Room querying one to many relationship

Question

How can we query all entities with their one to many relationship with Room?

Say we have a User Entity :

@Entity
class User(@PrimaryKey(autoGenerate = true) var id: Long?, var name: String)

and a Pet Entity :

@Entity(foreignKeys = [(ForeignKey(entity = User::class,
    parentColumns = ["id"],
    childColumns = ["userId"],
    onDelete = ForeignKey.CASCADE))])
public class Pet(@PrimaryKey(autoGenerate = true),
    var name: String,
    var userId: Long?,
    @Ignore var user: User?)

Normally we would query all pets like this:

@Dao
interface PetsDao {
    @Query("SELECT * FROM pets")
    fun getAll(): Flowable<List<Pet>>
}

But how can we make it so that the User is attached to the Pet object automatically? Like the query would be:

@Dao
interface PetsDao {
    @Query("SELECT * FROM pets p JOIN users u ON u.id = p.userId")
    fun getAllWithUser(): Flowable<List<Pet>>
}

And then we could use pet.user.name without having to query the user separately.

Answer 1

您应该使用@Embedded （资源： https : //developer.android.com/reference/android/arch/persistence/room/Relation.html ）