javascript - 如何获取文档数组中元素的数量 - MongoDB？

Question

我有以下 MongoDB 集合 (JSON):

{
    "_id" : ObjectId("570185458351bbac27bc9a20"),
    "email" : "test@gmail.com",
    "applicants" : [
            {
                    "id" : "570724e4ae4f8a5026156999",
                    "email" : "a@gmail.com",
            },
            {
                    "id" : "570724e4ae4f8a5026156333",
                    "email" : "a2@gmail.com",
            },
            {
                    "id" : "570724e4ae4f8a5026156111",
                    "email" : "a3@gmail.com",
            },
            {
                    "id" : "570724e4ae4f8a5026156222",
                    "email" : "a4@gmail.com",
            }
    ],
}, 
{
    "_id" : ObjectId("570185458351bbac27bc9a20"),
    "email" : "test@gmail.com",
    "applicants" : [
            {
                    "id" : "570724e4ae4f8a5026156555",
                    "email" : "a@gmail.com",
            },
            {
                    "id" : "570724e4ae4f8a5026156666",
                    "email" : "a2@gmail.com",
            },
    ],
},
{
    "_id" : ObjectId("570185458351bbac27bc9a20"),
    "email" : "test2@gmail.com",
    "applicants" : [
            {
                    "id" : "570724e4ae4f8a5026156555",
                    "email" : "a@gmail.com",
            },
            {
                    "id" : "570724e4ae4f8a5026156666",
                    "email" : "a2@gmail.com",
            },
    ],
 }

我想获取 email = test@gmail.com 的文档的所有数组中元素的计数。我怎样才能得到那个计数？

我正在使用以下方法获取电子邮件 test@gmail.com 的文档数量:

   collection.count({"email" : tmpEmail},    function (err, count) {
      res.json(count);
      console.log("Number: " + count);
    }); 

我如何继续计算电子邮件为 test@gmail.com 的文档的所有申请人数组中的元素数量？上面示例的可能是:6.

编辑:

根据其中一个答案，我将查询修改为以下内容:

答案一:

collection.aggregate(
        {$match: {"email": req.user.username, "status" : "true"}},
        {$unwind: "$applicants"},
        {$group: {_id:null, count: {$sum :1}},  function (err, count) {
            res.json(count);
            console.log("Number of New Applicants: " + count);
            }
        });

答案2:

collection.aggregate(
        [{$match:{"email" : req.user.username, "status" : "true"}}, 
         {$project:{_id:0, email:1, totalApplicants:{$size:"$applicants"}}},  
         {$group:{_id:"$employer", count:{$sum:"$totalApplicants"}}}],
         function (err, count){
             res.json(count);
             console.log("Number of New Applicants: " + count);
         });

Answer 1

您可以改用聚合查询:

collection.aggregate(
    [{$match: {"email": req.user.username, "status" : "true"}},
    {$unwind: "$applicants"},
    {$group: {_id:null, count: {$sum :1}}}],  function (err, result) {
        console.log(result);
        console.log("Number of New Applicants: " + result[0].count);
        if(result.length > 0)
            res.json(result[0]);
        else
            res.json({count:0});
        }
    });

这将生成一份文档，其中 count 将具有您所需的结果