title: Taking sum/average of columns in mongoosejs
body: <p>I have a MongoDB collection "Marks" with fields student_id,Subject,Mark.
  I want to the average value of marks scored by a particular student.
  My project is in NodeJs and I am using MongooseJS for object modeling.
  How to query the this using MOngooseJS?</p>
  
  <p><strong>EXAMPLE</strong></p>
  
  <pre><code>     Student_id   Subject  Mark
       111          AAA      9
       111          BBB      5
       111          CCC      7
       222          AAA      10
       222          CCC      6
       222          BBB      8
  </code></pre>
  
  <p>I want average of marks scored by student(id) 111 i.e((9+5+7)/3)=7)</p>