How to Manually Iterate a Cursor in MongoDB

There are different ways to manually iterate a cursor in MongoDB. Users can manually iterate a cursor in MongoDB by:

Assigning Cursor to a Var Keyword

In MongoDB, the find() method returns the cursor, now to access the document we need to iterate the cursor.

To iterate a cursor manually, simply assign the cursor return by the find() method to the var keyword Or JavaScript variable. 

Note: In the Mongo shell, if the cursor is not assigned to a var keyword then the Mongo shell automatically iterates the cursor up to 20 documents. If a cursor is inactive for 10 min then the MongoDB server will automatically close that cursor.

Syntax:

var name = db.collection_name.find()
name

Example :

var mycursor = db.student.find({studentId:3}).pretty()
mycursor

Here, we iterate the cursor manually to find the document whose studentId is 3. So, we assigned the cursor returned by the find() method to the JavaScript variable(i.e. mycursor).

Using the next() Method

We can also use the next() cursor method to access the next document. Let us discuss with the help of an example:

Example:

var mycursor = db.student.find({studentId:{$gt:1}});
> while(mycursor.hasNext()){
... print(tojson(mycursor.next()));
... }

In this example, studentId 2 and 3 documents are displayed because, in the first line, we exclusively took the cursor to start with the studentId > 1. So it skipped 1st document and retrieved the remaining documents. Here, the print(tojson()) method is used to display the result. You can also use printjson() method to display the result.

Using the forEach() method

We can also use the forEach() method to iterate the cursor. This function applies a JavaScript function to each document from the cursor.

Syntax:

db.collection.find().forEach(<function>)

Example:

var mycursor = db.student.find({studentId:3}).pretty()
mycursor.forEach(printjson)

Here, first we store the cursor returned by the find() method(i.e., studentId:3) in the mycursor variable. Now, we use the forEach() method to iterate the cursor and display the resultant document using printjson.

Using Iterator Index

In Mongo shell, you are allowed to iterate the cursor and display the resultant document in the array using the toArray() method.

Syntax:

cursor.toArray()

Example:

var mycursor = db.student.find().pretty()
var docs = mycursor.toArray()
var resultdoc = docs[0]
resultdoc

Here, first, we assign the returned cursor to the var keyword(i.e. mycursor), in next we create an array from the resultant cursor using the toArray() method and assign the result to the var keyword(i.e. docs). Now we access the documents according to their index e.g. var resultdoc = docs[0], here we display a document whose index is 0.

Alternate Method:

You can also this method to access a document using index on the cursor.  

var mycursor = db.student.find().pretty()
var resultdoc = mycursor[0]
resultdoc

The MongoDB cursor is a pointer that references the documents of the collection returned by the find() method.

The cursor is used to access the documents. By default, the cursor iterates automatically, but can also be iterated manually by the user.