Introduction to MongoDB cursor.returnKey() Method

cursor.returnKey() is a method in MongoDB that allows returning only the keys of matching documents in a query instead of the whole document itself. This can improve query performance in certain situations.

Syntax

The syntax of the cursor.returnKey() method is as follows:

db.collection.find().returnKey()

Use Cases

The primary use case for cursor.returnKey() is when you want to return only the keys of a document instead of the entire document in a query. This situation typically arises in the following two cases:

  1. In some cases, the value of the key is smaller than the size of the entire document, so returning only the key can save bandwidth and resources.
  2. If a query involves a large amount of data and needs to be sorted, returning only the keys can improve query performance.

It is important to note that returning only the keys will result in the query results lacking the contents of the document itself, so further querying or using other methods may be necessary to obtain the desired information.

Examples

Here are two examples of the cursor.returnKey() method:

Example 1

Suppose we have a collection named users where each document contains two keys, name and age. We want to find all users who are over 30 years old and return only their names.

db.users.find({ age: { $gt: 30 } }, { name: 1, _id: 0 }).returnKey()

In this example, { name: 1, _id: 0 } specifies to only return the name and not the _id, and returnKey() method indicates to return only the keys and not the document itself.

Example 2

Suppose we have a collection named orders where each document contains three keys, order_id, customer_id, and order_date. We want to find all orders within the last 30 days and sort them in descending order by order_date.

db.orders
  .find({ order_date: { $gt: ISODate("2022-02-06T00:00:00.000Z") } })
  .sort({ order_date: -1 })
  .returnKey()

In this example, the find() method uses a query to return orders within the last 30 days, the sort() method sorts them in descending order by order_date, and the returnKey() method indicates to return only the keys and not the document itself.

Conclusion

The cursor.returnKey() method can improve performance in queries where only the keys of a document are needed. However, because returning only the keys will result in the query results lacking the contents of the document itself, further querying or using other methods may be necessary to obtain the desired information.