Question

我有一个系列＆＃34;学生＆＃34;。

{
    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"},
    ]
}

我希望得到主题＆＃34;数学＆＃34;学生。

我正在使用aggregation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"}

但它显示所有字段，我想要结果（仅显示主题和分数）：

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

怎么做？请帮帮我。

Answer 1

由于不支持排除，我不确定应该怎么做，我会使用另一个投影来包含所需的字段。（这可能是一种更好的方法）。

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"
            }
        ]
    }
]

这是你在找什么？

如何在对象中获得一些字段？

1 个答案: