MongoDB:通过@DBRef查询 [英] MongoDB: query by @DBRef
问题描述
我有一个用于存储用户通知的类层次结构:
@Document
public class Notification<T> {
@Id
private String id;
@DBRef
private T tag;
...
}
@Document
public class NotificationA extends Notification<WrappedA> {
}
@Document
public class NotificationB extends Notification<WrappedB> {
}
...
这对于返回多态数组很有用,允许我将任何类型的数据存储在标签"字段中.当包装的对象包含@DBRef字段时,问题开始出现:
@Document
public class WrappedA {
@Id
private String id;
@DBRef
private JetAnotherClass referenced;
...
}
对标签"字段的查询工作正常:
db.NotificationA.find( {"tag.$id": ObjectId("507b9902...32a")} )
但是我需要查询JetAnotherClass的字段(两个级别的@DBRef字段).我已经尝试过使用点符号和子对象,但是它返回null:
点表示法:
db.NotificationA.findOne( {"tag.$referenced.$id": ObjectId("508a7701...29f")} )
子对象:
db.NotificationA.findOne( {"tag.$referenced": { "_id": ObjectId("508a7701...29f") }} )
有帮助吗? 预先感谢!
由于您看起来只通过_id
查询,所以我相信您可以做到:
db.NotificationA.findOne({"tag.$id": ObjectId("blah")});
但是:
但是我需要查询JetAnotherClass的字段(两个级别的@DBRef字段).
DBRef不是联接,它们只是描述_id
的自我,如果您不知道它将创建帮助对象的链接集合,那么您不必自己在客户端编写此代码. /p>
您可以在此处找到有关DBRef的更多信息: http://docs.mongodb.org/manual/applications/数据库引用/
基本上,您可以从同一文档中查询DBRef中的子字段,即:DBRef.$_id
,但是您不能在服务器端解析该DBRef并查询结果字段.
I have a class hierarchy designed for store user notifications:
@Document
public class Notification<T> {
@Id
private String id;
@DBRef
private T tag;
...
}
@Document
public class NotificationA extends Notification<WrappedA> {
}
@Document
public class NotificationB extends Notification<WrappedB> {
}
...
This is useful for returning polymorphic arrays, allowing me to store any kind of data in the "tag" field. The problem starts when the wrapped objects contains @DBRef fields:
@Document
public class WrappedA {
@Id
private String id;
@DBRef
private JetAnotherClass referenced;
...
}
Queries on the fields of "tag" works fine:
db.NotificationA.find( {"tag.$id": ObjectId("507b9902...32a")} )
But I need to query on the fields of JetAnotherClass (two levels of @DBRef fields). I've tried with dot notation and also with subobjects but it returns null:
Dot notation:
db.NotificationA.findOne( {"tag.$referenced.$id": ObjectId("508a7701...29f")} )
Subobjects:
db.NotificationA.findOne( {"tag.$referenced": { "_id": ObjectId("508a7701...29f") }} )
Any help? Thanks in advance!
Since you look like you are only querying by _id
I believe you can do:
db.NotificationA.findOne({"tag.$id": ObjectId("blah")});
However:
But I need to query on the fields of JetAnotherClass (two levels of @DBRef fields).
DBRefs are not JOINs, they are merely a self describing _id
in the event that you do not know the linking collection it will create a helper object so you don't have to code this yourself on the client side.
You can find more on DBRefs here: http://docs.mongodb.org/manual/applications/database-references/
Basically you can query the sub fields within the DBRef from the same document, i.e.: DBRef.$_id
but you cannot, server-side, resolve that DBRef and query on the resulting fields.
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