[
/* 1 */
{
    "_id" : 1,
    "semester" : 1,
    "grades" : [
        70,
        87,
        90
    ]
}

/* 2 */
{
    "_id" : 2,
    "semester" : 1,
    "grades" : [
        90,
        88,
        92
    ]
}

/* 4 */
{
    "_id" : 5,
    "semester" : 2,
    "grades" : [
        88,
        88,
        92
    ]
}
]

[
 {
   "uniqeGrades" : [70,87,90,82,92,85,100,88]
  }
]