MongoDB之DBref(關聯插入,查詢,刪除) 執行個體深入

，A,B,C三個Collection互相關聯。 其中的數字為document的value值。

關於DBref的入門可以看 http://blog.csdn.net/crazyjixiang/article/details/6616678 這篇文章。

我們先建立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為ObjectId("4e3f33de6266b5845052c02c")作為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為 ObjectId("4e3f3de16266b5845052c036") 作為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 }

目前為止3個collection 的關係已經建成。

查詢

> 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" }

其實好好想想引用不是必須的。

MongoDB 權威指南說了這麼一句：

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.