Calculate average of ratings in array and then include the field to original document in MongoDB?

You can use $avg operator along with aggregate framework. Let us first create a collection with documents −

> db.averageOfRatingsInArrayDemo.insertOne(
...   {
...      "StudentDetails":[
...          {
...             "StudentId":1,
...             "StudentScore":45
...         },
...         {
...            "StudentId":2,
...            "StudentScore":58
...         },
...         {
...            "StudentId":3,
...            "StudentScore":67
...         }
...      ]
...   }
... );
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cd427dc2cba06f46efe9ee4")
}

Following is the query to display all documents from a collection with the help of find() method −

> db.averageOfRatingsInArrayDemo.find().pretty();

This will produce the following output −

{
   "_id" : ObjectId("5cd427dc2cba06f46efe9ee4"),
   "StudentDetails" : [
      {
         "StudentId" : 1,
         "StudentScore" : 45
      },
      {
         "StudentId" : 2,
         "StudentScore" : 58
      },
      {
         "StudentId" : 3,
         "StudentScore" : 67
      }
   ]
}

Following is the query to calculate average of ratings in array and then include field to original document in MongoDB −

> db.averageOfRatingsInArrayDemo.aggregate([ {$addFields : {StudentScoreAverage : {$avg : "$StudentDetails.StudentScore"}}} ]);

This will produce the following output −

{ "_id" : ObjectId("5cd427dc2cba06f46efe9ee4"), "StudentDetails" : [ { "StudentId" : 1, "StudentScore" : 45 }, { "StudentId" : 2, "StudentScore" : 58 }, { "StudentId" : 3, "StudentScore" : 67 } ], "StudentScoreAverage" : 56.666666666666664 }