如何使用 Room DAO 访问数据以从多个表中检索数据

Question

我很努力但没有结果，我想获得关于产品的所有信息，除了类别名称和单位名称，因为我想在里面显示所有这些信息 my adapter 最好的做法是什么，请检查我的代码。

如何编写查询以访问多个表中的数据。我需要定义哪种类型的 class 来涵盖所有这些数据。

@Entity(tableName = "product")
data class Product(
    @PrimaryKey(autoGenerate = true)
    val product_id: Int,

    val name: String,
    val sellPrice: Double,
    val barcode: String?,
    val buyPrice: Double?,
    val quantity: Double?,
    val packing: Double?,
    val reducePrice: Double?,
    val description: String?,

    val fk_category: Int?,
    val fk_unit: Int?
)

@Entity(tableName = "category")
data class Category(
    @PrimaryKey(autoGenerate = true)
    val category_id: Int,

    val name: String
)

@Entity(tableName = "unit")
data class Unit(
    @PrimaryKey(autoGenerate = true)
    val unit_id: Int,

    val name: String
)

data class ProductAndCategory(
    @Embedded
    val category: Category,
    @Relation(
        parentColumn = "category_id",
        entityColumn = "fk_category"
    )
    val product: Product
)

data class ProductAndUnit(
    @Embedded
    val unit: Unit,
    @Relation(
        parentColumn = "unit_id",
        entityColumn = "fk_unit"
    )
    val product: Product
)

@Dao
interface ProductDao {

    @Query("SELECT * FROM product")
    fun getAllProducts(): LiveData<List<Product>>

    @Transaction
    @Query("SELECT * FROM product INNER JOIN category on product.fk_category = category.category_id")
    fun getAllProductAndCategory(): LiveData<List<ProductAndCategory>>

    @Query("SELECT * FROM product WHERE product_id = :productID")
    suspend fun getProduct(productID: Int): List<Product>

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insertProduct(product: Product)

    @Update
    suspend fun updateProduct(product: Product)

    @Delete
    suspend fun deleteProduct(product: Product)

    @Query("DELETE FROM product")
    suspend fun deleteAllProducts()

    @Transaction
    @Query("SELECT * FROM product INNER JOIN category on product.fk_category = category.category_id WHERE product_id= :productID")
    suspend fun getProductAndCategory(productID: Int): List<ProductAndCategory>

    @Transaction
    @Query("SELECT * FROM product INNER JOIN unit on product.fk_unit = unit.unit_id WHERE product_id= :productID")
    suspend fun getProductAndUnit(productID: Int): List<ProductAndUnit>
}

Answer 1

您可以通过两种方式执行此操作。

您可以嵌入产品并为类别和单位设置一个@Relation (2 @Realtion)，或者您可以嵌入所有三个。

所以你可以:-

data class ProductAndCategoryAndUnit(
    @Embedded
    val product: Product,
    @Relation(entity = Category::class, parentColumn = "fk_category",entityColumn = "category_id")
    val category: Category,
    @Relation(entity = Unit::class, parentColumn = "fk_unit",entityColumn = "unit_id")
    val unit: Unit
)

类似 :-

@Query("SELECT * FROM product")
@Transaction
fun getProductAndCategoryAndUnit(): List<ProductAndCategoryAndUnit>

或交替（但请阅读注释）：-

data class ProductCategoryUnit (
    @Embedded
    val product: Product,
    @Embedded
    val category: Category,
    @Embedded
    val unit: Unit
)

类似 :-

@Query("SELECT * FROM product JOIN category ON product.fk_category = category.category_id JOIN unit ON product.fk_unit = unit.unit_id")
    fun getProductCategoryUnit(): List<ProductCategoryUnit>

第二个注释

此替代方案将对所有名为 name 的 3 列产生问题（它们会产生歧义）。您可以使用 @Embedded(prefix = "?") 来消除歧义，但这需要在 SQL 中为列添加别名。一种更简单的方法是确保列名不会有歧义。这可以很容易地使用@ColumnInfo 的名称参数作为单位的示例来实现（类别需要类似的）：-

@Entity(tableName = "unit")
data class Unit(
    @PrimaryKey(autoGenerate = true)
    val unit_id: Long,
    @ColumnInfo(name = "unit_name") // ADDED to disambiguate for joins
    val name: String
)

注意 id 应该是 Long 而不是 Int。

两者的工作示例

根据您的代码，下面对两者进行了演示，请注意，为了方便和简洁起见，演示是在主线程上运行，因此 LiveData 和挂起的函数已相应更改。长整数也被用于 id 而不是 Int。

使用的产品数据class：-

@Entity(tableName = "product")
data class Product(
    @PrimaryKey(autoGenerate = true)
    val product_id: Long,

    val name: String,
    val sellPrice: Double,
    val barcode: String?,
    val buyPrice: Double?,
    val quantity: Double?,
    val packing: Double?,
    val reducePrice: Double?,
    val description: String?,
    val fk_category: Long?,
    val fk_unit: Long?
)

使用的分类数据class:-

@Entity(tableName = "category")
data class Category(
    @PrimaryKey(autoGenerate = true)
    val category_id: Long,
    @ColumnInfo(name = "category_name") // ADDED to disambiguate for joins
    val name: String
)

数据class单位和POJO一样。

用于演示的 ProductDao :-

@Dao
interface ProductDao {

    @Query("SELECT * FROM product")
    fun getAllProducts(): /*LiveData<*/List<Product>/*>*/

    /*
    @Transaction
    @Query("SELECT * FROM product INNER JOIN category on product.fk_category = category.category_id")
    fun getAllProductAndCategory(): LiveData<List<ProductAndCategory>>
    */

    @Insert(onConflict = OnConflictStrategy.IGNORE)
    fun insert(unit: Unit): Long
    @Insert(onConflict = OnConflictStrategy.IGNORE)
    fun insert(category: Category): Long


    @Query("SELECT * FROM product WHERE product_id = :productID")
    suspend fun getProduct(productID: Int): List<Product>

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    /*suspend*/ fun insertProduct(product: Product)

    @Update
    suspend fun updateProduct(product: Product)

    @Delete
    suspend fun deleteProduct(product: Product)

    @Query("DELETE FROM product")
    suspend fun deleteAllProducts()

    /*
    @Transaction
    @Query("SELECT * FROM product INNER JOIN category on product.fk_category = category.category_id WHERE product_id= :productID")
    suspend fun getProductAndCategory(productID: Int): List<ProductAndCategory>
    */

    /*
    @Transaction
    @Query("SELECT * FROM product INNER JOIN unit on product.fk_unit = unit.unit_id WHERE product_id= :productID")
    suspend fun getProductAndUnit(productID: Int): List<ProductAndUnit>
    */

    @Query("SELECT * FROM product")
    @Transaction
    fun getProductAndCategoryAndUnit(): List<ProductAndCategoryAndUnit>

    @Query("SELECT * FROM product WHERE product_id=:productID")
    fun getProductAndCategoryAndUnitById(productID: Long): List<ProductAndCategoryAndUnit>

    @Query("SELECT * FROM product JOIN category ON product.fk_category = category.category_id JOIN unit ON product.fk_unit = unit.unit_id")
    fun getProductCategoryUnit(): List<ProductCategoryUnit>

}

TheDatabase class 是一个很好的@Database，所以不包括在内。

最后是 Activity 根据 :-

class MainActivity : AppCompatActivity() {
    lateinit var db: TheDatabase
    lateinit var dao: ProductDao
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        db = TheDatabase.getInstance(this)
        dao = db.getProductDao()

        var u1 = dao.insert(Unit(0L, "Unit1"))
        var u2 = dao.insert(Unit(0L, "Unit2"))

        var c1 = dao.insert(Category(0L,"Cat1"))
        var c2 = dao.insert(Category(0L,"Cat2"))

        dao.insertProduct(Product(0L,"Produuct1",1.1,"Barcode001",2.2,10.5,0.0,0.0,"Desc4Product1",c2,u1))
        dao.insertProduct(Product(0L,"Product2",1.1,"Barcode002",2.2,10.5,0.0,0.0,"Desc4product2",c1,u2))

        // Inserts Catergory,Unit and Product (utilising the id returned by the inserts for the category and Unit)
        dao.insertProduct(
            Product(
                0L,"" +
                        "Product3",
                1.1,"" +
                        "Barcode003",
                2.2,
                10.5,
                0.0,
                0.0,
                "Desc4Product3",
                dao.insert(Category(0L,"Cat4")),
                dao.insert(Unit(0L,"Unit4"))
            )
        )

        // Extract and Log via the @Embedded and 2 @Realtion's
        for(pcu: ProductAndCategoryAndUnit in dao.getProductAndCategoryAndUnit()) {
            Log.d("PRODUCTINFO","Product is ${pcu.product.name} .... Category is ${pcu.category.name} Unit is ${pcu.unit.name}")
        }
        // Extract and Log via the 3 @Embedded's
        for(pcu: ProductCategoryUnit in dao.getProductCategoryUnit()) {
            Log.d("PRODUCTINFO","Product is ${pcu.product.name} .... Category is ${pcu.category.name} Unit is ${pcu.unit.name}")
        }
    }
}

日志中的结果：-

D/PRODUCTINFO: Product is Produuct1 .... Category is Cat2 Unit is Unit1
D/PRODUCTINFO: Product is Product2 .... Category is Cat1 Unit is Unit2
D/PRODUCTINFO: Product is Product3 .... Category is Cat4 Unit is Unit4


D/PRODUCTINFO: Product is Produuct1 .... Category is Cat2 Unit is Unit1
D/PRODUCTINFO: Product is Product2 .... Category is Cat1 Unit is Unit2
D/PRODUCTINFO: Product is Product3 .... Category is Cat4 Unit is Unit4

即两种方法的结果是相同的。

如何使用 Room DAO 访问数据以从多个表中检索数据

How to accessing data using Room DAOs to retrieve data from multiple tables

android

kotlin

android-room

android-adapter

android-room-relation