how to get slice of type struct defined inside another struct using go and mongodb

Question

mongodb collections

employee
{
  _id:ObjectId(),
  "emp_name":"qwert",
  "emp_id":111,
  "emp_dept":"XYZ"
}
{
_id:ObjectId(),
  "emp_name":"asdfg",
  "emp_id":121,
  "emp_dept":"XYZ"
}
department{
_id:ObjectId(),
"dept_id":11,
"dept_name":"XYZ",
"description":"decs",
}

My Go Code is

 type Employee struct {
    EmployeeName  string             `json:"emp_name" bson:"emp_name"`
    EmployeeID    int                `json:"emp_id" bson:"emp_id"`
    EmployeeDept  String             `json:"emp_dept" bson:"emp_dept"`
    EmpDepartment Department         `json:"department" bson:"department"` 
}
 type Department struct {
    DepartmentID    int                `json:"dept_id" bson:"dept_id"`
    DepartmentName  string             `json:"dept_name" bson:"dept_name"` 
    Description     string             `json:"description" bson:"description"` 
    EmployeeList    []Employee         `json:"employee_list" bson:"employee_list"`
}

collection := session.DB("db").C("department")
pipeline := collection.Pipe([]bson.M{
        {"$match": bson.M{
        "dept_name": "xyz",
          }},
        {"$lookup": bson.M{
        "from":         "employee",
        "localField":   "dept_name",
        "foreignField": "emp_dept",
        "as":           "employee_list",
    }},
        {"$unwind": "$employee_list"},
    })

err = pipeline.One(&departmentEmployees)

It shows Department Details with Employee Slice empty

I want to get deartment info of 'XYZ' dept with list of all employees working inside that department. I have uploaded my collections and struct as Example

Answer 1

Your Department struct has a EmployeeList []Employee field, so if you plan to unmarshal the aggregation result into Department values, you have to remove the $unwind stage.

Something like this should work:

collection := session.DB("db").C("department")
pipeline := collection.Pipe([]bson.M{
    {"$match": bson.M{
        "dept_name": "xyz",
    }},
    {"$lookup": bson.M{
        "from":         "employee",
        "localField":   "dept_name",
        "foreignField": "emp_dept",
        "as":           "employee_list",
    }},
})

var departments []*Department
err = pipeline.One(&departments)

Also you have a EmployeeDept String field in Employee , I don't know what your String type is, maybe it's just string ?