Relation

<init>

Relation(
    entity: KClass<*>, 
    parentColumn: String, 
    entityColumn: String, 
    associateBy: Junction, 
    projection: Array<String>)

A convenience annotation which can be used in a POJO to automatically fetch relation entities. When the POJO is returned from a query, all of its relations are also fetched by Room.

@Entity
  public class Song {
      @ PrimaryKey
      int songId;
      int albumId;
      String name;
      // other fields
  }
  public class AlbumNameAndAllSongs {
      int id;
      String name;
      @Relation(parentColumn = "id", entityColumn = "albumId")
      List<Song> songs;
  }
 
  @Dao
  public interface MusicDao {
      @Query("SELECT id, name FROM Album")
      List<AlbumNameAndAllSongs> loadAlbumAndSongs();
  }
  

For a one-to-many or many-to-many relationship, the type of the field annotated with Relation must be a java.util.List or java.util.Set.

By default, the Entity type is inferred from the return type. If you would like to return a different object, you can specify the entity() property in the annotation.

public class Album {
      int id;
      // other fields
  }
  public class SongNameAndId {
      int songId;
      String name;
  }
  public class AlbumAllSongs {
      @Embedded
      Album album;
      @Relation(parentColumn = "id", entityColumn = "albumId", entity = Song.class)
      List<SongNameAndId> songs;
  }
  @Dao
  public interface MusicDao {
      @Query("SELECT * from Album")
      List<AlbumAllSongs> loadAlbumAndSongs();
  }
  

In the example above, SongNameAndId is a regular POJO but all of fields are fetched from the entity defined in the @Relation annotation (Song). SongNameAndId could also define its own relations all of which would also be fetched automatically.

If you would like to specify which columns are fetched from the child Entity, you can use projection() property in the Relation annotation.

public class AlbumAndAllSongs {
      @Embedded
      Album album;
      @Relation(
              parentColumn = "id",
              entityColumn = "albumId",
              entity = Song.class,
              projection = {"name"})
      List<String> songNames;
  }
  

If the relationship is defined by an associative table (also know as junction table) then you can use associateBy() to specify it. This is useful for fetching many-to-many relations.

Note that @Relation annotation can be used only in POJO classes, an Entity class cannot have relations. This is a design decision to avoid common pitfalls in Entity setups. You can read more about it in the main Room documentation. When loading data, you can simply work around this limitation by creating POJO classes that extend the Entity.

associateBy

val associateBy: Junction

The entity or view to be used as a associative table (also known as a junction table) when fetching the relating entities.

entity

val entity: KClass<*>

The entity or view to fetch the item from. You don't need to set this if the entity or view matches the type argument in the return type.

entityColumn

val entityColumn: String

The column to match in the entity().

In a one-to-one or one-to-many relation, this value will be matched against the column defined in parentColumn() ()}. In a many-to-many using associateBy() then this value will be matched against the Junction#entityColumn()

parentColumn

val parentColumn: String

Reference column in the parent POJO.

In a one-to-one or one-to-many relation, this value will be matched against the column defined in entityColumn(). In a many-to-many using associateBy() then this value will be matched against the Junction#parentColumn()

projection

val projection: Array<String>

If sub columns should be fetched from the entity, you can specify them using this field.

By default, inferred from the the return type.