如何在对象中获取一些字段？

Question

我有一个集合“学生”。

{ 
    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"}, 
    ] 
} 

我想获得学生的学科“数学”的所有分数。

即时通讯使用aggregation得到

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

而结果：

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

但它显示的所有字段，我要的结果（只显示主题和得分）：

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

如何做到这一点？ 请帮帮我。

Answer 1

我不知道应该如何做，因为排除不支持，我会使用另一个投影包含所需的字段。 （这可能是一个更好的方法）。

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？