MongoDB limit

Summary: in this tutorial, you’ll learn how to use the MongoDB limit() method to specify the number of documents returned by a query.

Introduction to MongoDB limit() method

The find() method may return a lot of documents to the application. Typically, the application may not need that many documents.

To limit the number of returned documents, you use the limit() method:

db.collection.find(<query>).limit(<documentCount>)

The <documentCount> is in the range of -2³¹ and 2³¹. If you specify a value for the <documentCount> that is out of this range, the behavior of the limit() is unpredictable.

If the <documentCount> is negative, the limit() returns the same number of documents as if the <documentCount> is positive. In addition, it closes the cursor after returning a single batch of documents.

If the result set does not fit into a single batch, the number of returned documents will be less than the specified limit.

If the <documentCount> is zero, then is is equivalent to setting no limit.

Note that the limit() is analogous to the LIMIT clause in SQL.

To get the predictable result set using the limit(), you need to sort the result set first before applying the method like this:

cursor
   .sort({...})
   .limit(<documentCount>)

In practice, you often use the limit() with the skip() method to paginate a collection.

The skip() method specifies from where the query should start returning the documents:

cursor.skip(<offset>)

The following shows the documents of the page pageNo with the documentCount documents per page:

db.collection.find({...}
).sort({...}
).skip(pageNo > 0 ? ( ( pageNo - 1 ) * documentCount) : 0
).limit(documentCount);

The skip(<offset>) requires the MongoDB server to scan from the beginning of the result set before starting to return the documents. When the <offset> increases, the skip() will become slower.

Note that the limit() and skip() is analogous to the LIMIT OFFSET clause in SQL.

MongoDB limit() examples

We’ll use the following products collection:

db.products.insertMany([
    { "_id" : 1, "name" : "xPhone", "price" : 799, "releaseDate" : ISODate("2011-05-14T00:00:00Z"), "spec" : { "ram" : 4, "screen" : 6.5, "cpu" : 2.66 }, "color" : [ "white", "black" ], "storage" : [ 64, 128, 256 ] },
    { "_id" : 2, "name" : "xTablet", "price" : 899, "releaseDate" : ISODate("2011-09-01T00:00:00Z"), "spec" : { "ram" : 16, "screen" : 9.5, "cpu" : 3.66 }, "color" : [ "white", "black", "purple" ], "storage" : [ 128, 256, 512 ] },
    { "_id" : 3, "name" : "SmartTablet", "price" : 899, "releaseDate" : ISODate("2015-01-14T00:00:00Z"), "spec" : { "ram" : 12, "screen" : 9.7, "cpu" : 3.66 }, "color" : [ "blue" ], "storage" : [ 16, 64, 128 ] },
    { "_id" : 4, "name" : "SmartPad", "price" : 699, "releaseDate" : ISODate("2020-05-14T00:00:00Z"), "spec" : { "ram" : 8, "screen" : 9.7, "cpu" : 1.66 }, "color" : [ "white", "orange", "gold", "gray" ], "storage" : [ 128, 256, 1024 ] },
    { "_id" : 5, "name" : "SmartPhone", "price" : 599, "releaseDate" : ISODate("2022-09-14T00:00:00Z"), "spec" : { "ram" : 4, "screen" : 9.7, "cpu" : 1.66 }, "color" : [ "white", "orange", "gold", "gray" ], "storage" : [ 128, 256 ] },
    { "_id" : 6, "name" : "xWidget", "spec" : { "ram" : 64, "screen" : 9.7, "cpu" : 3.66 }, "color" : [ "black" ], "storage" : [ 1024 ] },
    { "_id" : 7, "name" : "xReader", "price" : null, "spec" : { "ram" : 64, "screen" : 6.7, "cpu" : 3.66 }, "color" : [ "black", "white" ], "storage" : [ 128 ] }
])

1) Using MongoDB limit() to get the most expensive product

The following example uses the limit() method to get the most expensive product in the products collection. It includes the _id, name, and price fields in the returned documents:

db.products.find({}, {
    name: 1,
    price: 1
}).sort({
    price: -1
}).limit(1);

Output:

{ "_id" : 2, "name" : "xTablet", "price" : 899 }

In this example, we sort the products by prices in descending order and use limit() to select the first one.

The products collection has two products at the same price 899. The returned document depends on the order of documents stored on the disk.

To get the predictable result, the sort should be unique. For example:

db.products.find({}, {
    name: 1,
    price: 1
}).sort({
    price: -1,
    name: 1
}).limit(1);

Output:

{ "_id" : 3, "name" : "SmartTablet", "price" : 899 }

This example sorts the document by prices in descending order. And then it sorts the sorted result set by names in ascending order. The limit() returns the first document in the final result set.

2) Using MongoDB limit() and skip() to get the paginated result

Suppose you want to divide the products collection into pages, each has 2 products.

The following query uses the skip() and limit() to get the documents of the second page:

db.products.find({}, {
    name: 1,
    price: 1
}).sort({
    price: -1,
    name: 1
}).skip(2).limit(2);

Output:

{ "_id" : 1, "name" : "xPhone", "price" : 799 }
{ "_id" : 4, "name" : "SmartPad", "price" : 699 }

Summary

• Use limit() to specify the number of returned documents for a query.
• Use sort() and limit() to select top / bottom N documents.
• Use skip() and limit() to paginate the documents in a collection.