node.js - 如何在对象中获取一些字段？

Question

我有一个集合“Students”。

{
    student:"Jone Doe",
    class:"A",
    subjects:
    [
        {subject:"Math",teacher:"Linda","score":"82"},
        {subject:"English",teacher:"Jone","score":"52"},
        {subject:"History",teacher:"Maria","score":"32"},
    ]
}
{
    student:"Baby Doe",
    class:"B",
    subjects:
    [
        {subject:"Math",teacher:"Hilary","score":"52"},
        {subject:"English",teacher:"Notham","score":"52"},
        {subject:"History",teacher:"Hamet","score":"32"},
    ]
}

我想获得学生“数学”科目的所有分数。

我正在使用聚合来获取

db.collection('studens').aggregate([
      {$match: {"subjects.subject" : "Math" } },
      {$project: {
        _id:0,
         subjects: {
            $filter: {
               input: "$subjects",
               as: "subject",
               cond: { $lte: [ "$$subject.subject", "Math" ] }
            }
         }
      }
   }
]);

结果:

{subject:"Math",teacher:"Linda","score":"82"}
{subject:"Math",teacher:"Hilary","score":"52"}

但是它显示了所有字段，我想要结果(只显示主题和分数):

{subject:"Math","score":"82"}
{subject:"Math","score":"52"}

如何做到这一点？请帮助我。

Answer 1

我不确定应该怎么做，因为不支持排除，我会使用另一个投影来包含所需的字段。 (可能有更好的方法)。

让我们试试这个:

db.collection('studens').aggregate([
      {$match: {"subjects.subject" : "Math" } },
      {$project: {
            _id:0,
            subjects: {
                 $filter: {
                    input: '$subjects',
                    as: 'subjects',
                    cond: {$eq: ['$$subjects.subject', 'Math']}
                 }
            }
      }
   },{$project: {subjects: { subject:1, score:1}}}
]);

结果:

"result" : [ 
    {
        "subjects" : [ 
            {
                "subject" : "Math",
                "score" : "82"
            }
        ]
    }, 
    {
        "subjects" : [ 
            {
                "subject" : "Math",
                "score" : "52"
            }
        ]
    }
]

这是您要找的吗？