如何连接ROOM中的三个表

Question

我需要加入三个Entities

1.

@Entity(tableName = "accounts") data class AccountModel(
    @PrimaryKey(autoGenerate = true)
    val id: Int = 0,
    var name: String = "name",
    var currency: String = "currency",
    var number: String? = null,
    var color: Int = 0,
    var value: Double = 0.0,
    var start_value: Double = 0.0,
    var note: String? = null,
    var accountType: String? = null)

@Entity(tableName = "categories")data class CategoryModel (
    @PrimaryKey(autoGenerate = true)
    var id: Int? = null,
    var name: String,
    var color: Int,
    var is_income: Boolean
)

@Entity(tableName = "transactions")
data class TransactionModel(
    @PrimaryKey(autoGenerate = true)
    val id: Int = 0,
    var name: String? = null,
    var date: Long = 0L,
    var value: Double = 0.0,
    var account_id: Int? = null,          <-
    var category_id: Int? = null,         <-
    var description: String? = null,
    var is_income: Boolean = true
)

它是什么类型的关系？我可以一对一。是一对多吗？我应该使用吗Junction？

Answer 1

这是一对一的关系。您可以添加下一个课程：

data class TransactionWithAccountAndCategory(
    @Embedded val transaction: Transaction,
    @Relation(
         parentColumn = "account_id",
         entityColumn = "id"
    )
    val account: AccountModel
    @Relation(
         parentColumn = "category_id",
         entityColumn = "id"
    )
    val category: CategoryModel
)

并使用下一个方法获取它：

@Transaction
@Query("SELECT * FROM transactions")
fun getTransactions(): List<TransactionWithAccountAndCategory>

因此，您不必使用Junction，因为只有与附加表的多对多关系才需要它（并且根据您的实体，您没有这种情况）。