NodeJS + Mongo native - 检查查询之前是否存在集合 [英] NodeJS + Mongo native – check if collection exists before query

问题描述

我有一个函数，试图从MongoDB中的设置集合中获取特定的值。设置集合中设置对象的标记（包含设置值）为{'settings'：'settings'}。模式是：

I've got a function, trying to get a specific value from settings collection in MongoDB. The marker for settings object, containing settings values, in settings collection is {'settings':'settings'}. The schema is:

collection:setting
|--object
   |--{'settings':'settings'}
   |--{'valueA':'valueA'}
   |--...

问题是当我第一次查询设置对象时，集合的设置根本不存在。因此，

The problem is when I first time query settings object, the collection 'settings' simply does not exists. So,

exports.getInstruments = function (callback) {
db.collection("settings", function(error, settings) {
    settings.find({ "settings" : "settings" }), (function(err, doc) {
           callback(doc.instruments);
    }); 
]);  
}

只是挂起，不会调用回调。如果收集不存在，我应该返回或未定义，else - doc.instrumens。

just hangs and callback is not invoked. If collection does not exist, I should return "" or undefined, else - doc.instrumens.

推荐答案

除了一些语法问题，主要的问题是 find 传递 Cursor 到您的回调函数，而不是第一个匹配的文档。如果你只需要一个文档，你应该使用 findOne 。

Aside from some syntax problems, the main problem is that find passes a Cursor to your callback function, not the first matching document. If you're expecting just one doc, you should use findOne instead.

>

This should work:

exports.getInstruments = function (callback) {
    db.collection("settings", function(error, settings) {
        settings.findOne({ "settings" : "settings" }, function(err, doc) {
            callback(doc && doc.instruments);
        });
    });
};

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