Mongodb findone方法不起作用

Question

function updateMongo(url, cached) {
    console.log("Update mongo called");
    MongoClient.connect('mongodb://127.0.0.1:27017/dbnerds', function(err, db) {
        if(err) {
            return console.log(err);
        }
        var cache = db.collection('cache');
        cache.insert({url: url, image: null}, {safe: false}, null);
        cache.findOne({ url: url }, function(err, result) {
            console.log(result);
        });
        db.close();
    });
};

As you can see from my code, that I insert url into mongodb, but when I do findOne operation, it seems it does not work. The console did not print out any result. What is wrong with here?

Answer 1

The problem is because there is no guarantee that the call to .insert() has finished when the call to .findOne() is executed.

Try using the following approach instead:

function updateMongo(url, cached) {
    console.log("Update mongo called");
    MongoClient.connect('mongodb://127.0.0.1:27017/dbnerds', function(err, db) {
        if(err) {
            return console.log(err);
        }
        var cache = db.collection('cache');
        cache.insert({url: url, image: null}, {safe: false}, function(err, n) {
            // Make the call to findOne until insert has finished
            cache.findOne({ url: url }, function(err, result) {
                console.log(result);
                // close the DB after you are done
                db.close();
            });
        });
    });
};

Notice how I moved the call to findOne() to be the callback function executed after the insert has finished.