[英]MEAN Stack - Delay in query and request?
I'm trying to learn Node.js and writing a web app with the MEAN stack. 我正在尝试学习Node.js,并使用MEAN堆栈编写一个Web应用程序。
As I send on an array of strings containing item data (say, ["one", "two", "three"]
) to the server side, I have the following code to iterate through the array, check if a document with the element already exists in MongoDB, then accordingly create new documents in the database. 当我向服务器端发送包含项目数据(例如
["one", "two", "three"]
)的字符串数组时,我具有以下代码来遍历该数组,检查是否有包含元素已经存在于MongoDB中,然后相应地在数据库中创建新文档。
server side code: 服务器端代码:
// items is an array of strings sent from the client side
for (var i = 0; i < items.length; i++) {
var item_name = items[i];
// For each item, find an existing one
Item.findOne({'name': item_name }, function(err, item) {
// If not already existing, add to database.
if (!item) {
var new_item = new Item();
new_item.name = item_name;
new_item.save(function(err) {
if (err) throw err;
});
}
});
}
}
The problem is that new_item.name
always equals the last element in the array, ie "three"
问题是
new_item.name
始终等于数组中的最后一个元素,即"three"
My guess is that the query in the database takes longer than the loop to run, so by the time we're in the callback of findOne()
, we've already iterated to the end of the array. 我的猜测是数据库中的查询所花的时间比循环要长,因此,当我们进入
findOne()
的回调时,我们已经迭代到数组的末尾。
Is this true? 这是真的? If so, what would be a work around for this problem?
如果是这样,该问题将如何解决?
I understand that this is potentially be a common question that many may have asked before, but after some time of searching I'm still not able to solve the problem. 我了解到,这可能是许多人以前曾问过的一个常见问题,但是经过一段时间的搜索,我仍然无法解决问题。 Please advise.
请指教。
There's already a comment that says to do this. 已经有一条评论说要这样做。 You can create closure (as below) or create a separate function that's outside of the loop.
您可以创建闭包(如下所示),也可以创建循环外的单独函数。 Here's a little blog post about closures http://jondavidjohn.com/blog/2011/09/javascript-closure-explained-using-events
这是有关封包的小博文,网址为http://jondavidjohn.com/blog/2011/09/javascript-closure-explained-using-events
// items is an array of strings sent from the client side
for (var i = 0; i < items.length; i++) {
(function(item_name) {
// For each item, find an existing one
Item.findOne({'name': item_name }, function(err, item) {
// If not already existing, add to database.
if (!item) {
var new_item = new Item();
new_item.name = item_name;
new_item.save(function(err) {
if (err) throw err;
});
}
});
})(items[i]);
}
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