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Iterate a Cursor in mongosh

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  • Manually Iterate the Cursor
  • Iterator Index
  • Cursor Behaviors
  • Cursor Information

The db.collection.find() method returns a cursor. To access the documents, you need to iterate the cursor. However, in mongosh, if the returned cursor is not assigned to a variable using the var keyword, then the cursor is automatically iterated up to 20 times [1] to print up to the first 20 documents in the results.

The following examples describe ways to manually iterate the cursor to access the documents or to use the iterator index.

Manually Iterate the Cursor

In mongosh, when you assign the cursor returned from the find() method to a variable using the var keyword, the cursor does not automatically iterate.

You can call the cursor variable in the shell to iterate up to 20 times [1] and print the matching documents, as in the following example:

var myCursor = db.users.find( { type: 2 } );
myCursor

You can also use the cursor method next() to access the documents, as in the following example:

var myCursor = db.users.find( { type: 2 } );
while (myCursor.hasNext()) {
   print(tojson(myCursor.next()));
}

As an alternative print operation, consider the printjson() helper method to replace print(tojson()):

var myCursor = db.users.find( { type: 2 } );
while (myCursor.hasNext()) {
   printjson(myCursor.next());
}

You can use the cursor method forEach() to iterate the cursor and access the documents, as in the following example:

var myCursor =  db.users.find( { type: 2 } );
myCursor.forEach(printjson);

See JavaScript cursor methods and your driver documentation for more information on cursor methods.

[1](1, 2) You can set the DBQuery.shellBatchSize attribute to change the number of documents from the default value of 20.

Iterator Index

In mongosh, you can use the toArray() method to iterate the cursor and return the documents in an array, as in the following:

var myCursor = db.inventory.find( { type: 2 } );
var documentArray = myCursor.toArray();
var myDocument = documentArray[3];

The toArray() method loads into RAM all documents returned by the cursor; the toArray() method exhausts the cursor.

Additionally, some Drivers provide access to the documents by using an index on the cursor (i.e. cursor[index]). This is a shortcut for first calling the toArray() method and then using an index on the resulting array.

Consider the following example:

var myCursor = db.users.find( { type: 2 } );
var myDocument = myCursor[1];

The myCursor[1] is equivalent to the following example:

myCursor.toArray() [1];

Cursor Behaviors

Cursors Opened Within a Session

Starting in MongoDB 5.0, cursors created within a client session close when the corresponding server session ends with the killSessions command, if the session times out, or if the client has exhausted the cursor.

By default, server sessions have an expiration timeout of 30 minutes. To change the value, set the localLogicalSessionTimeoutMinutes parameter when starting up mongod.

Cursors Opened Outside of a Session

Cursors that aren't opened under a session automatically close after 10 minutes of inactivity, or if client has exhausted the cursor. To override this behavior in mongosh, you can use the cursor.noCursorTimeout() method:

var myCursor = db.users.find().noCursorTimeout();

After setting the noCursorTimeout option, you must either close the cursor manually with cursor.close() or by exhausting the cursor's results.

See your driver documentation for information on setting the noCursorTimeout option.

Cursor Isolation

As a cursor returns documents, other operations may interleave with the query.

Cursor Batches

The MongoDB server returns the query results in batches. The amount of data in the batch will not exceed the maximum BSON document size. To override the default size of the batch, see batchSize() and limit().

Operations of type find(), aggregate(), listIndexes, and listCollections return a maximum of 16 megabytes per batch. batchSize() can enforce a smaller limit, but not a larger one.

find() and aggregate() operations have an initial batch size of 101 documents by default. Subsequent getMore operations issued against the resulting cursor have no default batch size, so they are limited only by the 16 megabyte message size.

For queries that include a sort operation without an index, the server must load all the documents in memory to perform the sort before returning any results.

As you iterate through the cursor and reach the end of the returned batch, if there are more results, cursor.next() will perform a getMore operation to retrieve the next batch. To see how many documents remain in the batch as you iterate the cursor, you can use the objsLeftInBatch() method, as in the following example:

var myCursor = db.inventory.find();
var myFirstDocument = myCursor.hasNext() ? myCursor.next() : null;
myCursor.objsLeftInBatch();

Cursor Results for Non-Existent mongos Databases

Starting in MongoDB 7.2, aggregation pipeline queries that attempt to use non-existent databases on mongos deployments return validation errors.

In previous versions, these aggregation queries return empty cursors.

Cursor Information

The db.serverStatus() method returns a document that includes a metrics field. The metrics field contains a metrics.cursor field with the following information:

  • number of timed out cursors since the last server restart

  • number of open cursors with the option DBQuery.Option.noTimeout set to prevent timeout after a period of inactivity

  • number of "pinned" open cursors

  • total number of open cursors

Consider the following example which calls the db.serverStatus() method and accesses the metrics field from the results and then the cursor field from the metrics field:

db.serverStatus().metrics.cursor

The result is the following document:

{
   "timedOut" : <number>
   "open" : {
      "noTimeout" : <number>,
      "pinned" : <number>,
      "total" : <number>
   }
}

Tip

See also:

db.serverStatus()

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