先看一遍 带有翻译的官方的Room数据库讲解 看完官方小姐姐的讲解的在看下面的就简单了很多

主要的点就是一定要有关联键 关联键的值一定要一样  其他的就没有啥重要的了基本都是CURD（如果你不了解Room的基础，请先去看看基础用法）最后有DEMO

 1、开始创建Entity

1、创建一个人的实体

import androidx.room.Entity
import androidx.room.PrimaryKey

@Entity
data class Owner(
    val name: String

) {
//关联键
    @PrimaryKey
    var ownerId: Long = 2
}

2、创建一个狗狗类用于对应

 

@Entity
data class Dog(
    val name: String,
    val cuteness: Int,
    val barkVolume: Int,
    val breed: String

) {
    @PrimaryKey(autoGenerate = true)
    var dogId: Long=0
//这个变量在后面类中可以看到（关联键）
    var dogOwnerId: Long=2

}

3、又创建了一个Cat类

import androidx.room.Entity
import androidx.room.PrimaryKey

@Entity
data class Cat(
    val catName: String
) {
    @PrimaryKey(autoGenerate = true)
    var catId: Long = 0
//关联键
    var catOwnerId: Long=2

}

 2、创建Dao

按照上面的三个依次创建了3个Dao（我只是创建了添加的方法）

 

//人的dao
@Dao
interface OwnerDao {
    @Insert
    fun insert(owner: Owner)

}


//Cat的dao层
@Dao
interface CatDao {
    @Insert
    fun insert(cat: Cat)

}

//Dog的dao层
@Dao
interface DogDao {
    @Insert
    fun insert(dogDao: Dog)

}

 3、关键的来了

创建一个Entity把Dog、Cat、Owner链接起来的类

import androidx.room.Embedded
import androidx.room.Relation

data class OwnerWithDogs(
    @Embedded val owner: Owner,
    @Relation(
        parentColumn = "ownerId",//关联键
        entityColumn = "dogOwnerId"//关联键
    )
    val dogs: List<Dog>,
    @Relation(
        parentColumn = "ownerId",//关联键
        entityColumn = "catOwnerId"//关联键
    )
    val cats: List<Cat>

)

创建Dao

import androidx.room.Dao
import androidx.room.Query
import androidx.room.Transaction
import com.example.myroomtest.room.entity.OwnerWithDogs
import io.reactivex.Single

@Dao
interface DogsAndOwnersDao {
    @Transaction
    @Query("SELECT * FROM Owner")//SQ写的是查询主键的类（也就是那个人Owner类）
    fun getDogsAndOwners(): Single<List<OwnerWithDogs>> 
//在接受结果的时候使用的OwnerWithDogs关联类接收

}

 创建数据库的初始类（简单自己封装的）

object DBInstance {
    //    private static final String DB_NAME = "/sdcard/LianSou/room_test.db";
    private val DB_NAME = "test.db"
    var appDataBase: MyDataBase? = null
    fun getInstance(): MyDataBase? {
        if (appDataBase == null) {
            synchronized(DBInstance::class.java) {
                if (appDataBase == null) {
                    appDataBase = Room.databaseBuilder(
                        FinancialExchangeApplication.context,
                        MyDataBase::class.java,
                        DB_NAME
                    )
                        //不推荐打开这句，但是为了demo展示，我只能打开了
                        .allowMainThreadQueries()

                        .build()
                }
            }
        }
        return appDataBase
    }

}

 自己封装了一个dao层的工具类

@Database(
    entities = [Owner::class, Dog::class, Cat::class],
    version = 1,
    exportSchema = false
)
abstract class MyDataBase : RoomDatabase() {

    abstract fun getDao(): DogsAndOwnersDao
    abstract fun getDog(): DogDao
    abstract fun getOwen(): OwnerDao
    abstract fun getCat(): CatDao


}

引入依赖

 //Room的依赖引用
    implementation "androidx.room:room-runtime:2.2.5"
    implementation "androidx.room:room-ktx:2.2.5"
    kapt "androidx.room:room-compiler:2.2.5"
    androidTestImplementation "androidx.room:room-testing:2.2.5"
    implementation "androidx.room:room-rxjava2:2.2.5"  //room的RxJ
    implementation "io.reactivex.rxjava2:rxandroid:2.1.0"  //线程调度器

 

 就是这样完事了

附上Git（Demo）的地址