# 承诺（promises）
> 原文：[Built-in Promises](http://mongoosejs.com/docs/promises.html)
## Built-in Promises
Mongoose异步操作，像`.save()`和查询，返回[Promises/A+ conformant promises](https://promisesaplus.com/)。这意味着你可以像`MyModel.findOne({}).then()`和 `MyModel.findOne({}).exec()`（如果你使用[co](https://www.npmjs.com/package/co)包）。
为了向后兼容，Mongoose 4返回默认mpromise承诺。
```
var gnr = new Band({
name: "Guns N' Roses",
members: ['Axl', 'Slash']
});
var promise = gnr.save();
assert.ok(promise instanceof require('mpromise'));
promise.then(function (doc) {
assert.equal(doc.name, "Guns N' Roses");
});
```
### Queries are not promises
Mongoose查询不是承诺。然而，他们也有一个`.then()`功能为产量和异步/等待。如果你需要一个完全成熟的承诺，用`.exec()`功能。
```
var query = Band.findOne({name: "Guns N' Roses"});
assert.ok(!(query instanceof require('mpromise')));
// 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 require('mpromise'));
promise.then(function (doc) {
// use doc
});
```
### Plugging in your own Promises Library
> Mongoose 4.1.0 新特性
而mpromise满足基本使用的情况下，高级用户可能想插上自己喜欢的[ES6风格](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise)[承诺类库](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise)像[bluebird](https://www.npmjs.com/package/bluebird)，或只使用本地的ES6承诺。只需要设置`mongoose.Promise`到你喜欢的ES6风格承诺构造函数，mongoose会使用它。
随着本土承诺ES6Mongoose试验，[bluebird](https://www.npmjs.com/package/bluebird)和[q](https://www.npmjs.com/package/q)。任何承诺类库，理论上导出一个ES6风格的构造函数应该工作，但理论往往不同于实践。如果你发现一个bug，开放式问题在[GitHub](https://github.com/Automattic/mongoose/issues)上。
```
var query = Band.findOne({name: "Guns N' Roses"});
// Use native promises
mongoose.Promise = global.Promise;
assert.equal(query.exec().constructor, global.Promise);
// 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
mongoose。承诺的属性集的mongoose用承诺。然而，这并不影响底层的MongoDB的驱动程序;。如果你使用了底层的驱动程序，例如`Model.collection.db.insert()`，你需要做一些额外的工作来改变其所承诺的类库。注意，下面的代码假定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'));
});
```
