MongoDB的DBREF 使用.

首先要记一下根据 DBREF 的ObjectId 以及根据 ref 集合为条件查询问题.

在不同的可视化客户端里面显示的问题.

//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"))
}

换了一个客户端

//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")
    }
}

以下两个shell均可查询

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

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

以下是我的测试代码,使用morphia

@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
　　}

}