[英]Room querying one to many relationship
How can we query all entities with their one to many relationship with Room? 我们如何查询所有具有Room一对一关系的实体?
Say we have a User Entity : 假设我们有一个用户实体 :
@Entity
class User(@PrimaryKey(autoGenerate = true) var id: Long?, var name: String)
and a Pet Entity : 和一个宠物实体 :
@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: 通常我们会像这样查询所有宠物:
@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? 但是,如何使用户自动附加到Pet对象上呢? Like the query would be:
就像查询将是:
@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. 然后我们可以使用pet.user.name,而不必分别查询用户。
您应该使用@Embedded
(资源: https : //developer.android.com/reference/android/arch/persistence/room/Relation.html )
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