我尝试在pojo中获取N：M个数据库实体。
以下是实体：
人员：
js汽车：
@Entity(
tableName = "person_table",
indices = [Index("person_id")]
)
class Person() {
@PrimaryKey(autoGenerate = true)
@ColumnInfo(name = "person_id")
var id: Long = 0
}
和结实体：
@Entity(
tableName = "car_table",
indices = [Index("car_id")]
)
class Car() {
@PrimaryKey(autoGenerate = true)
@ColumnInfo(name = "car_id")
var id: Long = 0
}
这是我的Pojo：
@Entity(
tableName = "person_car_join",
primaryKeys = ["fk_person_id", "fk_car_id"],
indices = [
Index("fk_person_id"),
Index("fk_car_id")
],
foreignKeys = [
ForeignKey(
entity = Person::class,
childColumns = arrayOf("fk_person_id"),
parentColumns = arrayOf("person_id")
), ForeignKey(
entity = Car::class,
childColumns = arrayOf("fk_car_id"),
parentColumns = arrayOf("car_id")
)
]
)
class PersonCarJoin(
@ColumnInfo(name = "fk_person_id") var fkPersonId: Long,
@ColumnInfo(name = "fk_car_id") var fkCarId: Long
)
然后我尝试通过以下查询获取它们：
class PersonWithCarsPojo {
@Embedded
lateinit var person: Person
@Relation(
entityColumn = "car_id",
parentColumn = "person_id",
associateBy = Junction(
PersonCarJoin::class,
parentColumn = "fk_person_id",
entityColumn = "fk_car_id"
)
)
var cars: List<Car> = arrayListOf()
}
但是我只让所有只有1辆车的人进入汽车列表。我在做什么错了？
谢谢。
答案 0 :(得分：0)
确定要添加关系吗？
或者您是否至少有相应的Room库的 2.2.0版本，其中添加了associateBy和@Junction。 Version 2.2.0 - October 9, 2019
根据您的代码（但使用Java而不是Kotlin），然后执行以下操作：-
mPersonCarsDao = mRTDB.personCarsDao();
mPersonCarsDao.insertPeople(
new Person("Fred"),
new Person("Jane"),
new Person("Anne")
);
mPersonCarsDao.insertCars(
new Car("A"),
new Car("B"),
new Car("C")
);
mPersonCarsDao.insertPersonCarJoins(
new PersonCarJoin(1,2), // Fred -> B
new PersonCarJoin(1,3), // Fred -> C
new PersonCarJoin(2,1), // Jane -> A
new PersonCarJoin(2,3), // Jane -> C
new PersonCarJoin(3,3) // Anne -> C
);
List<PersonWithCarsPojo> personWithCarsPojos = mPersonCarsDao.getAllPeopleWithCars();
for (PersonWithCarsPojo pwcp: personWithCarsPojos) {
for (Car c: pwcp.cars) {
Log.d("PWCPINFO","Person is " + pwcp.getPerson().getName() + " Car is " + c.getCar());
}
}
输出：-
2019-10-09 07:22:40.163 D/PWCPINFO: Person is Fred Car is B
2019-10-09 07:22:40.163 D/PWCPINFO: Person is Fred Car is C
2019-10-09 07:22:40.163 D/PWCPINFO: Person is Jane Car is A
2019-10-09 07:22:40.163 D/PWCPINFO: Person is Jane Car is C
2019-10-09 07:22:40.163 D/PWCPINFO: Person is Anne Car is C
PersonCarsDao是：-
@Dao
public interface PersonCarsDao {
@Insert
long[] insertPeople(Person... people);
@Insert
long insertPerson(Person person);
@Insert
long[] insertCars(Car... cars);
@Insert
long insertCar(Car car);
@Insert
long insertPersonCarJoin(PersonCarJoin personCarJoin);
@Insert
long[] insertPersonCarJoins(PersonCarJoin... personCarJoins);
@Query("SELECT * FROM car_table")
List<Car> getAllCars();
@Query("SELECT * FROM person_table")
List<Person> getAllPeople();
@Query("SELECT * FROM car_table WHERE car_id = :car_id")
Car getCarById(long car_id);
@Query("SELECT * FROM person_table WHERE person_id = :person_id")
Person getPersonById(long person_id);
@Query("SELECT * FROM person_table")
List<PersonWithCarsPojo> getAllPeopleWithCars();
}
其他（与Kotlin一起使用）
以下是上面的完整代码，但在Kotlin中有效。
Person.kt （已添加personName列）
@Entity(
tableName = "person_table",
indices = [Index("person_id")]
)
class Person() {
@PrimaryKey(autoGenerate = true)
@ColumnInfo(name = "person_id")
var id: Long = 0
var personName: String = ""
}
Car.kt （添加的carName列）
@Entity(
tableName = "car_table",
indices = [Index("car_id")]
)
class Car {
@PrimaryKey(autoGenerate = true)
@ColumnInfo(name = "car_id")
var id: Long = 0
var carName: String = ""
}
PersonCarJoin.kt （不变）
@Entity(
tableName = "person_car_join",
primaryKeys = ["fk_person_id", "fk_car_id"],
indices = [
Index("fk_person_id"),
Index("fk_car_id")
],
foreignKeys = [
ForeignKey(
entity = Person::class,
childColumns = arrayOf("fk_person_id"),
parentColumns = arrayOf("person_id")
), ForeignKey(
entity = Car::class,
childColumns = arrayOf("fk_car_id"),
parentColumns = arrayOf("car_id")
)
]
)
class PersonCarJoin(
@ColumnInfo(name = "fk_person_id") var fkPersonId: Long,
@ColumnInfo(name = "fk_car_id") var fkCarId: Long
)
PersonWithCarsPojo （不变）
class PersonWithCarsPojo {
@Embedded
lateinit var person: Person
@Relation(
entityColumn = "car_id",
parentColumn = "person_id",
associateBy = Junction(
PersonCarJoin::class,
parentColumn = "fk_person_id",
entityColumn = "fk_car_id"
)
)
var cars: List<Car> = arrayListOf()
}
PersonCarDoa （已创建，但包含您的getAllPersonsWithCars）
@Dao
interface PersonCarDao {
@Insert
fun insertPerson(person: Person): Long
@Insert
fun insertpeople(vararg People: Person)
@Insert
fun insertCar(car: Car): Long
@Insert
fun insertCars(vararg cars: Car)
@Insert
fun insertPersonCarJoin(personCarJoin: PersonCarJoin): Long
@Query("SELECT * FROM person_table")
fun getAllPersonsWithCars(): List<PersonWithCarsPojo>
}
AppDatabase.kt （已创建）
@Database(version = 1, entities = [Person::class,Car::class,PersonCarJoin::class])
abstract class AppDatabase : RoomDatabase() {
abstract fun personCarDao(): PersonCarDao
}
MainActivity.kt
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val db = Room.databaseBuilder(applicationContext,AppDatabase::class.java,"personcardb")
.allowMainThreadQueries()
.build()
val personCarDao = db.personCarDao()
val person = Person()
person.personName = "Fred"
personCarDao.insertPerson(person)
person.personName = "Jane"
personCarDao.insertPerson(person)
person.personName = "Anne"
personCarDao.insertPerson(person)
val car = Car()
car.carName = "A"
personCarDao.insertCar(car)
car.carName = "B"
personCarDao.insertCar(car)
car.carName = "C"
personCarDao.insertCar(car)
val pcj1 = PersonCarJoin(1,2)
personCarDao.insertPersonCarJoin(pcj1)
val pcj2 = PersonCarJoin(1,3)
personCarDao.insertPersonCarJoin(pcj2)
val pcj3 = PersonCarJoin(2,1)
personCarDao.insertPersonCarJoin(pcj3)
val pcj4 = PersonCarJoin(2,3)
personCarDao.insertPersonCarJoin(pcj4)
val pcj5 = PersonCarJoin(3,3)
personCarDao.insertPersonCarJoin(pcj5)
val theList: List<PersonWithCarsPojo> = personCarDao.getAllPersonsWithCars()
for (pwc: PersonWithCarsPojo in theList) {
for(c: Car in pwc.cars) {
Log.d("PWCINFO","Person is " + pwc.person.personName + " Car is " + c.carName)
}
}
}
}
2019-10-09 20:25:08.364 D/PWCINFO: Person is Fred Car is B
2019-10-09 20:25:08.364 D/PWCINFO: Person is Fred Car is C
2019-10-09 20:25:08.364 D/PWCINFO: Person is Jane Car is A
2019-10-09 20:25:08.364 D/PWCINFO: Person is Jane Car is C
2019-10-09 20:25:08.364 D/PWCINFO: Person is Anne Car is C
您可以看到3人和3辆车，但有5辆输出车（弗雷德有2辆车，珍妮也有1辆车）