理解Node / Mongo中的find [英] Understanding find in Node/Mongo

问题描述

我正在尝试学习节点。
考虑这段代码（基于官方MongoDB Node.js驱动程序）

I am trying to learn node. Consider this code (based on official MongoDB Node.js driver)

  // Retrieve all the documents in the collection
  collection.find().toArray(function(err, documents) {
    assert.equal(1, documents.length);
    assert.deepEqual([1, 2, 3], documents[0].b);

    db.close();
  });

我有两个问题：

• 是 find 同步还是异步？



• 如果它是异步的， .toArray 函数调用让我感到困惑，因为通常情况下我会期待某些内容

• is find synchronous or asynchronous?

• If it is asynchronous, the .toArray function call is confusing me, because normally I would expect something along the lines of

collection.find(function(err, results){});

具体来说我感兴趣什么机制允许你在异步函数的结果上调用 .toArray ？因为我得到的异步函数很少返回一些东西（我认为除了promises），而是调用传递给它们的回调。有人可以通过find和 .toArray 澄清这种情况吗？

Specifically I am interested what mechanism allows you to call a .toArray on result of asynchronous function? Because asynchronous functions as I get it rarely return something (I think except promises), rather invoke callbacks passed to them. Can someone clarify this situation with find and .toArray?

例如，在这个问题的接受答案中：如何获得回调MongoDB collection.find（），你可以看到作者调用找到我想象的方式，并收到游标在回调函数中。这对我很好，这就是我期望它的工作方式。
但链接异步调用的结果查找（如果是异步？）， toArray 有点让我困惑。

For example in the accepted answer of this question: How to get a callback on MongoDB collection.find(), you can see author calls find the way I envisioned, and received cursor in callback function. That is fine with me, that is how I expected it to work. But chaining result of asynchronous call find (if it is asynch?), with toArray a bit confuses me.

我的推测是 find 返回一个句柄类的东西，数据此时尚未从DB加载，只有当您调用 toArray 实际数据到达时。我是对的吗？

My speculation is find returns a handle kind of thing, the data at this point hasn't been loaded from DB, only when you call toArray the actual data arrives. Am I right?

推荐答案

我承认你，这个案子有点奇怪。这里是mongodb-native的v2.2。

I concede you, this case is a bit weird. Here is for the v2.2 of mongodb-native.

首先， find 有两种不同的用法。你可以给出回调函数。但在无论如何中，它会返回同步一个对象。更准确地说，它是一个光标。
我们可以期待一个异步机制在传递回调但不在这里。

First of all, find has two different usages. You can either give a callback function or not. But in any case, it returns synchronously an object. More precisely it's a cursor. We could expect a asynchronous mechanism when passing a callback but not here.

collection.find({ }, function (err, cursor) {
  assert(!err);
});
console.log('This happens after collection.find({ }, callback)');

OR

const cursor = collection.find({});
console.log('Also happening after');

另一方面， toArray 是一个异步功能也有两种不同的用法。这次，返回的对象因参数而异。

On the other hand, toArray is an asynchronous function and has also two different usages. This time, the returned object is different depending on the arguments.

等效：

cursor.toArray(function (err, documents) {
  assert.equal(1, documents.length);
});

AND

cursor.toArray()
  .then(documents => {
    assert.equal(1, documents.length);
  });

在第一次调用中， toArray 返回 undefined 而在第二个中，它返回 Promise 。

In the first call, toArray returns undefined whereas in the second, it returns a Promise.

这篇关于理解Node / Mongo中的find的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！