Android Room Kotlin内部联接

Question

我有两个实体帐户：

@Entity(tableName = "accounts",foreignKeys = arrayOf(
        ForeignKey(
                entity = Currency::class,
                parentColumns = arrayOf("id"),
                childColumns = arrayOf("currencyId"),
                onDelete = ForeignKey.CASCADE
        )
))
data class Account (
        @PrimaryKey(autoGenerate = true)
        var id:Int=0,
        @ColumnInfo(name="name")
        var accountName:String,
        @ColumnInfo(name = "balance")
        var initialBalance:Double,
        var currencyId:Int,
        var date:Date,
        var isDefault:Boolean=true
){
    constructor():this(0,"",0.0,0,Date(),false)
}

和货币：

@Entity(tableName = "currencies")
data class Currency(
        @PrimaryKey(autoGenerate = true)
        var id:Int=0,
        @ColumnInfo(name="name")
        var currencyName:String,
        @ColumnInfo(name="code")
        var currencyCode:String
)
{
    constructor():this(0,"","")

    override fun toString(): String =currencyCode
}

我想在中嵌入一个currency对象account。如您所见，currencies和之间存在一对多关系accounts。当我查询accounts实体时，我也想查看其货币。我尝试@Embedded在account实体中添加一个字段，但显然无法正常工作，这是我误解了，该字段返回null，“ No exception just null”。而且，如果可以将currency对象“扁平化”到对象内部account，效果会更好。

所有这些的重点是，我想显示所有帐户RecyclerView及其货币信息。我现在感到困惑@Embedded，@Relation任何帮助将不胜感激。

编辑
我不知道这是否有帮助：
这是我的AccountDao：

@Dao
interface AccountDao {
    @Insert
    fun insertAll(items:Array<Account>)

    @Update
    fun update(item:Account)

    @Delete
    fun delete(item:Account)

    @Query("select * from accounts")
    fun getAll():LiveData<Array<Account>>
}

Answer 1

我设法让它发挥作用。解决方案是创建一个单独的（POJO 或 POKO 等）类，我将其命名为AccountModel：

class AccountModel{
    var accountId:Int = 0
    var accountName:String = ""
    var accountInitialBalance:Double = 0.0
    var accountCreationDate: Date = Date()
    var currencyId:Int = 0
    var currencyCode:String = ""
    var isDefaultAccount:Boolean = false
    constructor()
    constructor(id:Int,name:String,balance:Double,date:Date,currencyId:Int,currencyCode:String,isDefault:Boolean){
        this.accountId = id
        this.accountName = name
        this.accountInitialBalance = balance
        this.accountCreationDate = date
        this.currencyId = currencyId
        this.currencyCode = currencyCode
        this.isDefaultAccount= isDefault
    }
    fun toAccount():Account = Account(this.accountId,this.accountName,this.accountInitialBalance,this.currencyId,this.accountCreationDate,this.isDefaultAccount)
}

然后，构建查询以执行正常操作，就像对普通 SQL 数据库inner join执行的操作一样。inner join像这样：

@Query("select accounts.id as accountId," +
            "accounts.name as accountName," +
            "accounts.balance as accountInitialBalance," +
            "accounts.currencyId," +
            "accounts.date as accountCreationDate," +
            "accounts.isDefault as isDefaultAccount," +
            "currencies.code as currencyCode " +
            "from accounts inner join currencies on accounts.currencyId=currencies.id")
    fun getAll():LiveData<Array<AccountModel>>

显然，您可以使用as x将此列投影到x返回对象中的字段，正如您所知，在数据库中该列是，accounts.id但在我的中AccountModel它是一个accountId. Google Room 真正令人印象深刻的是，即使我添加了一个非常聪明的对象，
我也能得到一个LiveData。AccountModelAccount