Node Js mongoose：如何从与其他集合相关的数据库集合中检索数据

Question

我想实现一个函数来处理学生、课程和教师数据（从 MongoDB 集合中加载），以便生成有关注册课程的学生的信息（并将其存储在另一个 MongoDB 集合中）。该函数报告每个学生的：

primary key
their name
the number of courses they are enrolled in

我想将此例程的输出存储在一个名为 coursereport 的 MongoDB 集合中，如下所示：

[
  {
    "_id": "jeff",
    "value": {
      "name": "Jeff Holland",
      "numbercourses": 2
    }
  },
  {
    "_id": "john.shore",
    "value": {
      "name": "John Shore",
      "numbercourses": 2
    }
  },
  {
    "_id": "lee2331",
    "value": {
      "name": "Lee Aldwell",
      "numbercourses": 1
    }
  },
  {
    "_id": "rcotter",
    "value": {
      "name": "Ray Cotter",
      "numbercourses": 2
    }
  },
  {
    "_id": "scott",
    "value": {
      "name": "Scott Mills",
      "numbercourses": 3
    }
  }
]

我的数据：

{
  "students": [
    {
      "_id": "john.shore",
      "name": {"first": "John", "last": "Shore"},
      "email": "john.shore@gmail.com",
      "major": "Electrical Engineering"
    },
    {
      "_id": "jeff",
      "name": {"first": "Jeff", "last": "Holland"},
      "email": "jeff@yahoo.com",
      "major": "Business"
    },
    {
      "_id": "scott",
      "name": {"first": "Scott", "last": "Mills"},
      "email": "scott@hotmail.com",
      "major": "Humanities/Art"
    },
    {
      "_id": "rcotter",
      "name": {"first": "Ray", "last": "Cotter"},
      "email": "rcotter@msn.com",
      "major": "Computer Science"
    },
    {
      "_id": "lee2331",
      "name": {"first": "Lee", "last": "Aldwell"},
      "email": "lee2331@aol.com",
      "major": "Graphic Design"
    }
  ],
  "courses": [
    {
      "_id": "HIST-1010",
      "name": "History of the World 1010",
      "description": "A bunch of really interesting things that actually happened",
      "students": ["scott", "john.shore"],
      "ratings": [3, 5, 4, 5, 4, 4, 2, 4]
    },
    {
      "_id": "ENGCOMP-1010",
      "name": "English Composition 1010",
      "description": "If you can't write well, you've got nothing!",
      "students": ["scott", "lee2331", "rcotter", "john.shore", "jeff"],
      "ratings": [4, 4, 5, 4, 5, 1, 5]
    },
    {
      "_id": "ART-1050",
      "name": "Artistic Interpretation 1050",
      "description": "Discover your inner beholder",
      "students": ["rcotter", "scott", "jeff"],
      "ratings": [3, 4, 3, 4, 4, 3, 4, 4]
    }
  ],   
  "instructors": [
    {
      "_id": "wally.r.binns",
      "name": {"first": "Wally", "middle": "r", "last": "Binns"},
      "email": "wally.r.binns@ssu.edu",
      "bio": "I was born in the middle of my mother's doctoral dissertation on Faraday Cage isolation. I've been an academic ever since...",
      "publications": [{
        "title": "Inverted Celestial Poetry",
        "source": "http://www.pubcentral.com/poetry/inverted-celestial-poetry"
      }],
      "courses": ["ENGLIT-2500"]
    },
    {
      "_id": "gerald.waterford.iii",
      "name": {"first": "Gerald", "last": "Waterford", "suffix": "III"},
      "email": "gerald.waterford.iii@ssu.edu",
      "bio": "My father's father was a great man. My father, not so much. I am restoring the family honor.",
      "publications": [{
        "title": "Grow, grow, little Dandelion",
        "source": "http://www.hopefulstories.com/my-dandelion"
      }, {"title": "The teapot and the spoon", "source": "http://www.dishsoap.com/teapot-spoon"}],
      "courses": ["ENGCOMP-1010", "HIST-1010"]
    },
    {
      "_id": "kim.b",
      "name": {"prefix": "Mrs.", "first": "Kim", "last": "Binnley"},
      "email": "kim.b@ssu.edu",
      "bio": "My mother told me 'Don't let those dopes push you around'. My life has been a constant struggle against dopeness ever since. Sigh...",
      "publications": [],
      "courses": ["ART-1050"]
    }
  ]
}

我从那个开始：

async function produceReport(db, callback) {
  const students = db.collection('students');
  const courses = db.collection('courses');
  const instructors = db.collection('instructors');

  // implement missing code that will create 'coursereport' mongo collection with student courses data

  callback();
}

module.exports = produceReport;

谁能帮帮忙

Answer 1

你必须使用$lookup

定义为：

要在输入文档中的字段与“已加入”集合的文档中的字段之间执行相等匹配，

所以你必须加入students 和courses 以及后来的输出值instructors..

看看这个查询：

db.students.aggregate([
  {
    $lookup: {
      from: "courses",
      localField: "item",
      foreignField: "email",
      as: "students_courses"
    }
  },
  {
    "$lookup": {
      "from": "instructors",
      "localField": "students_courses._id",
      "foreignField": "courses",
      "as": "students_courses"
    }
  },
  {
    "$set": {
      "_id": "$_id",
      "value": {
        "name": {
          "$concat": [
            "$name.first",
            " ",
            "$name.last"
          ]
        },
        "numbercourses": {
          "$size": "$students_courses"
        }
      }
    }
  },
  {
    "$project": {
      "_id": 1,
      "value": 1
    }
  }
])

第一阶段是两个集合的连接，其中指定的字段并分类为一个名为students_courses 的字段。 然后，从students_courses 与_ids 再次加入。

使用数据，您可以创建一个名为 value 的字段，并计算其他字段中的课程数。

和$project 仅显示您想要的字段。

查看示例here

Answer 2

db.students.aggregate([
  {
    $lookup: {
      from: "courses",
      localField: "_id",
      foreignField: "students",
      as: "students_courses"
    }
  },
  {
    $project: {
      _id: 1,
      value: {
        name: {
          $concat: [
            "$name.first",
            " ",
            "$name.last"
          ]
        },
        numbercourses: {
          "$size": "$students_courses"
        }
      }
    }
  }
])