Example for http://stackoverflow.com/questions/34212638/how-to-use-add-aggregate-in-mongodb/34213108?noredirect=1#comment56171294_34213108

example.js

test> db.salary.find()
{
  "_id": ObjectId("5669fee5cdeb623e44ea3f41"),
  "emp_id": 3,
  "Employee_Name": "Farquleet Shadab",
  "Employee_Gender": "Male",
  "Employee_Salary": [
    {
      "month_year": "Jan-2015",
      "salary_paid": 100
    },
    {
      "month_year": "Feb-2015",
      "salary_paid": 100
    }
  ]
}
{
  "_id": ObjectId("566a6e9a35ed09e99590b9ab"),
  "emp_id": 4,
  "Employee_Name": "John Robert",
  "Employee_Gender": "Male",
  "Employee_Salary": [
    {
      "month_year": "Jan-2015",
      "salary_paid": 200
    },
    {
      "month_year": "Feb-2015",
      "salary_paid": 200
    }
  ]
}

// aggregate:
db.salary.aggregate([
    {
        $unwind: "$Employee_Salary"
    }
    ,{
        $match:
        {
            "Employee_Salary.month_year" : "Jan-2015"
        }
    }
    ,{
        $group:
        {
            _id:null,
            total: { $sum: "$Employee_Salary.salary_paid" }
        }
    }
])

// Result:
{
  "waitedMS": NumberLong("0"),
  "result": [
    {
      "_id": null,
      "total": 300
    }
  ],
  "ok": 1
}