# 承诺（promises）

> 原文：[Built-in Promises](http://mongoosejs.com/docs/promises.html)\
> 翻译：小虾米（QQ:509129）

## 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。

```javascript
  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。

```javascript
    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。

```javascript
  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。

```javascript
    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'));
    });
```


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