Android 房间 Kotlin 内部连接

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 account 中嵌入一个 currency 对象。如您所见，我在 currencies 和 accounts 之间建立了一对多的关系。当我查询 accounts 实体时，我也想查看其货币。我尝试在 account 实体中添加一个 @Embedded 字段，但它不起作用 显然我误解了一些东西 ，该字段返回 null "No exception just null"。如果可以在account对象中"flatten"currency对象，这样就更好了。

所有这一切的重点是，我想显示 RecyclerView 中的所有帐户及其货币信息。我现在在 @Embedded 和 @Relation 之间感到困惑。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

我设法让它工作了。解决方案是创建一个单独的（POJO 或 POKO 随便什么）class，我称之为 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，构造查询以执行正常的 inner join，就好像您正在对普通的 SQL 数据库执行 inner join 一样。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 将此列投影到返回对象中的 x 字段，如您所知，在数据库中该列是 accounts.id 但在我的 AccountModel 这是一个 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 Account 对象，我也能够获得 AccountModel 的 LiveData。

Answer 2

我不会推荐上述方法，因为您最终会编写相同的属性（重复自己），也就是样板代码。

按以下方式使用 @Embedded and Relation 注释，您的代码很可能如下所示：

data class AccountWithCurrency (
    @Embedded
    var account: Account? = null,
    @Relation(parentColumn = "id", entityColumn = "currencyId")
    var currency: List<Currency>? = null,
){
constructor() : this(Account(), emptyList())
}

Android 房间 Kotlin 内部连接

Android Room Kotlin inner join

android

kotlin

android-room