MongoDB Middle Level (关联多表查询)

DBRef is a more formal specification for creating references between documents. DBRefs (generally) include a collection name as well as an object id. Most developers only use DBRefs if the collection can change from one document to the next. If your referenced collection will always be the same, the manual references outlined above are more efficient.

^_^[root@:/usr/local/mongodb/bin]#./mongo
MongoDB shell version: 1.8.2
connecting to: test
> var a = {name:"C++"}                                                                                           
> db 
test
> db.language.save(a)
> db.language.find()
{ "_id" : ObjectId("4da32c897d2de864e0448e06"), "name" : "C++" }
> var b = {name:"javascript"}
> db.language.save(b)
> db.language.find()
{ "_id" : ObjectId("4da32c897d2de864e0448e06"), "name" : "C++" }
{ "_id" : ObjectId("4da32cb17d2de864e0448e07"), "name" : "javascript" }
> lan = {name:"obj1",computer:[new DBRef('language',a._id)]}
{
        "name" : "obj1",
        "computer" : [
                {
                        "$ref" : "language",
                        "$id" : ObjectId("4da32c897d2de864e0448e06")
                }
        ]
}
> lan.computer[0]
{ "$ref" : "language", "$id" : ObjectId("4da32c897d2de864e0448e06") }
> lan.computer[0].fetch()
{ "_id" : ObjectId("4da32c897d2de864e0448e06"), "name" : "C++" }
> db.language.insert(lan)                                                                                        
> db.language.find()
{ "_id" : ObjectId("4da32c897d2de864e0448e06"), "name" : "C++" }
{ "_id" : ObjectId("4da32cb17d2de864e0448e07"), "name" : "javascript" }
{ "_id" : ObjectId("4da33b487d2de864e0448e08"), "name" : "obj1", "computer" : [ { "$ref" : "language", "$id" : ObjectId("4da32c897d2de864e0448e06") } ] }
> db.language.findOne({name:"obj1"}).computer[0].fetch()                                                         
{ "_id" : ObjectId("4da32c897d2de864e0448e06"), "name" : "C++" }
> lan2 = {name:"obj2",computer:[new DBRef('language',b._id)]}     
{
        "name" : "obj2",
        "computer" : [
                {
                        "$ref" : "language",
                        "$id" : ObjectId("4da32cb17d2de864e0448e07")
                }
        ]
}
> db.language.insert(lan2)
> db.language.find()
{ "_id" : ObjectId("4da32c897d2de864e0448e06"), "name" : "C++" }
{ "_id" : ObjectId("4da32cb17d2de864e0448e07"), "name" : "javascript" }
{ "_id" : ObjectId("4da33b487d2de864e0448e08"), "name" : "obj1", "computer" : [ { "$ref" : "language", "$id" : ObjectId("4da32c897d2de864e0448e06") } ] }

相关数据存放在一起，针对性的查询可以消除join，性能比分散存储要高且方便