentColumn = "server_id")
var siteWithGroup: List<SiteWithGroup>
)
- 请注意，我使用了与原始名称不同的名称，因为我相信它更能描述底层 POJO。
MyDatabse 绑定实体和道的@Database 抽象类（为方便起见，仅提供一个）
@Database(entities = arrayOf(Server::class,Site::class,Group::class),version = 1)
abstract class MyDatabase: RoomDatabase() {
abstract fun getAllDao(): AllDao
}
MainActivity 用于在主线程上运行的演示/bervity/convenience
class MainActivity : AppCompatActivity() {
val TAG = "MYDBINFO"
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val db = Room.databaseBuilder(applicationContext,MyDatabase::class.java,"MyDatabase")
.allowMainThreadQueries()
.build()
val allDao = db.getAllDao()
val s1 = Server(null,1,2,3)
val s2 = Server(null,4,5,6)
allDao.insertServer(s1)
allDao.insertServer(s2)
val site1 = Site(null,"Site1 - Server1")
val site2 = Site(null,"Site2 - Server1")
val site3 = Site(null,"Site3 - Server2")
val site4 = Site(null,"Site4 - Server2")
val site5 = Site(null,"Site5 - Server 1")
allDao.insertSite(site1)
allDao.insertSite(site2)
allDao.insertSite(site3)
allDao.insertSite(site4)
allDao.insertSite(site5)
val g1 = Group(null,"Group1 - Site1 (impicitly Server 1)")
val g2 = Group(null,10,"Group2!!! - Site1 (impicitly Server 1)") // OOOPS no server with ID 10
val g2_2 = Group(null,"Group2_2 - Site1 (impicitly Server 1)")
val g3 = Group(null,"Group3 - Site2 (impicitly Server 1)")
allDao.insertGroup(g1)
allDao.insertGroup(g2)
allDao.insertGroup(g2_2)
allDao.insertGroup(g3)
val servers = allDao.getAllServers()
Log.d(TAG,"Server List")
for (s: Server in servers) {
Log.d(TAG,"\tServerID = " + s.serverId + " CompanyID =" + s.companyId + " AccountID = " + s.accountId + " DeviceID = " + s.deviceId)
}
val sites = allDao.getAllSites()
Log.d(TAG,"Site List")
for (si: Site in sites) {
Log.d(TAG,"\tSiteID = " + si.siteId + " Description = " + si.description + " Server ID = " + si.serverId)
}
val groups = allDao.getAllGroups()
for(g: Group in groups) {
Log.d(TAG,"\tGroup ID = " + g.groupId + " Description = " + g.description + " ServerID = " + g.serverId + " SiteID = " + g.siteId)
}
val sitesWithGroupsList = allDao.getAllSitesWithGroups()
Log.d(TAG,"Site With Groups List")
for(swgl: SiteWithGroup in sitesWithGroupsList) {
Log.d(TAG,"\tSiteID = " + swgl.site.siteId + " ServerID = " + swgl.site.serverId + " Description " + swgl.site.description)
for(grp: Group in swgl.groups) {
Log.d(TAG,"\t\tGroup ID = " + grp.groupId + " SiteID = " + grp.siteId + " Description = " + grp.description + " ServerID = " + grp.serverId)
}
}
Log.d(TAG,"****Servers with Sites with Groups using @Relation")
val swswg = allDao.getAllServersWithSitesWithGroups()
for(s: ServerWithSiteWithGroup in swswg) {
Log.d(TAG,"ServerID is " + s.server.serverId)
Log.d(TAG,"Sites in Server =" + s.siteWithGroup.size)
for(swg: SiteWithGroup in s.siteWithGroup) {
Log.d(TAG,"\tSite is " + swg.site.description)
Log.d(TAG,"\tGroups in Site =" + swg.groups.size)
for (g: Group in swg.groups) {
Log.d(TAG,"\t\tGroup is " + g.description)
}
}
}
Log.d(TAG,"****Servers with Sites with Groups using Joins")
val swswg2 = allDao.getAllServersWithSitesWithGroupsUsingJoin()
for(s: ServerWithSiteWithGroup in swswg2) {
Log.d(TAG,"\t\tGroup is " + g.description)
}
}
}
// ORIGINAL PROBLEM QUERY
Log.d(TAG,"****Servers With Sites With Groups using joins and where clauses (problem query)")
val swswgsel = allDao.getSiteWithGroup(1,1)
for (swswg3: ServerWithSiteWithGroup in swswgsel) {
Log.d(TAG,"ServerID is " + swswg3.server.serverId)
Log.d(TAG,"Sites in Server =" + swswg3.siteWithGroup.size)
for(swg: SiteWithGroup in swswg3.siteWithGroup) {
Log.d(TAG,"\t\tGroup is " + g.description)
}
}
}
}
}
- 以上代码依次流过
- 日志记录可以轻松监控结果。
- 设计为运行一次（多次运行可能会引起一些混乱）
- 为方便起见，我们使用了 LiveData。
结果最初运行时的日志
- 您的原始查询位于日志末尾
:-
03-28 16:56:24.018 D/MYDBINFO: Server List
03-28 16:56:24.018 D/MYDBINFO: ServerID = 1 DeviceID = 2 AccountID = 1 CompanyID = 3
03-28 16:56:24.018 D/MYDBINFO: ServerID = 2 DeviceID = 5 AccountID = 4 CompanyID = 6
03-28 16:56:24.019 D/MYDBINFO: Site List
03-28 16:56:24.019 D/MYDBINFO: SiteID = 1 Description = Site1 - Server1 ServerID = 1
03-28 16:56:24.019 D/MYDBINFO: SiteID = 2 Description = Site2 - Server1 ServerID = 1
03-28 16:56:24.019 D/MYDBINFO: SiteID = 3 Description = Site3 - Server2 ServerID = 2
03-28 16:56:24.019 D/MYDBINFO: SiteID = 4 Description = Site4 - Server2 ServerID = 2
03-28 16:56:24.019 D/MYDBINFO: SiteID = 5 Description = Site5 - Server 1 ServerID = 1
03-28 16:56:24.021 D/MYDBINFO: Group List
03-28 16:56:24.022 D/MYDBINFO: GroupID = 1 Description = Group1 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.022 D/MYDBINFO: GroupID = 2 Description = Group2!!! - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 10(May not be correct)
03-28 16:56:24.022 D/MYDBINFO: GroupID = 3 Description = Group2_2 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.022 D/MYDBINFO: GroupID = 4 Description = Group3 - Site2 (impicitly Server 1)Parent SiteID = 2 Parent ServerID = 2(May not be correct)
03-28 16:56:24.024 D/MYDBINFO: Site With Groups List
03-28 16:56:24.024 D/MYDBINFO: SiteID = 1 Description = Site1 - Server1 ServerID = 1
03-28 16:56:24.024 D/MYDBINFO: GroupID = 1 Description = Group1 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.024 D/MYDBINFO: GroupID = 2 Description = Group2!!! - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 10(May not be correct)
03-28 16:56:24.024 D/MYDBINFO: GroupID = 3 Description = Group2_2 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.024 D/MYDBINFO: SiteID = 2 Description = Site2 - Server1 ServerID = 1
03-28 16:56:24.024 D/MYDBINFO: GroupID = 4 Description = Group3 - Site2 (impicitly Server 1)Parent SiteID = 2 Parent ServerID = 2(May not be correct)
03-28 16:56:24.024 D/MYDBINFO: SiteID = 3 Description = Site3 - Server2 ServerID = 2
03-28 16:56:24.024 D/MYDBINFO: SiteID = 4 Description = Site4 - Server2 ServerID = 2
03-28 16:56:24.025 D/MYDBINFO: SiteID = 5 Description = Site5 - Server 1 ServerID = 1
03-28 16:56:24.025 D/MYDBINFO: ****Servers with Sites with Groups using @Relation
03-28 16:56:24.027 D/MYDBINFO: ServerID = 1 DeviceID = 2 AccountID = 1 CompanyID = 3
03-28 16:56:24.028 D/MYDBINFO: Sites in Server =3
03-28 16:56:24.028 D/MYDBINFO: SiteID = 1 Description = Site1 - Server1 ServerID = 1
03-28 16:56:24.028 D/MYDBINFO: Groups in Site =3
03-28 16:56:24.028 D/MYDBINFO: GroupID = 1 Description = Group1 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.028 D/MYDBINFO: GroupID = 2 Description = Group2!!! - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 10(May not be correct)
03-28 16:56:24.028 D/MYDBINFO: GroupID = 3 Description = Group2_2 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.028 D/MYDBINFO: SiteID = 2 Description = Site2 - Server1 ServerID = 1
03-28 16:56:24.028 D/MYDBINFO: Groups in Site =1
03-28 16:56:24.028 D/MYDBINFO: GroupID = 4 Description = Group3 - Site2 (impicitly Server 1)Parent SiteID = 2 Parent ServerID = 2(May not be correct)
03-28 16:56:24.028 D/MYDBINFO: SiteID = 5 Description = Site5 - Server 1 ServerID = 1
03-28 16:56:24.028 D/MYDBINFO: Groups in Site =0
03-28 16:56:24.028 D/MYDBINFO: ServerID = 2 DeviceID = 5 AccountID = 4 CompanyID = 6
03-28 16:56:24.028 D/MYDBINFO: Sites in Server =2
03-28 16:56:24.028 D/MYDBINFO: SiteID = 3 Description = Site3 - Server2 ServerID = 2
03-28 16:56:24.028 D/MYDBINFO: Groups in Site =0
03-28 16:56:24.028 D/MYDBINFO: SiteID = 4 Description = Site4 - Server2 ServerID = 2
03-28 16:56:24.028 D/MYDBINFO: Groups in Site =0
03-28 16:56:24.028 D/MYDBINFO: ****Servers with Sites with Groups using Joins
03-28 16:56:24.029 D/MYDBINFO: ServerID = 1 DeviceID = 2 AccountID = 1 CompanyID = 3
03-28 16:56:24.029 D/MYDBINFO: Sites in Server =3
03-28 16:56:24.029 D/MYDBINFO: SiteID = 1 Description = Site1 - Server1 ServerID = 1
03-28 16:56:24.029 D/MYDBINFO: Groups in Site =3
03-28 16:56:24.029 D/MYDBINFO: GroupID = 1 Description = Group1 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.029 D/MYDBINFO: GroupID = 2 Description = Group2!!! - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 10(May not be correct)
03-28 16:56:24.029 D/MYDBINFO: GroupID = 3 Description = Group2_2 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.029 D/MYDBINFO: SiteID = 2 Description = Site2 - Server1 ServerID = 1
03-28 16:56:24.029 D/MYDBINFO: Groups in Site =1
03-28 16:56:24.029 D/MYDBINFO: GroupID = 4 Description = Group3 - Site2 (impicitly Server 1)Parent SiteID = 2 Parent ServerID = 2(May not be correct)
03-28 16:56:24.029 D/MYDBINFO: SiteID = 5 Description = Site5 - Server 1 ServerID = 1
03-28 16:56:24.029 D/MYDBINFO: Groups in Site =0
03-28 16:56:24.029 D/MYDBINFO: ServerID = 10 DeviceID = 2 AccountID = 1 CompanyID = 3
03-28 16:56:24.029 D/MYDBINFO: Sites in Server =0
03-28 16:56:24.029 D/MYDBINFO: ServerID = 1 DeviceID = 2 AccountID = 1 CompanyID = 3
03-28 16:56:24.029 D/MYDBINFO: Sites in Server =3
03-28 16:56:24.030 D/MYDBINFO: SiteID = 1 Description = Site1 - Server1 ServerID = 1
03-28 16:56:24.030 D/MYDBINFO: Groups in Site =3
03-28 16:56:24.030 D/MYDBINFO: GroupID = 1 Description = Group1 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.030 D/MYDBINFO: GroupID = 2 Description = Group2!!! - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 10(May not be correct)
03-28 16:56:24.030 D/MYDBINFO: GroupID = 3 Description = Group2_2 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.030 D/MYDBINFO: SiteID = 2 Description = Site2 - Server1 ServerID = 1
03-28 16:56:24.030 D/MYDBINFO: Groups in Site =1
03-28 16:56:24.030 D/MYDBINFO: GroupID = 4 Description = Group3 - Site2 (impicitly Server 1)Parent SiteID = 2 Parent ServerID = 2(May not be correct)
03-28 16:56:24.030 D/MYDBINFO: SiteID = 5 Description = Site5 - Server 1 ServerID = 1
03-28 16:56:24.030 D/MYDBINFO: Groups in Site =0
03-28 16:56:24.030 D/MYDBINFO: ServerID = 2 DeviceID = 2 AccountID = 1 CompanyID = 3
03-28 16:56:24.030 D/MYDBINFO: Sites in Server =2
03-28 16:56:24.030 D/MYDBINFO: SiteID = 3 Description = Site3 - Server2 ServerID = 2
03-28 16:56:24.030 D/MYDBINFO: Groups in Site =0
03-28 16:56:24.030 D/MYDBINFO: SiteID = 4 Description = Site4 - Server2 ServerID = 2
03-28 16:56:24.030 D/MYDBINFO: Groups in Site =0
03-28 16:56:24.030 D/MYDBINFO: ****Servers With Sites With Groups using joins and where claues (problem query)
03-28 16:56:24.031 D/MYDBINFO: ServerID = 1 DeviceID = 2 AccountID = 1 CompanyID = 3
03-28 16:56:24.031 D/MYDBINFO: Sites in Server =3
03-28 16:56:24.031 D/MYDBINFO: SiteID = 1 Description = Site1 - Server1 ServerID = 1
03-28 16:56:24.031 D/MYDBINFO: Groups in Site = 3
03-28 16:56:24.031 D/MYDBINFO: GroupID = 1 Description = Group1 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.031 D/MYDBINFO: GroupID = 2 Description = Group2!!! - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 10(May not be correct)
03-28 16:56:24.031 D/MYDBINFO: GroupID = 3 Description = Group2_2 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.031 D/MYDBINFO: SiteID = 2 Description = Site2 - Server1 ServerID = 1
03-28 16:56:24.032 D/MYDBINFO: Groups in Site = 1
03-28 16:56:24.032 D/MYDBINFO: GroupID = 4 Description = Group3 - Site2 (impicitly Server 1)Parent SiteID = 2 Parent ServerID = 2(May not be correct)
03-28 16:56:24.032 D/MYDBINFO: SiteID = 5 Description = Site5 - Server 1 ServerID = 1
03-28 16:56:24.032 D/MYDBINFO: Groups in Site = 0
额外 您可能希望考虑以下使用正确关系的方法：-
@Transaction
@Query("SELECT * FROM servers " +
"JOIN sites ON sites.server_id = servers.server_id " +
"JOIN groups ON groups.site_id = sites.site_id " +
"WHERE servers.server_id = :serverId AND sites.site_id = :siteId AND groups.group_id = :groupId")
fun getSuggested(serverId: Long,siteId: Long,groupId: Long): List<ServerWithSiteWithGroup>
无论如何，对于相同的测试用例，这实际上实现了相同的结果:-
03-28 16:56:24.032 D/MYDBINFO: ????Servers With Sites With Groups and WHERE clause (suggested)
03-28 16:56:24.033 D/MYDBINFO: ServerID = 1 DeviceID = 2 AccountID = 1 CompanyID = 3
03-28 16:56:24.033 D/MYDBINFO: Sites in Server = 3
03-28 16:56:24.033 D/MYDBINFO: SiteID = 1 Description = Site1 - Server1 ServerID = 1
03-28 16:56:24.033 D/MYDBINFO: Groups in Site = 3
03-28 16:56:24.033 D/MYDBINFO: GroupID = 1 Description = Group1 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.033 D/MYDBINFO: GroupID = 2 Description = Group2!!! - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 10(May not be correct)
03-28 16:56:24.033 D/MYDBINFO: GroupID = 3 Description = Group2_2 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
03-28 16:56:24.033 D/MYDBINFO: SiteID = 2 Description = Site2 - Server1 ServerID = 1
03-28 16:56:24.033 D/MYDBINFO: Groups in Site = 1
03-28 16:56:24.033 D/MYDBINFO: GroupID = 4 Description = Group3 - Site2 (impicitly Server 1)Parent SiteID = 2 Parent ServerID = 2(May not be correct)
03-28 16:56:24.033 D/MYDBINFO: SiteID = 5 Description = Site5 - Server 1 ServerID = 1
03-28 16:56:24.033 D/MYDBINFO: Groups in Site = 0
更多
在指定所有三个参数时，您可能不期望结果，而是期望单个服务器/站点/组。如果是这样，那么我相信使用 @Relation 不是要走的路。相反，没有关系的 POJO 是要走的路。
考虑后面查询中的 SQL，例如：-
SELECT *,sites.description AS site_description,groups.description AS group_description FROM groups
INNER JOIN sites ON sites.site_id = groups.site_id
INNER JOIN servers ON servers.server_id = sites.server_id
WHERE servers.server_id = 1 AND sites.site_id = 1 AND groups.group_id = 1;
有了上面的数据（注意 sites.description AS site_description,groups.description AS group_description 是为了从不同的表中消除/区分相同的列名）。
您可能会预料到（当 args 为 1,1 时）：-
正如上面的日志，这不是 Room 提供的。
但是考虑 POJO :-
class AltServerSiteGroup {
var server_id: Long = 0
var device_id: Long = 0
var account_id: Long = 0
var company_id: Long = 0
var site_id: Long = -1
var site_description: String = ""
var group_id: Long = -1
var group_description: String = ""
}
- 注意我改为使用 Long 而不是 Int（在
Server中也是如此）。
然后考虑道:-
@Transaction
@Query("SELECT *,groups.description AS group_description FROM groups " +
"INNER JOIN sites ON sites.site_id = groups.site_id " +
"INNER JOIN servers ON servers.server_id = sites.server_id " +
"WHERE servers.server_id = :serverId AND sites.site_id = :siteId AND groups.group_id = :groupId")
fun getAlt(serverId: Long,groupId: Long): List<AltServerSiteGroup>
- 如果可能只返回 1 个值，则可以仅使用
AltServerSiteGroup而不是List<AltServerSiteGroup>（如果 ID 是主键，情况就是如此）。
所以通过将以下内容添加到 MainActivity :-
Log.d(TAG,"Alternative Perhaps what is wanted")
val alt = allDao.getAlt(1,1)
for(s: AltServerSiteGroup in alt) {
logserver(Server(s.server_id,s.account_id,s.device_id,s.company_id))
logsite(Site(s.site_id,s.server_id,s.site_description))
loggroup(Group(s.group_id,s.site_id,s.group_description))
}
- 添加了记录实体的函数，因此 logserver(Server(....)) 等
那么该部分的结果将是：-
03-28 19:10:41.930 D/MYDBINFO: Alternative Perhaps what is wanted
03-28 19:10:41.931 D/MYDBINFO: ServerID = 1 DeviceID = 2 AccountID = 1 CompanyID = 3
03-28 19:10:41.931 D/MYDBINFO: SiteID = 1 Description = Site1 - Server1 ServerID = 1
03-28 19:10:41.931 D/MYDBINFO: GroupID = 1 Description = Group1 - Site1 (impicitly Server 1)Parent SiteID = 1 Parent ServerID = 1(May not be correct)
即符合选择条件的单个服务器/站点/组。
您可以向 AltServerSiteGroup 添加函数以返回提取的 Server/Site/Group 对象，但请注意，这些对象与完整/完整对象不同，因为它们只有单个 Server-Site-Group，例如:-
class AltServerSiteGroup {
var server_id: Long = 0
var device_id: Long = 0
var account_id: Long = 0
var company_id: Long = 0
var site_id: Long = -1
var site_description: String = ""
var group_id: Long = -1
var group_description: String = ""
fun getServer(): Server {
return Server(server_id,account_id,device_id,company_id)
}
fun getSite(): Site {
return Site(site_id,server_id,site_description)
}
fun getGroup(): Group {
return Group(group_id,site_id,group_description)
}
}
-
说明
-
简而言之，Room 将根据
@Relation构建完整 对象，从而添加额外的不需要的 站点和组。 -
如果你查看Java（生成）中Dao的代码（使用项目窗口中的Android视图），注意文件名后缀为_Impl （所以对于{{1 }}，如上所用，在生成的java中它是
AllDao)，你会看到room做了什么，并且为AllDao_Impl构建的代码比getAlt的代码短很多{1}}。