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。
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- > 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：
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- > 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
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- > 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
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- <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>
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- > 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.
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