Android官方ORM数据库Room技术解决方案 @Embed

（一）附录1简介了Android Room的基本使用。在附录1例子中，User对象元素均为普通的Java基本数据类型，但是实际的开发中，通常建立的持久化存储对象复杂，且通常是结构化的Java对象，互相之间存在引用或者内嵌关系。
Android Room支持数据库表Java对象通过注解符@Embedded内嵌一个Java对象。这样就像过去的ORM数据库一样，比如构造一个名为Info的Java对象，作为一个成员变量添加到User里面：

package zhangphil.demo;
 
import android.arch.persistence.room.ColumnInfo;
import android.arch.persistence.room.Embedded;
import android.arch.persistence.room.Entity;
import android.arch.persistence.room.PrimaryKey;
 
/**
 * Created by Phil on 2017/11/22.
 */
 
@Entity(tableName = "user_table")
public class User {
    @PrimaryKey(autoGenerate = true)
    public int userId;
 
    @ColumnInfo(name = "userName")
    public String name;
 
    @ColumnInfo(name = "userAge")
    public int age;
 
    @ColumnInfo(name = "updateTime")
    public long updateTime;
 
    @Embedded
    public Info info;
}

（二）Info对象本身也是一个Android Room的@Entity。也有自己的列名和主键等完整的Android Room数据表要素，Info.java：

package zhangphil.demo;
 
import android.arch.persistence.room.ColumnInfo;
import android.arch.persistence.room.Entity;
import android.arch.persistence.room.PrimaryKey;
 
/**
 * Created by Phil on 2017/11/23.
 */
 
@Entity(tableName = "info_table")
public class Info {
    @PrimaryKey(autoGenerate = true)
    public int infoId;
 
    @ColumnInfo(name = "blog")
    public String blog;
 
    @ColumnInfo(name = "content")
    public String content;
}

（三）需要注意的是，通过Android Room的@Embedded符号内嵌的数据表，列表名将自动二次添加到“宿主”对象中。本例是User钟内嵌了Info，那么User数据库表user_table中将会被Android Room自动添加Info里面的列名字段。User的user_table中原有userId，name，age，updateTime四列，由于@Embedded了Info，那么Info里面的blog，content将会自动添加到User数据表中。但是Info的主键infoId将在User中被忽略不再被作为主键。

（四）写一个MainActivity.java测试：

package zhangphil.demo;
 
import android.arch.persistence.room.Room;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
 
import java.util.List;
 
public class MainActivity extends AppCompatActivity {
 
    private String TAG = "输出";
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
 
        new Thread(new Runnable() {
            @Override
            public void run() {
                databaseOperation();
            }
        }).start();
    }
 
    private void databaseOperation() {
        UserDatabase mUserDatabase = Room.databaseBuilder(getApplicationContext(), UserDatabase.class, "users").build();
        UserDao mUserDao = mUserDatabase.getUserDao();
 
        //写数据库
        writeUserDatabase(mUserDao, "zhangphil", 18);
        readDatabase(mUserDao);
 
        //关闭数据库
        mUserDatabase.close();
    }
 
    private void readDatabase(UserDao dao) {
        Log.d(TAG, "读数据库...");
        List<User> users = dao.getAllUsers();
        for (User u : users) {
            Log.d(TAG, u.userId + "," + u.name + "," + u.age + ","+u.info.blog+","+u.info.content);
        }
        Log.d(TAG, "读数据库完毕.");
    }
 
    private void writeUserDatabase(UserDao dao, String name, int age) {
        Info info = new Info();
        info.blog = "http://blog.csdn.net/zhangphil";
        info.content = "Android";
 
        User user = new User();
        user.name = name;
        user.age = age;
        user.updateTime = System.currentTimeMillis();
 
        user.info = info;
 
        dao.insertUser(user);
    }
}

代码运行结果，logcat输出：

11-24 09:20:00.716 30805-30851/zhangphil.demo D/输出: 读数据库...
11-24 09:20:00.723 30805-30851/zhangphil.demo D/输出: 1,zhangphil,18,http://blog.csdn.net/zhangphil,Android
11-24 09:20:00.723 30805-30851/zhangphil.demo D/输出: 读数据库完毕.