Promise

内置 Promise

¥Built-in Promises

Mongoose 异步操作（例如 .save() 和查询）返回 thenables。这意味着如果你使用 async/await，则可以执行 MyModel.findOne({}).then() 和 await MyModel.findOne({}).exec() 之类的操作。

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

你可以找到具体操作的返回类型 在 API 文档中 ，也可以阅读有关 Mongoose 中的 promise 的更多信息。

¥You can find the return type of specific operations in the api docs You can also read more about promises in Mongoose.

const gnr = new Band({
  name: 'Guns N\' Roses',
  members: ['Axl', 'Slash']
});

const promise = gnr.save();
assert.ok(promise instanceof Promise);

promise.then(function(doc) {
  assert.equal(doc.name, 'Guns N\' Roses');
});

查询不是 promise

¥Queries are not promises

Mongoose 查询 不是 promise。为了方便起见，它们有 .then() 函数用于 co 和 async/await。如果你需要完整的 promise，请使用 .exec() 函数。

¥Mongoose queries are not promises. They have a .then() function for co and async/await as a convenience. If you need a fully-fledged promise, use the .exec() function.

const 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
const promise = Band.findOne({ name: 'Guns N\' Roses' }).exec();
assert.ok(promise instanceof Promise);

promise.then(function(doc) {
  // use doc
});

可以进行查询

¥Queries are thenable

虽然查询不是 promise，但查询是 thenables。这意味着它们有 .then() 函数，因此你可以将查询用作带有 Promise 链或 异步等待 的 Promise

¥Although queries are not promises, queries are thenables. That means they have a .then() function, so you can use queries as promises with either promise chaining or async await

Band.findOne({ name: 'Guns N\' Roses' }).then(function(doc) {
  // use doc
});

你应该将 exec() 与 await 一起使用吗？

¥Should You Use exec() With await?

使用 await 进行查询有两种选择：

¥There are two alternatives for using await with queries:

  • await Band.findOne();

  • await Band.findOne().exec();

就功能而言，这两者是等效的。但是，我们建议使用 .exec()，因为这可以为你提供更好的堆栈跟踪。

¥As far as functionality is concerned, these two are equivalent. However, we recommend using .exec() because that gives you better stack traces.

const doc = await Band.findOne({ name: 'Guns N\' Roses' }); // works

const badId = 'this is not a valid id';
try {
  await Band.findOne({ _id: badId });
} catch (err) {
  // Without `exec()`, the stack trace does **not** include the
  // calling code. Below is the stack trace:
  //
  // CastError: Cast to ObjectId failed for value "this is not a valid id" at path "_id" for model "band-promises"
  //   at new CastError (/app/node_modules/mongoose/lib/error/cast.js:29:11)
  //   at model.Query.exec (/app/node_modules/mongoose/lib/query.js:4331:21)
  //   at model.Query.Query.then (/app/node_modules/mongoose/lib/query.js:4423:15)
  //   at process._tickCallback (internal/process/next_tick.js:68:7)
  err.stack;
}

try {
  await Band.findOne({ _id: badId }).exec();
} catch (err) {
  // With `exec()`, the stack trace includes where in your code you
  // called `exec()`. Below is the stack trace:
  //
  // CastError: Cast to ObjectId failed for value "this is not a valid id" at path "_id" for model "band-promises"
  //   at new CastError (/app/node_modules/mongoose/lib/error/cast.js:29:11)
  //   at model.Query.exec (/app/node_modules/mongoose/lib/query.js:4331:21)
  //   at Context.<anonymous> (/app/test/index.test.js:138:42)
  //   at process._tickCallback (internal/process/next_tick.js:68:7)
  err.stack;
}
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Mastering Async/Await