//某客户端显示这样,直接CMD查询也是这样显示.这样我无法看懂find条件
{

    "_id" : ObjectId("58ae865f7dde420cd0eae39f"),

    "className" : "com.thesys.morphia.dbref.DbrefTest$Book",

    "price" : "60.0",

    "author" : DBRef("Author", ObjectId("58ae865f7dde420cd0eae39e"))

}

//同一个数据显示的格式,根据这样尝试查询
{

    "_id" : ObjectId("58ae865f7dde420cd0eae39f"),

    "className" : "com.thesys.morphia.dbref.DbrefTest$Book",

    "price" : "60.0",

    "author" : {

        "$ref" : "Author",

        "$id" : ObjectId("58ae865f7dde420cd0eae39e")

    }

}

db.Book.find({"author.$ref":"Author"})

db.Book.find({"author.$id":ObjectId("58ae865f7dde420cd0eae39e")})

@RunWith(JUnit4.class)

public class DbrefTest {

    static final Morphia morphia = new Morphia();

    static final Datastore datastore = morphia.createDatastore(new MongoClient(),"BookTest");

    @Entity

    static class Book{

        @Id

        private ObjectId id;//id

        private String name;//书名

        private double price;//价格

        @Reference

        private Author author;

    }

    @Entity

    static class Author{

        @Id

        private ObjectId id;//id

        private String name;//作者名称

        private String nationality;//国籍

    }

    public static void main(String[] args) {

        Author author = new Author();

        author.setName("大仲马");

        author.setNationality("法国");

        datastore.save(author);

        Book book = new Book();

        book.setAuthor(author);

        datastore.save(book);

    }

    @Test

    public void testQuery(){

        Query<Book> query = datastore.createQuery(Book.class);

        System.out.println(query.count());

        List<Book> books = query.asList();

        System.out.println(books.get(0).getAuthor().getName());

    }

    @Test

    public void testUpdate(){

        //注意根据ID查询不能传入String 必须是ObjectId

        Query<Book> query = datastore.createQuery(Book.class).field("_id").equal(new ObjectId("58ae7f837dde423968454e62"));

//        System.out.println(query.get().price);//直接获取结果集第一个

//        datastore.get(Book.class,new ObjectId("58ae7f837dde423968454e62")).price //直接根据ID查询一个文档

        UpdateOperations<Book> updateOperation = datastore.createUpdateOperations(Book.class).set("name","三个火枪手").set("price", 55);

        datastore.updateFirst(query, updateOperation);//.更新第一个找到的

    }

　　//聚合管道,投射

　　@Test

　　public void testAggregation(){
　　　　//把price 重命名为 name

　　　　AggregationPipeline aggregationPipeline = datastore.createAggregation(Book.class).project(Projection.projection("name","price"));

　　　　Iterator<Book> books = aggregationPipeline.aggregate(Book.class);

　　　　System.out.println(books.next().getName());//打印出name : 60.0

　　}

}