Menentukan dan membuat kueri hubungan many-to-many
Tetap teratur dengan koleksi
Simpan dan kategorikan konten berdasarkan preferensi Anda.
Hubungan many-to-many antara dua entity adalah hubungan saat setiap
instance parent entity tidak berkaitan sama sekali atau berkaitan dengan beberapa instance entity
turunan, dan hal sebaliknya juga berlaku.
Dalam contoh aplikasi streaming musik, pertimbangkan lagu dalam playlist yang ditentukan
pengguna. Setiap playlist dapat berisi banyak lagu, dan setiap lagu dapat menjadi bagian dari
banyak playlist yang berbeda. Oleh karena itu, ada hubungan many-to-many
antara entity Playlist dan entity Song.
Ikuti langkah-langkah berikut untuk menentukan dan membuat kueri hubungan many-to-many di database Anda:
Menentukan hubungan: Buat entity dan
entity asosiatif (tabel referensi silang) untuk mewakili hubungan
many-to-many.
Buat kueri entity: Tentukan cara Anda ingin membuat kueri entity terkait dan membuat class data untuk merepresentasikan output yang diinginkan.
Menentukan hubungan
Untuk menentukan hubungan many-to-many, buat class untuk kedua entity
Anda terlebih dahulu. Hubungan many-to-many berbeda dengan jenis hubungan lainnya
karena secara umum tidak ada referensi ke entity induk dalam entity
turunan. Sebagai gantinya, buat class ketiga untuk merepresentasikan entity
asosial, atau tabel referensi silang, antara dua entity.
Tabel referensi silang harus memiliki kolom untuk kunci utama dari setiap entity
dalam hubungan many-to-many yang direpresentasikan dalam tabel. Dalam contoh ini, setiap
baris dalam tabel referensi silang berkaitan dengan pasangan instance Playlist
dan instance Song tempat lagu yang dirujuk disertakan dalam
playlist yang dirujuk.
Langkah berikutnya bergantung pada cara Anda ingin membuat kueri entity terkait ini.
Jika Anda ingin membuat kueri playlist dan daftar lagu terkait untuk
setiap playlist, buat class data baru yang berisi satu objek Playlist
dan daftar semua objek Song yang disertakan dalam playlist.
Jika Anda ingin membuat kueri lagu dan daftar playlist yang sesuai untuk setiap
lagu, buat class data baru yang berisi satu objek Song dan
daftar semua objek Playlist tempat lagu disertakan.
Dalam kedua kasus tersebut, buat model hubungan antara entity dengan menggunakan properti
associateBy dalam anotasi @Relation di setiap
class ini untuk mengidentifikasi entity referensi silang yang menyediakan hubungan
antara entity Playlist dan entity Song.
Terakhir, tambahkan metode ke class DAO untuk menampilkan fungsi kueri yang
diperlukan aplikasi Anda.
getPlaylistsWithSongs: metode ini membuat kueri database dan menampilkan semua
objek PlaylistWithSongs yang dihasilkan.
getSongsWithPlaylists: metode ini membuat kueri database dan menampilkan semua
objek SongWithPlaylists yang dihasilkan.
Masing-masing metode ini memerlukan Room untuk menjalankan dua kueri, jadi tambahkan
anotasi @Transaction ke kedua metode tersebut sehingga seluruh
operasi dilakukan secara atomik.
Kotlin
@Transaction@Query("SELECT * FROM Playlist")fungetPlaylistsWithSongs():List<PlaylistWithSongs>@Transaction@Query("SELECT * FROM Song")fungetSongsWithPlaylists():List<SongWithPlaylists>
Java
@Transaction@Query("SELECT * FROM Playlist")publicList<PlaylistWithSongs>getPlaylistsWithSongs();@Transaction@Query("SELECT * FROM Song")publicList<SongWithPlaylists>getSongsWithPlaylists();
Konten dan contoh kode di halaman ini tunduk kepada lisensi yang dijelaskan dalam Lisensi Konten. Java dan OpenJDK adalah merek dagang atau merek dagang terdaftar dari Oracle dan/atau afiliasinya.
Terakhir diperbarui pada 2025-07-27 UTC.
[[["Mudah dipahami","easyToUnderstand","thumb-up"],["Memecahkan masalah saya","solvedMyProblem","thumb-up"],["Lainnya","otherUp","thumb-up"]],[["Informasi yang saya butuhkan tidak ada","missingTheInformationINeed","thumb-down"],["Terlalu rumit/langkahnya terlalu banyak","tooComplicatedTooManySteps","thumb-down"],["Sudah usang","outOfDate","thumb-down"],["Masalah terjemahan","translationIssue","thumb-down"],["Masalah kode / contoh","samplesCodeIssue","thumb-down"],["Lainnya","otherDown","thumb-down"]],["Terakhir diperbarui pada 2025-07-27 UTC."],[],[],null,["# Define and query many-to-many relationships\n\nA *many-to-many relationship* between two entities is a relationship where each\ninstance of the parent entity corresponds to zero or more instances of the child\nentity, and the reverse is also true.\n\nIn the music streaming app example, consider the songs in the user-defined\nplaylists. Each playlist can include many songs, and each song can be a part of\nmany different playlists. Therefore, there is a many-to-many relationship\nbetween the `Playlist` entity and the `Song` entity.\n\nFollow these steps to define and query many-to-many relationships in your\ndatabase:\n\n1. **[Define the relationship](#define)**: Establish the entities and the associative entity (cross-reference table) to represent the many-to-many relationship.\n2. **[Query the entities](#query)**: Determine how you want to query the related entities and create data classes to represent the intended output.\n\nDefine the relationship\n-----------------------\n\nTo define a many-to-many relationship, first create a class for each of your two\nentities. Many-to-many relationships are distinct from other relationship types\nbecause there is generally no reference to the parent entity in the child\nentity. Instead, create a third class to represent an [associative\nentity](https://en.wikipedia.org/wiki/Associative_entity), or *cross-reference table* , between the two entities.\nThe cross-reference table must have columns for the primary key from each entity\nin the many-to-many relationship represented in the table. In this example, each\nrow in the cross-reference table corresponds to a pairing of a `Playlist`\ninstance and a `Song` instance where the referenced song is included in the\nreferenced playlist. \n\n### Kotlin\n\n @Entity\n data class Playlist(\n @PrimaryKey val playlistId: Long,\n val playlistName: String\n )\n\n @Entity\n data class Song(\n @PrimaryKey val songId: Long,\n val songName: String,\n val artist: String\n )\n\n @Entity(primaryKeys = [\"playlistId\", \"songId\"])\n data class PlaylistSongCrossRef(\n val playlistId: Long,\n val songId: Long\n )\n\n### Java\n\n @Entity\n public class Playlist {\n @PrimaryKey public long playlistId;\n public String playlistName;\n }\n\n @Entity\n public class Song {\n @PrimaryKey public long songId;\n public String songName;\n public String artist;\n }\n\n @Entity(primaryKeys = {\"playlistId\", \"songId\"})\n public class PlaylistSongCrossRef {\n public long playlistId;\n public long songId;\n }\n\nQuery the entities\n------------------\n\nThe next step depends on how you want to query these related entities.\n\n- If you want to query *playlists* and a list of the corresponding *songs* for each playlist, create a new data class that contains a single `Playlist` object and a list of all of the `Song` objects that the playlist includes.\n- If you want to query *songs* and a list of the corresponding *playlists* for each, create a new data class that contains a single `Song` object and a list of all of the `Playlist` objects in which the song is included.\n\nIn either case, model the relationship between the entities by using the\n[`associateBy`](/reference/kotlin/androidx/room/Relation#associateBy()) property in the [`@Relation`](/reference/kotlin/androidx/room/Relation) annotation in each of these\nclasses to identify the cross-reference entity providing the relationship\nbetween the `Playlist` entity and the `Song` entity. \n\n### Kotlin\n\n data class PlaylistWithSongs(\n @Embedded val playlist: Playlist,\n @Relation(\n parentColumn = \"playlistId\",\n entityColumn = \"songId\",\n associateBy = Junction(PlaylistSongCrossRef::class)\n )\n val songs: List\u003cSong\u003e\n )\n\n data class SongWithPlaylists(\n @Embedded val song: Song,\n @Relation(\n parentColumn = \"songId\",\n entityColumn = \"playlistId\",\n associateBy = Junction(PlaylistSongCrossRef::class)\n )\n val playlists: List\u003cPlaylist\u003e\n )\n\n### Java\n\n public class PlaylistWithSongs {\n @Embedded public Playlist playlist;\n @Relation(\n parentColumn = \"playlistId\",\n entityColumn = \"songId\",\n associateBy = @Junction(PlaylistSongCrossref.class)\n )\n public List\u003cSong\u003e songs;\n }\n\n public class SongWithPlaylists {\n @Embedded public Song song;\n @Relation(\n parentColumn = \"songId\",\n entityColumn = \"playlistId\",\n associateBy = @Junction(PlaylistSongCrossref.class)\n )\n public List\u003cPlaylist\u003e playlists;\n }\n\nFinally, add a method to the DAO class to expose the query function your\napp needs.\n\n- `getPlaylistsWithSongs`: this method queries the database and returns all the resulting `PlaylistWithSongs` objects.\n- `getSongsWithPlaylists`: this method queries the database and returns all the resulting `SongWithPlaylists` objects.\n\nThese methods each require Room to run two queries, so add the\n[`@Transaction`](/reference/kotlin/androidx/room/Transaction) annotation to both methods so that the whole\noperation is performed atomically. \n\n### Kotlin\n\n @Transaction\n @Query(\"SELECT * FROM Playlist\")\n fun getPlaylistsWithSongs(): List\u003cPlaylistWithSongs\u003e\n\n @Transaction\n @Query(\"SELECT * FROM Song\")\n fun getSongsWithPlaylists(): List\u003cSongWithPlaylists\u003e\n\n### Java\n\n @Transaction\n @Query(\"SELECT * FROM Playlist\")\n public List\u003cPlaylistWithSongs\u003e getPlaylistsWithSongs();\n\n @Transaction\n @Query(\"SELECT * FROM Song\")\n public List\u003cSongWithPlaylists\u003e getSongsWithPlaylists();\n\n| **Note:** If the `@Relation` annotation does not meet your specific use case, you might need to use the `JOIN` keyword in your SQL queries to manually define the appropriate relationships. To learn more about querying multiple tables manually, read [Accessing data using Room\n| DAOs](/training/data-storage/room/accessing-data#query-multiple-tables)."]]