从查询中的查询访问 MongoDB 值

Question

为什么在第二个查询中会导致该字段未定义？代码如下：

Survey.findById(req.params.id, function(err, survey) {

        for ( var i=0; i<=survey.businesses.length-1; i++ ) {

            console.log(survey.businesses[i].votes); // This returns the expected value

            UserSurvey.find({ surveyId: req.params.id, selections: survey.businesses[i].id }, function(err, usurvey) {

                console.log(survey.businesses[i].votes); // businesses[i] is undefined

            });
        }

});

Answer 1

您的方法存在一些问题。我建议改为这样做：

Survey.findById(req.params.id, function(err, survey) {
    for ( var i=0; i<=survey.businesses.length-1; i++ ) {
        (function(business) {
            console.log(business); // This returns the expected value
            UserSurvey.find({ surveyId: req.params.id, selections: business.id }, function(err, usurvey) {
                console.log(business.votes); // businesses[i] is undefined
            });
        })(survey.businesses[i]);
    }
});

当您使用带有异步代码和闭包的循环时，在运行异步代码之前，可能会提前关闭闭包（i 的值会发生变化）。这意味着您可能正在访问错误的元素或完全无效的元素。将异步函数包装在自关闭函数中可确保包装函数使用正确的项目。