Built-in Promises

mongoose

mongoose

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Built-in Promises ](#built-in-promises)

Mongoose async operations, like .save() and queries, return ES6 promises. This means that you can do things like MyModel.findOne({}).then() and await MyModel.findOne({}).exec() (if you're using async/await.

    var gnr = new Band({
      name: "Guns N' Roses",
      members: ['Axl', 'Slash']
    });
    var promise = gnr.save();
    assert.ok(promise instanceof Promise);
    promise.then(function (doc) {
      assert.equal(doc.name, "Guns N' Roses");
    });

[

Queries are not promises ](#queries-are-not-promises)

Mongoose queries are not promises. However, they do have a .then() function for yield and async/await. If you need a fully-fledged promise, use the .exec() function.

    var query = Band.findOne({name: "Guns N' Roses"});
    assert.ok(!(query instanceof Promise));
    // A query is not a fully-fledged promise, but it does have a `.then()`.
    query.then(function (doc) {
      // use doc
    });
    // `.exec()` gives you a fully-fledged promise
    var promise = query.exec();
    assert.ok(promise instanceof Promise);
    promise.then(function (doc) {
      // use doc
    });

[

Plugging in your own Promises Library ](#plugging-in-your-own-promises-library)

If you're an advanced user, you may want to plug in your own promise library like bluebird. Just set mongoose.Promise to your favorite ES6-style promise constructor and mongoose will use it.

    var query = Band.findOne({name: "Guns N' Roses"});
    // Use bluebird
    mongoose.Promise = require('bluebird');
    assert.equal(query.exec().constructor, require('bluebird'));
    // Use q. Note that you **must** use `require('q').Promise`.
    mongoose.Promise = require('q').Promise;
    assert.ok(query.exec() instanceof require('q').makePromise);

[

Promises for the MongoDB Driver ](#promises-for-the-mongodb-driver)

The mongoose.Promise property sets the promises mongoose uses. However, this does not affect the underlying MongoDB driver. If you use the underlying driver, for instance Model.collection.db.insert(), you need to do a little extra work to change the underlying promises library. Note that the below code assumes mongoose >= 4.4.4.

    var uri = 'mongodb://localhost:27017/mongoose_test';
    // Use bluebird
    var options = { promiseLibrary: require('bluebird') };
    var db = mongoose.createConnection(uri, options);
    Band = db.model('band-promises', { name: String });
    db.on('open', function() {
      assert.equal(Band.collection.findOne().constructor, require('bluebird'));
    });