Junction.kt
/*
* Copyright 2019 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.room
import kotlin.reflect.KClass
/**
* Declares a junction to be used for joining a relationship.
*
* If a [Relation] should use an associative table (also know as junction table or join
* table) then you can use this annotation to reference such table. This is useful for fetching
* many-to-many relations.
*
* ```
* @Entity(primaryKeys = {"pId", "sId"})
* public class PlaylistSongXRef {
* val pId: Int,
* val sId: Int
* }
* public class PlaylistWithSongs {
* @Embedded
* val playlist: Playlist
* @Relation(
* parentColumn = "playlistId",
* entity = Song::class,
* entityColumn = "songId",
* associateBy = @Junction(
* value = PlaylistSongXRef::class,
* parentColumn = "pId",
* entityColumn = "sId")
* )
* val songs: List<String>
* }
*
* @Dao
* public interface MusicDao {
* @Query("SELECT * FROM Playlist")
* val getAllPlaylistsWithSongs(): List<PlaylistWithSongs>
* }
* ```
*
* In the above example the many-to-many relationship between `Song` and `Playlist` has
* an associative table defined by the entity `PlaylistSongXRef`.
*
* @see [Relation]
*/
@Target(allowedTargets = []) // Complex annotation target
@Retention(AnnotationRetention.BINARY)
public annotation class Junction(
/**
* An entity or database view to be used as a junction table when fetching the
* relating entities.
*
* @return The entity or database view to be used as a junction table.
*/
val value: KClass<*>,
/**
* The junction column that will be used to match against the [Relation.parentColumn].
*
* If not specified it defaults to [Relation.parentColumn].
*/
val parentColumn: String = "",
/**
* The junction column that will be used to match against the [Relation.entityColumn].
*
* If not specified it defaults to [Relation.entityColumn].
*/
val entityColumn: String = ""
)