node.js - 在 node.js 和 mongodb 中处理异步数据库查询

Question

我在从 node.js 异步查询 mongodb 时遇到了这个问题。这是我的代码

var values = [];
var positives = new Array();
var negatives = new Array();

var server1 = new mongodb.Server('localhost',27017, {auto_reconnect: true});
var db1 = new mongodb.Db('clicker', server1);   

db1.open(function(err, db) {
    if(!err) {

        db1.collection('feedback', function(err, collection) {
            for (var i=0;i <5; i++) {
                collection.find(
                    {value:1},
                    {created_on: 
                        {       
                            $gte:startTime + (i*60*1000 - 30*1000),
                            $lt: startTime + (i*60*1000 + 30*1000)
                        }
                    },
                    function(err_positive, result_positive) {
                        result_positive.count(function(err, count){
                                console.log("Total matches: " + count);
                                positives[i] = count;
                        });
                    }

                );              

                collection.find(
                    {value:0},
                    {created_on: 
                        {
                            $gte:startTime + (i*60*1000 - 30*1000),
                            $lt: startTime + (i*60*1000 + 30*1000)
                        }
                    },
                    function(err_negative, result_negative) {
                        result_negative.count(function(err, count){
                                console.log("Total matches: " + count);
                                negatives[i] = count;
                        });
                    }   
                );                                  
            }

        });

    } else {
        console.log('Error connecting to the database');
    }      

});

实际上，我正在尝试从数据库中获取一些值。然后我需要操纵这些值。只是我需要从正数中减去负数，然后用正数-负数初始化值数组。现在，由于结果是异步获得的。我应该如何操作这些值并将它们放入值数组中。

Answer 1

在我进一步解释之前，我想指出您的代码中存在错误:

function(err_positive, result_positive) {
    result_positive.count(function(err, count){
        console.log("Total matches: " + count);
        positives[i] = count;  // <--- BUG: i id always 5 because it
    });                        //           is captured in a closure
}

经典的闭包和循环问题。请参阅:Please explain the use of JavaScript closures in loops

现在，如何在循环中处理异步函数。基本思想是您需要跟踪完成了多少异步调用，并在最终调用返回后运行您的代码。例如:

var END=5;
var counter=end;
for (var i=0;i<END; i++) {
  collection.find(
    {value:1},
    {created_on: 
      {       
        $gte:startTime + (i*60*1000 - 30*1000),
        $lt: startTime + (i*60*1000 + 30*1000)
      }
    },
    (function(j){
      return function(err_positive, result_positive) {
        result_positive.count(function(err, count){
            console.log("Total matches: " + count);
            positives[j] = count;
        });

        counter--;
        if (!counter) {
          /*
           * Last result, now we have all positives.
           *
           * Add code that need to process the result here.
           *
           */
        }
      }
    })(i)
  ); 
}

但是，如果我们继续这样做，很明显我们最终会创建一堆临时变量并最终得到可怕的嵌套代码。但这是 javascript，我们可以将此模式的逻辑封装在一个函数中。这是我在 javascript 中实现的这个“等待所有完成”逻辑:Coordinating parallel execution in node.js

但由于我们使用的是 node.js，我们可以使用方便的异步模块形式 npm:https://npmjs.org/package/async

使用异步，你可以这样写你的代码:

var queries = [];

// Build up queries:
for (var i=0;i <5; i++) {
  queries.push((function(j){
    return function(callback) {
      collection.find(
        {value:1},
        {created_on: 
          {       
            $gte:startTime + (j*60*1000 - 30*1000),
            $lt: startTime + (j*60*1000 + 30*1000)
          }
        },
        function(err_positive, result_positive) {
          result_positive.count(function(err, count){
            console.log("Total matches: " + count);
            positives[j] = count;          
            callback();
          });
        }

      );
    }
  })(i));
  queries.push((function(j){
    return function(callback) {
      collection.find(
        {value:0},
        {created_on: 
          {
            $gte:startTime + (j*60*1000 - 30*1000),
            $lt: startTime + (j*60*1000 + 30*1000)
          }
        },
        function(err_negative, result_negative) {
          result_negative.count(function(err, count){
            console.log("Total matches: " + count);
            negatives[j] = count;
            callback();
          });
        }   
      );
    }
  })(i));  
}

// Now execute the queries:
async.parallel(queries, function(){
  // This function executes after all the queries have returned
  // So we have access to the completed positives and negatives:

  // For example, we can dump the arrays in Firebug:
  console.log(positives,negatives);
});