在使用聚合计算同一行时，MongoDb需要帮助

Question

我的任务是写一个查询，要求我列出主题类型的名称和属于每种主题类型的主题的总no。在数据库中，有两种不同的主题类型是核心和选修课。

在这里，数据库中包含两个不同主题类型的2个示例数据：

db.Subject.insert(
{ 
    "_id":ObjectId(),
    "subject":{ 
        "subCode":"CSCI203",
        "subTitle":"Algorithm and Data Structures",
        "credit":3,
        "type":"Core",
        "prerequisite": ["csci103"],
        "assessments": [
                { "assessNum": 1,
                  "weight":10,
                  "assessType":"Assignment",
                  "description":"Assignment 1 - Mathematic Concepts and Algorithm Complexity" },
                { "assignNum": 2,
                  "weight":15,
                  "assessType":"Assignment",
                  "description":"Assignment 2 - Linear and Non-linear Data Structure" },
                { "assessNum": 3,
                  "weight":15,
                  "assessType":"Assignment",
                  "description":"Assignment 3 - Greedy Algorithm and Branch-and-Bound" },
                { "assessNum": 4,
                  "weight": 60,
                  "assessType":"Examination",
                  "description":"Closed-book Final Examination" }
            ],
        "book": [
                { "ISBN":"13:978-0-13-231681-1",
                  "bookType":"textbook",
                  "bookTitle":"Introduction to the Design and Analysis of Algorithms",
                  "edition":3,
                  "yearPub":2012,
                  "publisher":"Pearson",
                  "author": [ "Levitin" ] },
                { "ISBN":"978-0-262-53305-8",
                  "bookType":"reference",
                    "bookTitle":"Introduction to Algorithms",
                  "edition":3,
                  "yearPub":2013,
                  "publisher":"The MIT Press",
                  "author": [ "Thomas H Cormen", "Charles E Leiserson", "Ronald L Riverst", "Clifford Stein" ] } ]
  }
}
)

db.Subject.insert(
{ 
    "_id":ObjectId(),
    "subject":{ 
        "subCode":"IACT201",
        "subTitle":"Professional Practice and Ethics",
        "credit":3,
        "type":"Elective",
        "assessments": [
                { "assessNum": 1,
                  "weight":35,
                  "assessType":"Assignment",
                  "description":"Assignment 1 - Ethical Dilemma Case Study" },
                { "assignNum": 2,
                  "weight":25,
                  "assessType":"Presentation",
                  "description":"Presentation of Case Study" },
                { "assessNum": 3,
                  "weight": 40,
                  "assessType":"Examination",
                  "description":"Closed-book Final Examination" }
            ]
  }
}
)

如何使用聚合创建一个查询，使我能够计算核心和选修科目的数量？我现在很迷茫，因为两者都属于主题类型。

下面是一个期望结果的例子：{“核心科目”：6“选修科目”：1}

Answer 1

如果只有2种类型，则可以用一行来表示：

db.collection.aggregate([{$sortByCount: "$subject.type"}])

据我所知，唯一的问题是你不能让这个案子变得麻木不仁。

Answer 2

您可以使用组

db.collection.aggregate([
  {
    $match: {
      $or: [
        {
          "subject.type": "Core"
        },
        {
          "subject.type": "Elective"
        }
      ]
    }
  },
  {
    $group: {
      _id: "$subject.type",
      count: {
        $sum: 1
      }
    }
  }
])

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