MongoDB之DBref(关联插入,查询,删除) 实例深入

http://blog.csdn.net/crazyjixiang/article/details/6668288

suppose I have the following datastructure:

var user = {_id: 'foo', age: 35};
var post = {_id: '...', author: {$ref: user, $id: 'foo'},...};

How can I query all posts which references user[foo]? I tried the following but not work:

db.post.find('author.$id': 'foo')

如图所示，A,B,C三个Collection互相关联。 其中的数字为document的value值。

关于DBref的入门可以看 http://blog.csdn.net/crazyjixiang/article/details/6616678 这篇文章。

我们先建立A collection。

> var a={value:"1"}
> var b={value:"2"}
> var c={value:"9"}
> var d={value:"10"}
> db.A.save(a)
> db.A.save(b)
> db.A.save(c)
> db.A.save(d)
> db.A.find()
{ "_id" : ObjectId("4e3f33ab6266b5845052c02b"), "value" : "1" }
{ "_id" : ObjectId("4e3f33de6266b5845052c02c"), "value" : "2" }
{ "_id" : ObjectId("4e3f33e06266b5845052c02d"), "value" : "9" }
{ "_id" : ObjectId("4e3f33e26266b5845052c02e"), "value" : "10" }

B collection以A collection的  _id为ObjectId("4e3f33de6266b5845052c02c")作为Apid

所以：

> var Ba={Apid:[new DBRef('A',ObjectId("4e3f33de6266b5845052c02c"))],value:3}
> db.B.save(Ba)
> var Ba={Apid:[new DBRef('A',ObjectId("4e3f33de6266b5845052c02c"))],value:4}
> db.B.insert(Ba)
> var Ba={Apid:[new DBRef('A',ObjectId("4e3f33de6266b5845052c02c"))],value:7}
> db.B.insert(Ba)
> var Ba={Apid:[new DBRef('A',ObjectId("4e3f33de6266b5845052c02c"))],value:8}
> db.B.insert(Ba)
> db.B.find()
{ "_id" : ObjectId("4e3f3dd96266b5845052c035"), "Apid" : [ { "$ref" : "A", "$id" : ObjectId("4e3f33de6266b5845052c02c") } ], "value" : 3 }
{ "_id" : ObjectId("4e3f3de16266b5845052c036"), "Apid" : [ { "$ref" : "A", "$id" : ObjectId("4e3f33de6266b5845052c02c") } ], "value" : 4 }
{ "_id" : ObjectId("4e3f3dec6266b5845052c037"), "Apid" : [ { "$ref" : "A", "$id" : ObjectId("4e3f33de6266b5845052c02c") } ], "value" : 7 }
{ "_id" : ObjectId("4e3f3df06266b5845052c038"), "Apid" : [ { "$ref" : "A", "$id" : ObjectId("4e3f33de6266b5845052c02c") } ], "value" : 8 }

C collection以B collection的  _id为 ObjectId("4e3f3de16266b5845052c036") 作为Apid

> var Ca={Bpid:[new DBRef('B',ObjectId("4e3f3de16266b5845052c036"))],value:5}
> db.C.save(Ca)
> var Ca={Bpid:[new DBRef('B',ObjectId("4e3f3de16266b5845052c036"))],value:6}
> db.C.save(Ca)
> db.C.find()
{ "_id" : ObjectId("4e3f42f36266b5845052c03d"), "Bpid" : [ { "$ref" : "B", "$id" : ObjectId("4e3f3de16266b5845052c036") } ], "value" : 5 }
{ "_id" : ObjectId("4e3f42f96266b5845052c03e"), "Bpid" : [ { "$ref" : "B", "$id" : ObjectId("4e3f3de16266b5845052c036") } ], "value" : 6 }

目前为止3个collection 的关系已经建成。

查询

<span style="font-size:16px;">> var a = db.B.findOne({"value":4})
> a.Apid.forEach(function(ref){printjson(db[ref.$ref].findOne({"_id":ref.$id}));})
{ "_id" : ObjectId("4e3f33de6266b5845052c02c"), "value" : "2" }</span>

> db.A.findOne({"_id":db.B.findOne().Apid[0].$id})
{ "_id" : ObjectId("4e3f33de6266b5845052c02c"), "value" : "2" }

其实好好想想引用不是必须的。

MongoDB 权威指南说了这么一句：

In short,the best time to use DBRefs are when you're storing heterogeneous references to documents in different collections.like when you want to take advantage of some additional DBRef-specific functionality in a driver or tool.