mongodb - slice 元素的查找`from`集合上的match字段不起作用

Question

我使用gogo作为mongodb的后端使用golang
我的mongodb集合是

employee
{
  _id:ObjectId(),
  "emp_name":"qwert",
  "emp_id":111,
  "emp_dept":"XYZ"
  "qualification":"PHD",
   "employee_status":"working"
}
{
_id:ObjectId(),
  "emp_name":"asdfg",
  "emp_id":121,
  "emp_dept":"XYZ"
 "qualification":"MBA"
 "employee_status":"working"
}
department{
_id:ObjectId(),
"dept_id":11,
"dept_name":"XYZ",
"description":"decs",
"department_status":"active"
}

我的Go代码是

 type DepartmentInfo struct {
    DepartmentID    int                `json:"dept_id" bson:"dept_id"`
    DepartmentName  string             `json:"dept_name" bson:"dept_name"` 
    Description     string             `json:"description" bson:"description"` 
    EmployeeList    []EmployeeInfo     `json:"employee_list" bson:"employee_list"`
}
 type EmployeeInfo struct {
    EmployeeID    int                `json:"emp_id" bson:"emp_id"`
    EmployeeName  string             `json:"emp_name" bson:"emp_name"` 
    Qualification string             `json:"qualification" bson:"qualification"` 
    Dept_Name     string             `json:"emp_dept" bson:"emp_dept"` 
    Address       string             `json:"address" bson:"address"`
}

collection := session.DB("db").C("department")
pipeline := collection.Pipe([]bson.M{
        {"$match": bson.M{ "dept_id": 104, }},
        {"$lookup": bson.M{
        "from":         "employee",
        "localField":   "dept_name",
        "foreignField": "emp_dept",
        "as":           "employee_list",
    }},
      {"$match": bson.M{ "employee_list.qualification": "PHD", }},  
    })
var departmentInfo Department
err = pipeline.One(&departmentInfo)

它会返回部门信息以及所有员工的名单，而不匹配其资格。那么如何对集合中 slice返回的 lookup应用匹配运算符？

Answer 1

您可以将$filter用作$project或$addFields阶段的一部分，而不是在最后一个阶段使用$ match。
因此，您的管道如下所示:

[{
    $match: {
        dept_id: '11'
    }
}, {
    $lookup: {
        from: 'employee',
        localField: 'dept_name',
        foreignField: 'emp_dept',
        as: 'employee_list'
    }
}, {
    $project: {
        dept_id: 1,
        dept_name: 1,
        description: 1,
        employeesWithPHD: {
            $filter: {
                input: '$employee_list',
                as: 'employee',
                cond: {
                    $and: [{
                            $eq: [
                                '$$employee.qualification',
                                'PHD'
                            ]
                        },
                        {
                            $eq: [
                                '$$employee.employee_status',
                                'working'
                            ]
                        }
                    ]
                }
            }
        }
    }
}]