Mongo: Group, push and sort

Question

I have a mongodb collection data as per below;I want to group by EmployeedID( i.e 0001) and then sort(by age)

{
"_id" : ObjectId("54d0512191a4da7736e9db43"),
"EmployeeID" : "0001",
"Speciality" : "xxx",
"Code" : "P",
"Age" : 8
}

/* 1 */
{
"_id" : ObjectId("54d0512191a4da7736e9db44"),
"EmployeeID" : "0002",
"Speciality" : "yyyyy",
"Code" : "P",
"Age" : 6
 }

/* 2 */
{
"_id" : ObjectId("54d0512191a4da7736e9db45"),
"EmployeeID" : "0001",
"Speciality" : "zzz",
 "Code" : "P",
 "Age" : 5
 }

I know I can group using the following way.

collection.aggregate([
{$match:{"EmployeeId":0001}},
{$group:{"_id":"$EmployeeID",
        "speciality":{$push:"$Speciality"},
        "Code":{$push:"$Code"},
        "Age":{$push:"$Age"}}}
 ])

But how can I using $sort here? SO my result can be something like below;

[{ "EmployeeID" : "0001",
"speciality" : [ "zzz","xxx"],
 "Code" :[ "P","P"],
 "Age" : [5,8]
 }]

Answer 1

You can sort the document prior to the grouping stage:

collection.aggregate([
{$sort: {_id: -1}},
{$match:{"EmployeeId":0001}},
{$group:{"_id":"$EmployeeID",
    "speciality":{$push:"$Speciality"},
    "Code":{$push:"$Code"},
    "Age":{$push:"$Age"}}}
])

Answer 2

Sorting prior to grouping may exceed mongo's memory when dealing with large collections. Fortunately, you can set allowDiskUse to true to allow mongo to write temporary files.