尝试将两个集合连接在一起，集合中的数组映射到其他集合。 MongoDB

Question

在有两个集合的 MongoDB 中，尝试获取带有“courseCode”的课程。 在课程集合中，“assignedTutors”是课程集合中的 users._id 数组。 想要“assignedTutors”ID 数组来获取用户集合中的用户信息。

问题：使用当前代码，我无法使用“signedTutors”ID 数组获取用户集合中的用户数据。在当前示例中，“assignedTutors”数组中有 3 个用户 ID，我想使用聚合从用户集合中获取用户数据。

如果有更好的方法来做到这一点，我也愿意。

课程合集：

{
    "_id": {
        "$oid": "66e765c04ce5539d7f7362c2"
    },
    "courseCode": "ACB123",
    "courseName": "ABC",
    "courseImgUrl": "https://ABC.png",
    "coursePrice": "123",
    "assignedTutors": ["66f9a09505e0ca3203870633", "66f9a07e05e0ca3203870630", "66f9a05005e0ca320387062d"],
    "description": "This course ABC",
    "published": true,
    "school": "ABC school"
}

用户收藏：

{
    "_id": {
        "$oid": "66f9a09505e0ca3203870633"
    },
    "userName": "tutor3",
    "password": "xxx",
    "email": "[email protected]",
    "userType": "tutor",
    "firstName": "Tutor",
    "lastName": "Three",
    "teachingCourse": ["66f9adbb05e0ca3203870732", "66e765c04ce5539d7f7362c2"],
    "certificates": [],
    "purchasedCourse": [],
    "__v": {
        "$numberInt": "0"
    }
}

当前代码：

    module.exports.getCourseByCode = function (courseCode) {
        return new Promise((resolve, reject) => {
          Course.aggregate([
            {
              $match: { courseCode: courseCode }
            },
            {
              $lookup: {
                from: 'users',
                localField: 'assignedTutors',
                foreignField: '_id',
                as: 'assignedTutors'
              }
            },
            {
              $unwind: '$assignedTutors'
            },
            {
              $lookup: {
                from: 'users',
                localField: 'assignedTutors._id',
                foreignField: '_id',
                as: 'assignedTutors.tutorData'
              }
            },
            {
              $project: {
                _id: 1,
                courseCode: 1,
                courseName: 1,
                courseImgUrl: 1,
                coursePrice: 1,
                school: 1,
                description: 1,
                published: 1,
                assignedTutors: { $arrayElemAt: ['$assignedTutors', 0] }
              }
            }
          ])
            .then((courses) => {
                console.log(courses)
              if (courses.length === 0) {
                reject(`Course with code ${courseCode} not found.`);
              } else {
                resolve(courses[0]);
              }
            })
            .catch((err) => {
              reject(`Error while retrieving course with code ${courseCode}: ${err.message}`);
            });
        });
      };

电流输出：

{
    "_id": {
        "$oid": "66e765c04ce5539d7f7362c2"
    },
    "courseCode": "ABC123",
    "courseName": "Test",
    "courseImgUrl": "abc.png",
    "coursePrice": "123",
    "assignedTutors": ["66f9a09505e0ca3203870633", "66f9a07e05e0ca3203870630", "66f9a05005e0ca320387062d"],
"description": "XYZ.",
"published": true,
"school": "ABC school"
}

我希望它返回课程以及用户集合上分配的导师数据。

{
    "_id": {
        "$oid": "66e765c04ce5539d7f7362c2"
    },
    "courseCode": "ABC123",
    "courseName": "Test",
    "courseImgUrl": "abc.png",
    "coursePrice": "123",
    "assignedTutors": [{
            "_id": {
                "$oid": "66f99b8005e0ca32038705f6"
            },
            "userName": "user1",
            "password": "zzz",
            "email": "[email protected]",
            "userType": "xxx",
            "firstName": "userF",
            "lastName": "userL",
            "teachingCourse": [],
            "certificates": [],
            "purchasedCourse": [],
            "__v": {
                "$numberInt": "0"
            }
        } {
            "_id": {
                "$oid": "66f99b8005e0ca32038705f6"
            },
            "userName": "user2",
            "password": "zzz",
            "email": "[email protected]",
            "userType": "xxx",
            "firstName": "userF",
            "lastName": "userL",
            "teachingCourse": [],
            "certificates": [],
            "purchasedCourse": [],
            "__v": {
                "$numberInt": "0"
            }
        }, {
            "_id": {
                "$oid": "66f99b8005e0ca32038705f6"
            },
            "userName": "user3",
            "password": "zzz",
            "email": "[email protected]",
            "userType": "xxx",
            "firstName": "userF",
            "lastName": "userL",
            "teachingCourse": [],
            "certificates": [],
            "purchasedCourse": [],
            "__v": {
                "$numberInt": "0"
            }
        }
    }],
"description": "XYZ.",
"published": true,
"school": "ABC school"
}

Answer 1

1. 这里的主要问题是您的

   assignedTutors

   数组包含字符串而不是 ObjectID。因此，查找将

   users.assignedTutors

   中的字符串与

   users._id

   中的 ObjectID 进行匹配。

   如果这是您可以在数据库中更改的内容，那么您应该这样做。否则，您需要将其转换为查找之前或在查找中使用 let+pipelines。我下面的示例在查找之前转换了字符串。

2. 之后，唯一需要改变的是投影：

   assignedTutors: { $arrayElemAt: ['$assignedTutors', 0] }

   。

   您的第二个查找有

   assignedTutors.tutorData

   ，因此您需要对其应用 

   $arrayElemAt

    运算符：

   assignedTutors: { $arrayElemAt: ["$assignedTutors.tutorData", 0] }

   。

   但是，不清楚为什么你需要它，因为你在第一次查找时就已经有了它，并且你说你想要所有的导师。

这是您当前的聚合管道，仅修复了上述两个错误：

db.course.aggregate([
  { $match: { courseCode: courseCode } },
  {
    $set: {
      assignedTutors: {
        $map: {
          input: "$assignedTutors",
          in: { $toObjectId: "$$this" }
        }
      }
    }
  },
  {
    $lookup: {
      from: "users",
      localField: "assignedTutors",
      foreignField: "_id",
      as: "assignedTutors"
    }
  },
  { $unwind: "$assignedTutors" },
  {
    $lookup: {
      from: "users",
      localField: "assignedTutors._id",
      foreignField: "_id",
      as: "assignedTutors.tutorData"
    }
  },
  {
    $project: {
      _id: 1,
      courseCode: 1,
      courseName: 1,
      courseImgUrl: 1,
      coursePrice: 1,
      school: 1,
      description: 1,
      published: 1,
      assignedTutors: { $arrayElemAt: ["$assignedTutors.tutorData", 0] }
    }
  }
])

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