MongoDB-dbref (associated insertion, query, and deletion) instance depth

, A, B, and C collections are correlated. The number is the value of document.

For more information about dbref, see the http://blog.csdn.net/crazyjixiang/article/details/6616678 article.

Create a collection first.

> 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 uses the _ ID of a collection as objectid ("4e3f33de6266b5841052c02c") as the apid

Therefore:

> 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 uses the _ id of B collection as objectid ("4e3f3de16266b5841052c036") as the 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 }

So far, the relationship between the three collections has been established.

Query

> 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" }

In fact, it is not necessary to think about references.

The MongoDB authoritative guide says:

In short, the best time to use dbrefs are when you're storing heterogeneous references to arguments in different collections. Like when you want to take advantage of some additional dbref-specific
Functionality in a driver or tool.