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Junction

public abstract @interface Junction
implements Annotation

androidx.room.Junction

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 {
     int pId;
     int sId;
 }
 public class PlaylistWithSongs {
     @Embedded
     Playlist playlist;
     @Relation(
             parentColumn = "playlistId",
             entity = Song.class,
             entityColumn = "songId",
             associateBy = @Junction(
                     value = PlaylistSongXRef.class,
                     parentColumn = "pId",
                     entityColumn = "sId)
     )
     List<String> songs;
 }
 @Dao
 public interface MusicDao {
     @Query("SELECT * FROM Playlist")
     List<PlaylistWithSongs> getAllPlaylistsWithSongs();
 }

In the above example the many-to-many relationship between Song and Playlist has an associative table defined by the entity PlaylistSongXRef.

See also:

Summary

Public methods

String entityColumn()

The junction column that will be used to match against the Relation.entityColumn().

String parentColumn()

The junction column that will be used to match against the Relation.parentColumn().

Class<?> value()

An entity or database view to be used as a junction table when fetching the relating entities.

Inherited methods

Public methods

entityColumn

public String entityColumn ()

The junction column that will be used to match against the Relation.entityColumn().

If not specified it defaults to Relation.entityColumn().

Returns
String

parentColumn

public String parentColumn ()

The junction column that will be used to match against the Relation.parentColumn().

If not specified it defaults to Relation.parentColumn().

Returns
String

value

public Class<?> value ()

An entity or database view to be used as a junction table when fetching the relating entities.

Returns
Class<?> The entity or database view to be used as a junction table.