mongodb 创建auto increment 自增函数

计数器表  
> db.counters.insert({_id: "userid",seq: 0}  );

> db.counters.find();  
{ "_id" : "userid", "seq" : 0 }  
 
取下个ID的函数  
> function getNextSequence(name) {       
   var ret = db.counters.findAndModify(  
       {  
           query: { _id: name },  
           update: { $inc: { seq: 1 } },    
           new: true,  
           upsert: true  
        }  
   );        
   return ret.seq;  
 };

插入数据、查看
> db.users.insert({_id: getNextSequence("userid"),name: "tank"});
WriteResult({ "nInserted" : 1 })
> db.users.find();  
{ "_id" : 1, "name" : "tank" }
>
> db.users.insert({_id: getNextSequence("userid"),name: "test"});
WriteResult({ "nInserted" : 1 })
> db.users.find();  
{ "_id" : 1, "name" : "tank" }
{ "_id" : 2, "name" : "test" }