MongoDB: Performing actions on a field of a collection - mongodb

Question

I am very new to MongoDB (only got started out of interest last week). I am trying to figure out something and I'm not entirely sure of the terminology to go about searching for it. So I decided to make a SO question.
I created a collection called Students. Students has the fields id, name, undergrad(which is a boolean), classes(which is an array) and units(which is 3 times the number of classes the student has).
Now, I wanted to see how I could perform actions on a particular field of Students. What I did was, inserted a couple documents and purposefully did not include the units field. And I wanted to $set the units field forEach document/student that did not have the field. I did the following:
var studentDoc = db.students.find({units: {$exists:false}})
studentDoc.forEach(function(stu){
db.student.update({_id:stu._id}, {$set:{units:{$size:"$classes"}}})
}
)
Question 1: Is what I've done even remotely correct?
Question 2: When I type studentDoc after setting the var studentDoc, it doesn't print anything. But when I write
var studentDoc = db.students.find({units:{$exists:false}}).toArray()
it prints studentDoc as an array but still doesn't seem to do anything in the forEach loop.
Question 3: How do I $set the units field as 3 * (size of classes array)
I hope I have been clear in my question. I have tried searching on the MongoDB docs and google, but haven't had any luck (probably because of my lack of knowledge to search for the correct things).
Any help would be great! You can even point me in the right direction, and that'll be great!
Thank you in advance for all your help!!

Answer 1

I'm not sure why foreach not getting in. You can do the same with below working code.
for(i=0;i<studentDoc.length();i++){
var stud_id = studentDoc[i]._id;
var doc = db.students.findOne({"_id":stud_id})["classes"];
if(doc){
var len = doc.length;
db.students.update({"_id":stud_id},{$set:{"units":len*3}})
}
}
As far as I know, You can't use $size:"$" queries in update statement. you will get error like below:
The dollar ($) prefixed field '$size' in 'units.$size' is not valid for storage.

Answer 2

The return value of db.collection.find() is cursor.
In the mongo shell, if the returned cursor is not assigned to a variable using the var keyword, the cursor is automatically iterated up to 20 times to access up to the first 20 documents that match the query. To iterate manually, assign the returned cursor to a variable using the var keyword. So
var studentDoc = db.students.find({units: {$exists:false}})
You should iterator studentDoc manually.
Whereas, the cursor.toArray() returns an array that contains all the documents from a cursor. The method iterates completely the cursor, loading all the documents into RAM and exhausting the cursor. Thus
var studentDoc = db.students.find({units:{$exists:false}}).toArray()
it prints studentDoc as an array.
If you want to use forEach, here is cursor.forEach().
db.students.find({units:{$exists:false}}).forEach()

Answer 3

var studentDoc = db.students.find({units: {$exists:false}})
here studentDoc is a cursor, it's not printable.
you can use forEach
studentDoc.forEach(printjson);
or iterator the cursor
while (studentDoc.hasNext()) {
printjson(studentDoc.next());
}