MongoDB之DBref(关联插入,查询,删除) 实例深入

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.youkuaiyun.com/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

> 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" }

> 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.