06、Mongodb 自增

MongoDB 原生是没有 SQL 那种可以让 Field 自动增长的功能, 在某些情况下, 可能需要这样的功能, 好在 MongoDB 官方给出解决方案, 可以让 MongoDB 的 Field 自动增长.

这篇笔记我只个搬运工…

官方提供了两种方法 Use Counters Collection 和 Optimistic Loop, 以 _id 为例来说明下这两种方法.

Use Counters Collection
Optimistic Loop

Use Counters Collection

1. Insert into the counters collection, the initial value for the userid:

db.counters.insert(
   {
      _id: "userid",
      seq: 0
   }
)

2. Create a getNextSequence function that accepts a name of the sequence. The function uses the findAndModify() method to atomically increment the seq value and return this new value:

function getNextSequence(name) {
   var ret = db.counters.findAndModify(
          {
            query: { _id: name },
            update: { $inc: { seq: 1 } },
            new: true // true 表示返回 update 后的结果, false 表示返回 update 前的结果
          }
   );

   return ret.seq;
}

3. Use this getNextSequence() function during insert().

db.users.insert(
   {
     _id: getNextSequence("userid"),
     name: "Sarah C."
   }
)

db.users.insert(
   {
     _id: getNextSequence("userid"),
     name: "Bob D."
   }
)

Optimistic Loop

1. Create a function named insertDocument that performs the “insert if not present” loop. The function wraps the insert() method and takes a doc and a targetCollection arguments.

function insertDocument(doc, targetCollection) {

    while (1) {

        var cursor = targetCollection.find( {}, { _id: 1 } ).sort( { _id: -1 } ).limit(1);

        var seq = cursor.hasNext() ? cursor.next()._id + 1 : 1;

        doc._id = seq;

        var results = targetCollection.insert(doc);

        if( results.hasWriteError() ) {
            if( results.writeError.code == 11000 /* dup key */ )
                continue;
            else
                print( "unexpected error inserting data: " + tojson( results ) );
        }

        break;
    }
}

2. Use the insertDocument() function to perform an insert:

var myCollection = db.users2;

insertDocument(
   {
     name: "Grace H."
   },
   myCollection
);

insertDocument(
   {
     name: "Ted R."
   },
   myCollection
)

☠