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。

Cpp代码

1. > var a={value:"1"} 
2. > var b={value:"2"} 
3. > var c={value:"9"} 
4. > var d={value:"10"} 
5. > db.A.save(a) 
6. > db.A.save(b)       
7. > db.A.save(c)  
8. > db.A.save(d) 
9. > db.A.find()                                                                                                
10. { "_id" : ObjectId("4e3f33ab6266b5845052c02b"), "value" : "1" } 
11. { "_id" : ObjectId("4e3f33de6266b5845052c02c"), "value" : "2" } 
12. { "_id" : ObjectId("4e3f33e06266b5845052c02d"), "value" : "9" } 
13. { "_id" : ObjectId("4e3f33e26266b5845052c02e"), "value" : "10" } 

所以：

Cpp代码

1. > var Ba={Apid:[new DBRef(‘A‘,ObjectId("4e3f33de6266b5845052c02c"))],value:3}                       
2. > db.B.save(Ba) 
3. > var Ba={Apid:[new DBRef(‘A‘,ObjectId("4e3f33de6266b5845052c02c"))],value:4} 
4. > db.B.insert(Ba)                                                             
5. > var Ba={Apid:[new DBRef(‘A‘,ObjectId("4e3f33de6266b5845052c02c"))],value:7} 
6. > db.B.insert(Ba)                                                             
7. > var Ba={Apid:[new DBRef(‘A‘,ObjectId("4e3f33de6266b5845052c02c"))],value:8} 
8. > db.B.insert(Ba)                                                             
9. > db.B.find() 
10. { "_id" : ObjectId("4e3f3dd96266b5845052c035"), "Apid" : [ { "$ref" : "A", "$id" : ObjectId("4e3f33de6266b5845052c02c") } ], "value" : 3 } 
11. { "_id" : ObjectId("4e3f3de16266b5845052c036"), "Apid" : [ { "$ref" : "A", "$id" : ObjectId("4e3f33de6266b5845052c02c") } ], "value" : 4 } 
12. { "_id" : ObjectId("4e3f3dec6266b5845052c037"), "Apid" : [ { "$ref" : "A", "$id" : ObjectId("4e3f33de6266b5845052c02c") } ], "value" : 7 } 
13. { "_id" : ObjectId("4e3f3df06266b5845052c038"), "Apid" : [ { "$ref" : "A", "$id" : ObjectId("4e3f33de6266b5845052c02c") } ], "value" : 8 } 

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

Cpp代码

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

查询

Cpp代码

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

Cpp代码

1. > db.A.findOne({"_id":db.B.findOne().Apid[0].$id}) 
2. { "_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.