Why is MongoDB unable to insert the same JSON twice in a row?

Question

Why is MongoDB unable to insert the same JSON twice in a row?

I'm using the Node.js native driver. The following works fine

db.collection("test").insert({hello:'world_safe'}, {safe: true}, function(err, result) {
    if (err) throw err;
    db.collection("test").insert({hello:'world_safe'}, {safe: true}, function(err, result) {
        if (err) throw err;
        db.close();
    });
});

I get the following in the database

{ "hello" : "world_safe", "_id" : ObjectId("4fe978c8b8a5937d62000001") } { "hello" : "world_safe", "_id" : ObjectId("4fe978c8b8a5937d62000002") }

However when I tweak as follows

var json = {hello:'world_safe'};
db.collection("test").insert(json, {safe: true}, function(err, result) {
    if (err) throw err;
    db.collection("test").insert(json, {safe: true}, function(err, result) {
        if (err) throw err;
        db.close();
    });
});

I get the following error

MongoError: E11000 duplicate key error index:

Why do I get the error message?

node.js
mongodb

Answer 1

The driver adds an _id key to your json object on the first insert, so on the second insert your json has a _id witch is duplicate.

https://github.com/mongodb/node-mongodb-native/blob/master/lib/mongodb/collection.js

// Add id to each document if it's not already defined
    if (!(Buffer.isBuffer(doc)) && doc['_id'] == null && self.db.forceServerObjectId != true) {
      doc['_id'] = self.pkFactory.createPk();
    }

Answer 2

I agree with CD, except the solution is easier than that:

/* ... before insert */

if(typeof(collection._id) != 'undefined')
        delete collection._id;

/* ... now insert */