《Android ORMLite ForeignCollection关联外部集合》
  
Android ORMLite ForeignCollection关联外部集合的功能，适合层级比較深，数据模型互相渗透、交叉的数据结构或集合。尤其方便解决复杂数据模型。简单期间，我们临时以 班级 <-> 学生 这种数据模型为例加以说明。一个班级里面有若干学生(一对多，1<-n)。反过来说，若干个学生集合到一个班级中（n->1）。 在Android ORMLite中。这种结构模型能够用@ForeignCollectionField。ForeignCollection建模。我们定义一个班级类AClass（之所以在 ‘Class‘前加一个‘A’，是由于Java语言中。‘Class‘是保留字，却刚好是我们想用的班级英文单词，真不巧，所曾经面加个‘A‘规避）。AClass包括id（主键，方便查询和更新），name以及指向一个外部Student的集合（ForeignCollection<Student>）。

相同。我们定义学生类Student。Student中埋入一个字段aclass指向外部的AClass。
  
 备注：
 Android ORMLite简单介绍文章：http://blog.csdn.net/zhangphil/article/details/46878075
 
 演示样例代码总共同拥有4个文件：MainActivity.java，主Activity，用于測试。ORMLiteDatabaseHelper.java , AClass.java , Student.java 是数据库相关的代码文件，用于建模。

结构层次如图：

測试用的MainActivity.java :

package zhangphil.ormlitetest;

import java.sql.SQLException;

import com.j256.ormlite.dao.Dao;
import com.j256.ormlite.dao.ForeignCollection;

import zhangphil.ormlitetest.database.AClass;
import zhangphil.ormlitetest.database.ORMLiteDatabaseHelper;
import zhangphil.ormlitetest.database.Student;
import android.support.v7.app.ActionBarActivity;
import android.widget.Toast;
import android.os.Bundle;

public class MainActivity extends ActionBarActivity {

	private Dao<AClass, Integer> mClassDao;
	private Dao<Student, Integer> mStudentDao;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);

		ORMLiteDatabaseHelper mDatabaseHelper = ORMLiteDatabaseHelper
				.getInstance(this);

		mClassDao = mDatabaseHelper.getClassDao();
		mStudentDao = mDatabaseHelper.getStudentDao();

		// 创建5个班级用以演示。
		for (int i = 1; i < 6; i++) {
			AClass aclass = new AClass();
			aclass.id = i;
			aclass.name = i + "班";
			try {
				mClassDao.createIfNotExists(aclass);
			} catch (SQLException e) {
				e.printStackTrace();
			}
		}

		// 找到id=1的1班。
		AClass class1 = null;
		try {
			class1 = mClassDao.queryForId(1);
		} catch (SQLException e) {
			e.printStackTrace();
		}

		// 创建19个学生，这19个学生都归属到1班。
		for (int i = 1; i < 20; i++) {
			Student s = new Student();
			s.id = i;
			s.name = "学生" + i;

			// 将新创建的这些学生所在班级指针指向1班。
			// 数据模型：id=1的1班和这19个学生是1对多的学生，换句话说，这19个学生是1班的学生。

			s.aclass = class1;

			try {
				mStudentDao.createIfNotExists(s);
			} catch (SQLException e) {
				e.printStackTrace();
			}
		}
	}

	@Override
	public void onStart() {
		super.onStart();

		/**
		 * 以下我们演示更新一个Student的信息， 然后向上，根据外键，从Student - > Class更新班级信息。
		 * 
		 * */

		// 如果我们更新当中id=1的这个学生所在的这个班级的name变成“一班”。
		Student s1 = null;
		try {
			s1 = mStudentDao.queryForId(1);

			s1.aclass.name = "一班";

			// 更新1班的名字从“1班”变成“一班”
			mClassDao.update(s1.aclass);
		} catch (SQLException e) {
			e.printStackTrace();
		}

		// 显示我们更新后的结果。

		AClass class1 = null;
		try {
			class1 = mClassDao.queryForId(1);
		} catch (SQLException e1) {
			e1.printStackTrace();
		}

		ForeignCollection<Student> students = class1.students;
		for (Student s : students) {
			Toast.makeText(this, s.toString(), Toast.LENGTH_SHORT).show();
		}
	}
}

下面是数据库相关的建模：

AClass.java :

package zhangphil.ormlitetest.database;

import com.j256.ormlite.dao.ForeignCollection;
import com.j256.ormlite.field.DataType;
import com.j256.ormlite.field.DatabaseField;
import com.j256.ormlite.field.ForeignCollectionField;
import com.j256.ormlite.table.DatabaseTable;

@DatabaseTable(tableName = "classes")
public class AClass {

	@DatabaseField(canBeNull = false, dataType = DataType.LONG, id = true)
	public long id;

	@DatabaseField(canBeNull = false, defaultValue = "http://www.mamicode.com/a class", dataType = DataType.STRING)
	public String name;

	@ForeignCollectionField(eager = false)
	public ForeignCollection<Student> students = null;

	public AClass() {

	}
}

Student.java :

package zhangphil.ormlitetest.database;

import com.j256.ormlite.field.DataType;
import com.j256.ormlite.field.DatabaseField;
import com.j256.ormlite.table.DatabaseTable;

@DatabaseTable(tableName = "students")
public class Student {

	public Student(String name, int student_id) {
		this.name = name;
		this.id = student_id;
	}

	@DatabaseField(canBeNull = false, dataType = DataType.INTEGER, id = true)
	public int id;

	@DatabaseField(canBeNull = false, dataType = DataType.STRING)
	public String name;

	@DatabaseField(canBeNull = false, foreign = true, foreignAutoRefresh = true)
	public AClass aclass;

	public Student() {

	}

	@Override
	public String toString() {
		return "id:" + id + " 姓名:" + name + " 所在班级:" + aclass.name;
	}
}

ORMLiteDatabaseHelper.java :

package zhangphil.ormlitetest.database;

import java.sql.SQLException;

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.util.Log;

import com.j256.ormlite.android.apptools.OrmLiteSqliteOpenHelper;
import com.j256.ormlite.dao.Dao;
import com.j256.ormlite.support.ConnectionSource;
import com.j256.ormlite.table.TableUtils;

public class ORMLiteDatabaseHelper extends OrmLiteSqliteOpenHelper {

	private static ORMLiteDatabaseHelper mDatabaseHelper = null;

	private Dao<AClass, Integer> mClassDao = null;
	private Dao<Student, Integer> mStudentDao = null;

	private final static String DataBase_NAME = "school.db";
	private final static int DataBase_VERSION = 1;

	public ORMLiteDatabaseHelper(Context context, String databaseName,
			CursorFactory factory, int databaseVersion) {
		super(context, DataBase_NAME, factory, DataBase_VERSION);
	}

	public static ORMLiteDatabaseHelper getInstance(Context context) {
		if (mDatabaseHelper == null) {
			mDatabaseHelper = new ORMLiteDatabaseHelper(context, DataBase_NAME,
					null, DataBase_VERSION);
		}

		return mDatabaseHelper;
	}

	@Override
	public void onCreate(SQLiteDatabase arg0, ConnectionSource connectionSource) {

		Log.d(this.getClass().getName(), "ORMLite数据库 -> onCreate");

		try {
			TableUtils.createTableIfNotExists(connectionSource, AClass.class);
			TableUtils.createTableIfNotExists(connectionSource, Student.class);
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	@Override
	public void onUpgrade(SQLiteDatabase database,
			ConnectionSource connectionSource, int oldVersion, int newVersion) {

		Log.i(this.getClass().getName(), "数据库 -> onUpgrade");

		try {
			// 删除旧的数据库表。
			TableUtils.dropTable(connectionSource, AClass.class, true);
			TableUtils.dropTable(connectionSource, Student.class, true);

			// 又一次创建新版的数据库。
			onCreate(database, connectionSource);
		} catch (SQLException e) {
			e.printStackTrace();
		}
	}

	public Dao<Student, Integer> getStudentDao() {
		if (mStudentDao == null) {
			try {
				mStudentDao = getDao(Student.class);
			} catch (java.sql.SQLException e) {
				e.printStackTrace();
			}
		}

		return mStudentDao;
	}

	public Dao<AClass, Integer> getClassDao() {
		if (mClassDao == null) {
			try {
				mClassDao = getDao(AClass.class);
			} catch (java.sql.SQLException e) {
				e.printStackTrace();
			}
		}

		return mClassDao;
	}

	@Override
	public void close() {
		super.close();
		mClassDao = null;
		mStudentDao = null;
	}
}