MongoDB Cursor

In MongoDB, a cursor is an object that represents the result set of a query and allows efficient access to documents returned by methods like find().

• Automatically created when a query like find() is executed.
• Allows iteration over query results, either automatically or manually.
• Retrieves documents efficiently without loading all data into memory.
• Suitable for handling large datasets.
• Provides methods to access, manipulate, and optimize query results.
• Cursors expire after 10 minutes of inactivity, so proper management is required.

Suppose we have the following setup

• Database: gfg
• Collection: student
• Documents: Three documents contain the details of the students

Here, we use the following query to display all the documents present in the student collection

db.student.find().pretty()

This find() method returns a cursor containing all documents present in the student collection.

Manually Iterating a Cursor in MongoDB

While MongoDB automatically iterates over the cursor to fetch documents, you can manually iterate it using various techniques. Here are the most common ways to iterate a cursor:

1. 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, cursors not assigned to a variable auto-display up to 20 documents, and inactive cursors are automatically closed after 10 minutes.

Syntax:

var name = db.collection_name.find()
name

Example :

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

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).

2. Using the next() Method

We can also use the next() cursor method to access the next document. We can use this method to manually iterate through the documents one by one.

Example:

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

• Only documents with studentId > 1 are retrieved.
• The first document is skipped, so studentId 2 and 3 are displayed.
• The cursor starts from the filtered result set.
• print(tojson()) is used to display the output.
• printjson() can also be used to display the result.

3. Using the forEach() Method

The forEach() method allows us to apply a JavaScript function to each document in the cursor. This is an easy way to process each document without explicitly using a loop.

Syntax:

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

Example:

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

• The cursor returned by find({ studentId: 3 }) is stored in the mycursor variable.
• forEach() is used to iterate over the cursor.
• printjson() is used to display the resulting document.

4. Using the toArray() Method

In Mongo shell, we 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()
var docs = mycursor.toArray()
var resultdoc = docs[0]
resultdoc

• The cursor returned by the query is stored in mycursor.
• The cursor is converted into an array using toArray() and stored in docs.
• Documents are accessed using array indexes.
• docs[0] retrieves the first document in the result set.

5. Alternate Method: Accessing Documents Using Index

We can also this method to access a document using index on the cursor.  
Example:

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

Commonly Used MongoDB Cursor Methods

MongoDB offers various cursor methods to enhance query functionality. Here are some of the most commonly used cursor methods:

1. count() Method

In order to get the correct documents, we need to know how many documents are present for that collection. To get that we can use the count() method which returns the total number of documents present in the given collection.

Syntax:

db.collection_name.countDocuments()

Example:

db.students.find().countDocuments()

Here, we find the total number of documents present in the student collection using the count() method.

2. limit() Method

The limit() method helps to fetch limited records from a collection. Suppose we have multiple documents, but we want to have the topmost or only 2 documents, then by using the limit() method, we can achieve that.

Syntax:

db.collection_name.find().limit(<number>)

Example:

db.student.find().limit(2).pretty()

Here, we only display the first two documents from the student collection.

3. sort() Method

When verifying documents in a database, sorted output makes analysis easier. The sort() method is used to arrange documents in ascending order by setting the field value to 1 or in descending order by setting it to -1.

Syntax:

db.collection_name.find().sort(<sort>)

Example:

db.student.find().sort({studentId:-1}).pretty()

We sort all the documents present in the student collection in descending order.

4. toArray() Method

The toArray() method converts the cursor into an array of documents, making it easy to access documents by index.

Syntax:

db.collection_name.find().toArray()

Example:

db.student.find().toArray()

5. next() Method

The next() method returns the next document from a cursor in MongoDB.

Syntax:

db.collection_name.find().next()

Example:

db.student.find().next()

Here, the next() method returns the first document from the collection.

Additional Cursor Methods in MongoDB

MongoDB provides additional advanced cursor methods for more specialized functionality: