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$lookup result mongodb的计数

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$lookup result mongodb的计数,mongodb,aggregation-framework,Mongodb,Aggregation Framework,将mongodb与NodeJS驱动程序一起使用 我有两个收藏。一个给系里，另一个给学生 Deparmtent的示例数据 { "_id" : ObjectId("5a24d20590d3d12155f3094e"), "name" : "CSE", "hod" : "abc", "students" : [ ObjectId("5a2129172c3e542acb78c1f5"), ObjectId("5a2129172c3e542acb78c1f7"), Object

将mongodb与NodeJS驱动程序一起使用

我有两个收藏。一个给系里，另一个给学生

Deparmtent的示例数据

{ 
 "_id" : ObjectId("5a24d20590d3d12155f3094e"), "name" : "CSE", "hod" : 
 "abc", 
 "students" : [ 
  ObjectId("5a2129172c3e542acb78c1f5"),
  ObjectId("5a2129172c3e542acb78c1f7"), 
  ObjectId("5a2129172c3e542acb78c1f9"), 
  ObjectId("5a2129172c3e542acb78c1fb")
 ]
}

{
 "_id" : ObjectId("5a24d20590d3d12155f3094f"), 
 "name" : "IT", 
 "hod" : "xyz", , 
 "students" : [ 
  ObjectId("5a2129172c3e542acb78c1f6"),
  ObjectId("5a2129172c3e542acb78c1f8"), 
  ObjectId("5a2129172c3e542acb78c1fa"),
  ObjectId("5a2129172c3e542acb78c1fc") 
 ] 
}
以及学生的样本数据

{
    "_id" : ObjectId("5a2129172c3e542acb78c1f4"),
    "rollNumber" : 11,
    "name" : "Thor",
    "branch" : ObjectId("5a24d20590d3d12155f3094e"),
}
以下是查询：

aggregate([
   {$lookup: 
     { from: "students", 
       localField: "_id", 
       foreignField: "branch", as: "studentData"
     }
   }
])
查询的输出

[ 
  { _id: 5a24d20590d3d12155f3094e,
    name: 'CSE',
    hod: 'abc',
    studentData: [ [Object], [Object], [Object], [Object], [Object] ] 
  },
  { _id: 5a24d20590d3d12155f3094f,
    name: 'IT',
    hod: 'xyz',
    studentData: [ [Object], [Object], [Object], [Object] ] } 
]
我有两个问题

  • 如何打印完整的学生数据
  • 如何获取每个系的学生数据计数
    • 要获取完整的学生数据，您可以执行以下操作

    db.student.aggregate(
        [
            {
            $lookup:
                {
                    from:"department",
                    localField:"branch",
                    foreignField:"_id",
                    as:"branch"
                }
            }
        ]
    )
    这将为您提供如下信息：

    {
    "_id" : 1.0,
    "rollNumber" : 110.0,
    "name" : "Thor",
    "branch" : [ 
        {
            "_id" : 1.0,
            "name" : "CSE",
            "hod" : "abc"
        }
    ]
}

    {
    "_id" : [ 
        {
            "_id" : 2.0,
            "name" : "IT",
            "hod" : "xyz"
        }
    ],
    "numOfStudent" : 1.0,
    "listOfStudents" : [ 
        "Ironman2"
    ]
}
{
    "_id" : [ 
        {
            "_id" : 1.0,
            "name" : "CSE",
            "hod" : "abc"
        }
    ],
    "numOfStudent" : 3.0,
    "listOfStudents" : [ 
        "Thor", 
        "Ironman", 
        "Ironman3"
    ]
}

    {
    "name" : "CSE",
    "hod" : "abc",
    "numOfStudents" : 2,
    "students" : [ 
        {
            "_id" : 1.0,
            "rollNumber" : 110.0,
            "name" : "Thor",
            "branch" : 1.0
        }, 
        {
            "_id" : 3.0,
            "rollNumber" : 111.0,
            "name" : "Ironman2",
            "branch" : 2.0
        }
    ]
}

    aggregate([
   {$lookup: 
     { from: "students", 
       localField: "_id", 
       foreignField: "branch", as: "studentData"
     }
   }, { $addFields: {studentCount: {$size: "$studentData"}}
}
])
    获取每个系的学生数据计数

    db.getCollection('student').aggregate(
    [
        {
        $lookup:
            {
                from:"department",
                localField:"branch",
                foreignField:"_id",
                as:"branch"
            }
        },
        {
            $group:
            {
                _id:"$branch",
                "numOfStudent":{$sum:1},
                "listOfStudents":{$push:"$name"}
            }
        }
    ]
)
    这将为您提供如下信息：

    {
    "_id" : 1.0,
    "rollNumber" : 110.0,
    "name" : "Thor",
    "branch" : [ 
        {
            "_id" : 1.0,
            "name" : "CSE",
            "hod" : "abc"
        }
    ]
}

    {
    "_id" : [ 
        {
            "_id" : 2.0,
            "name" : "IT",
            "hod" : "xyz"
        }
    ],
    "numOfStudent" : 1.0,
    "listOfStudents" : [ 
        "Ironman2"
    ]
}
{
    "_id" : [ 
        {
            "_id" : 1.0,
            "name" : "CSE",
            "hod" : "abc"
        }
    ],
    "numOfStudent" : 3.0,
    "listOfStudents" : [ 
        "Thor", 
        "Ironman", 
        "Ironman3"
    ]
}

    {
    "name" : "CSE",
    "hod" : "abc",
    "numOfStudents" : 2,
    "students" : [ 
        {
            "_id" : 1.0,
            "rollNumber" : 110.0,
            "name" : "Thor",
            "branch" : 1.0
        }, 
        {
            "_id" : 3.0,
            "rollNumber" : 111.0,
            "name" : "Ironman2",
            "branch" : 2.0
        }
    ]
}

    aggregate([
   {$lookup: 
     { from: "students", 
       localField: "_id", 
       foreignField: "branch", as: "studentData"
     }
   }, { $addFields: {studentCount: {$size: "$studentData"}}
}
])
    如果要存储学生的id而不是姓名，可以将
    $push:$name
    更改为
    $push:$\u id

    编辑 使用“部门”集合获得类似结果：

    db.department.aggregate([
{
    $lookup:
                {
                    from:"student",
                    localField:"students",
                    foreignField:"_id",
                    as:"studentsDetails"
                }
},
{
    $project:{
            _id:0,
            name:"$name",
            hod:"$hod",
            numOfStudents:{$size:"$studentsDetails"},
            students:"$studentsDetails"
        }
}
])
    这将为您提供如下信息：

    {
    "_id" : 1.0,
    "rollNumber" : 110.0,
    "name" : "Thor",
    "branch" : [ 
        {
            "_id" : 1.0,
            "name" : "CSE",
            "hod" : "abc"
        }
    ]
}

    {
    "_id" : [ 
        {
            "_id" : 2.0,
            "name" : "IT",
            "hod" : "xyz"
        }
    ],
    "numOfStudent" : 1.0,
    "listOfStudents" : [ 
        "Ironman2"
    ]
}
{
    "_id" : [ 
        {
            "_id" : 1.0,
            "name" : "CSE",
            "hod" : "abc"
        }
    ],
    "numOfStudent" : 3.0,
    "listOfStudents" : [ 
        "Thor", 
        "Ironman", 
        "Ironman3"
    ]
}

    {
    "name" : "CSE",
    "hod" : "abc",
    "numOfStudents" : 2,
    "students" : [ 
        {
            "_id" : 1.0,
            "rollNumber" : 110.0,
            "name" : "Thor",
            "branch" : 1.0
        }, 
        {
            "_id" : 3.0,
            "rollNumber" : 111.0,
            "name" : "Ironman2",
            "branch" : 2.0
        }
    ]
}

    aggregate([
   {$lookup: 
     { from: "students", 
       localField: "_id", 
       foreignField: "branch", as: "studentData"
     }
   }, { $addFields: {studentCount: {$size: "$studentData"}}
}
])
    您的$lookup完全正常。要将学生数添加到输出数据中，可以使用$addFields

    将此添加到聚合中：

    { $addFields: {studentCount: {$size: "$studentData"}}}
    }

    因此，您的查询应该是这样的：

    {
    "_id" : 1.0,
    "rollNumber" : 110.0,
    "name" : "Thor",
    "branch" : [ 
        {
            "_id" : 1.0,
            "name" : "CSE",
            "hod" : "abc"
        }
    ]
}

    {
    "_id" : [ 
        {
            "_id" : 2.0,
            "name" : "IT",
            "hod" : "xyz"
        }
    ],
    "numOfStudent" : 1.0,
    "listOfStudents" : [ 
        "Ironman2"
    ]
}
{
    "_id" : [ 
        {
            "_id" : 1.0,
            "name" : "CSE",
            "hod" : "abc"
        }
    ],
    "numOfStudent" : 3.0,
    "listOfStudents" : [ 
        "Thor", 
        "Ironman", 
        "Ironman3"
    ]
}

    {
    "name" : "CSE",
    "hod" : "abc",
    "numOfStudents" : 2,
    "students" : [ 
        {
            "_id" : 1.0,
            "rollNumber" : 110.0,
            "name" : "Thor",
            "branch" : 1.0
        }, 
        {
            "_id" : 3.0,
            "rollNumber" : 111.0,
            "name" : "Ironman2",
            "branch" : 2.0
        }
    ]
}

    aggregate([
   {$lookup: 
     { from: "students", 
       localField: "_id", 
       foreignField: "branch", as: "studentData"
     }
   }, { $addFields: {studentCount: {$size: "$studentData"}}
}
])
    你到底想在哪里打印数据？你正在打印完整的学生数据，它们只是以数组的形式。你到底想要什么？@AlexP。我希望它不是“[对象]”，而是可读的形式。您的ans正在给出正确的结果。当从“系”集合而不是“学生”集合进行查询时，是否有任何方法可以获取此reuslt？如果在系集合中存储学生ID列表，则可以完成此操作。我尝试了，但无法生成答案。已将学生ID保存在“部门集合”中的数组中。如何继续？你能用新的“部门收集”更新你的问题吗？我已经更新了答案。如果这就是你想要的，请将答案标记为正确！谢谢：）