mongoose 文档（十） Promises

Built-in Promises

Mongoose 异步操作，像 .save() 和 queries，返回 Promises/A+ conformant promises. This means that you can do things like MyModel.findOne({}).then() and yield MyModel.findOne({}).exec() (if you're using co).

这就是说你可以做一些像MyModel.findOne({}).then() 和 yield MyModel.findOne({}).exec()（如果你在用co）

为了向后兼容，mongoose 4默认返回mpromise promise。

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promise。可是它有 yield 和 async/await 的 .then() 方法。如果你需要健全的promise，用.exec()方法。

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
});

插入到你的 Promises 库

在mongoose 4.1.0更新

在mpromise满足基本使用的情况下，高级用户可能想插入他们喜爱的ES6风格的Promise库如bluebird，或只是使用原生的ES6 promise。设置mongoose.Promise 给你喜欢的ES6风格的promise构造函数然后mongoose会使用它。

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);

MongoDB驱动的promise

mongoose.Promise属性设置mongoose使用promise。可是，这不影响底层MongoDB驱动。如果你使用底层驱动，例如Mondel.collection.db.insert()，你需要做点额外工作来改变底层promise库。注意，下面的代码假设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'));
});