how to use foreign key in Room persistence library

Question

I am working with room persistence library in android, i would appreciate if someone can help me in using foreign key, how to get data by using foreign key.

Answer 1

Just to summarize the above posts for future readers:

The foreign key syntax in Kotlin is

@Entity(foreignKeys = arrayOf(ForeignKey(entity = ParentClass::class,                     parentColumns = arrayOf("parentClassColumn"),                     childColumns = arrayOf("childClassColumn"),                     onDelete = ForeignKey.CASCADE))) 

The foreign key syntax in Java is:

@Entity(foreignKeys = {@ForeignKey(entity = ParentClass.class,     parentColumns = "parentClassColumn",     childColumns = "childClassColumn",     onDelete = ForeignKey.CASCADE) }) 

Note: foreignKeys is an array, so in Java enclose @ForeignKey elements in { and }

You can refer to the official documentation for more information. https://developer.android.com/reference/androidx/room/ForeignKey

Answer 2

Here how you can define and access a One-to-many (Foreign Key) relationship in Android Jetpack Room. Here the Entities are Artist and Album and the foreign key is Album.artist

@Entity data class Artist(     @PrimaryKey     val id: String,     val name: String )  @Entity(     foreignKeys = [ForeignKey(         entity = Artist::class,         parentColumns = arrayOf("id"),         childColumns = arrayOf("artist"),         onDelete = ForeignKey.CASCADE     )] ) data class Album(     @PrimaryKey     val albumId: String,     val name: String,     @ColumnInfo(index = true)     val artist: String ) 

then the embedded object (read official Android documentation: Define relationships between objects)

data class ArtistAndAlbums(     @Embedded     val artist: Artist,     @Relation(         parentColumn = "id",         entityColumn = "artist"     )     val albums: List<Album> ) 

And finally the DAO

@Dao interface Library {     @Insert     suspend fun save(artist: Artist)      @Insert     suspend fun save(vararg album: Album)      @Transaction     @Query("SELECT * FROM artist")     suspend fun getAll(): List<ArtistAndAlbums>      @Transaction     @Query("SELECT * FROM artist WHERE id = :id")     suspend fun getByArtistId(id: String): ArtistAndAlbums } 

trying this out, (all these operations happens in a Coroutine)

// creating objects val artist = Artist(id="hillsongunited", name="Hillsong United" ) val artist2 = Artist(id="planetshakers", name="Planet Shakers" )  val album1 = Album(albumId = "empires", name = "Empires", artist = artist.id) val album2 = Album(albumId = "wonder", name = "Wonder", artist = artist.id) val album3 = Album(albumId = "people", name = "People", artist = artist.id)  val album4 = Album(albumId = "rain", name = "Rain", artist = artist2.id) val album5 = Album(albumId = "itschristmas", name = "Its Christmas", artist = artist2.id) val album6 = Album(albumId = "overitall", name = "Over It All", artist = artist2.id)  // saving to database SongDatabase.invoke(applicationContext).library().save(artist) SongDatabase.invoke(applicationContext).library().save(artist2) SongDatabase.invoke(applicationContext).library().save(album1, album2, album3, album4, album5, album6) 

Logging out all Artists

val all = SongDatabase.invoke(applicationContext).library().getAll() Log.d("debug", "All Artists $all ")  D/debug: All Artists [ArtistAndAlbums(artist=Artist(id=hillsongunited, name=Hillsong United), albums=[Album(albumId=empires, name=Empires, artist=hillsongunited), Album(albumId=wonder, name=Wonder, artist=hillsongunited), Album(albumId=people, name=People, artist=hillsongunited)]), ArtistAndAlbums(artist=Artist(id=planetshakers, name=Planet Shakers), albums=[Album(albumId=rain, name=Rain, artist=planetshakers), Album(albumId=itschristmas, name=Its Christmas, artist=planetshakers), Album(albumId=overitall, name=Over It All, artist=planetshakers)])] 

Logging out albums by a specific artist,

val hillsongAlbums = SongDatabase.invoke(applicationContext).library().getByArtistId(artist.id) Log.d("debug", "Albums by artist ID: $hillsongAlbums ")  D/debug: Albums by artist ID: ArtistAndAlbums(artist=Artist(id=hillsongunited, name=Hillsong United), albums=[Album(albumId=empires, name=Empires, artist=hillsongunited), Album(albumId=wonder, name=Wonder, artist=hillsongunited), Album(albumId=people, name=People, artist=hillsongunited)])