MongoDB - sort() Method

The sort() method specifies the order of documents returned from a query. It’s applied to the cursor before fetching results and takes a document of field: value pairs, where 1 means ascending and -1 means descending order. MongoDB generally performs:

• Stable Sort: If the sort operation returns the same results each time it is performed on the same dataset, it is considered a stable sort.
• Unstable Sort: If the results vary each time the sort is performed on the same dataset, it is considered an unstable sort.

Syntax:

db.Collection_Name.sort({field_name:1 or -1})

In the above syntax:

• field_name: The field you want to sort by (e.g., age, name, date).
• 1 for ascending order or -1 for descending order.

Behavior of Sort method in MongoDB

In MongoDB, the sort() method is used to sort the results of a query in either ascending or descending order based on one or more fields. The sort() method takes an object as an argument, where each field to sort by is a key and the value is either 1 for ascending order or -1 for descending order.

Query:

db.collection.find().sort({ field1: 1, field2: -1 })

Explanation:

• The find() method retrieves documents from the collection.
• The sort() method sorts the results by field1 in ascending order and by field2 in descending order.
• If sort() is not used, MongoDB returns documents in the order they are stored (usually insertion order).

Examples of MongoDB sort()

To understand sort method in MongoDB we need a collection called students on which we will perform various operations and queries. 

[
  {
    _id: ObjectId('600f1abb923681e7681ebdce'),
    name: 'Akshay',
    age: 19
  },
  { _id: ObjectId('600f1abb923681e7681ebdcf'), name: 'Bablu', age: 18 },
  {
    _id: ObjectId('600f1abb923681e7681ebdd0'),
    name: 'Rakesh',
    age: 21
  },
  {
    _id: ObjectId('600f1abb923681e7681ebdd1'),
    name: 'Gourav',
    age: 20
  }
]

Example 1: Sorting by Age in Ascending Order

db.student.find().sort({age:1})

Output:

[
  { _id: ObjectId('600f1abb923681e7681ebdcf'), name: 'Bablu', age: 18 },
  {
    _id: ObjectId('600f1abb923681e7681ebdce'),
    name: 'Akshay',
    age: 19
  },
  {
    _id: ObjectId('600f1abb923681e7681ebdd1'),
    name: 'Gourav',
    age: 20
  },
  {
    _id: ObjectId('600f1abb923681e7681ebdd0'),
    name: 'Rakesh',
    age: 21
  }
]

Explanation:

• This query sorts the documents by the age field in ascending order.
• As a result, the youngest student ("Bablu", age 18) comes first, followed by others in increasing order of age.

Example 2: Sorting by Age in Descending Order

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

Output:

[
  {
    _id: ObjectId('600f1abb923681e7681ebdd0'),
    name: 'Rakesh',
    age: 21
  },
  {
    _id: ObjectId('600f1abb923681e7681ebdd1'),
    name: 'Gourav',
    age: 20
  },
  {
    _id: ObjectId('600f1abb923681e7681ebdce'),
    name: 'Akshay',
    age: 19
  },
  { _id: ObjectId('600f1abb923681e7681ebdcf'), name: 'Bablu', age: 18 }
]

Explanation:

• Here, the documents are sorted by age in descending order.
• The oldest student ("Rakesh", age 21) comes first, followed by the others in decreasing order of age.

Example 3: Sorting by Name in Ascending Order.

db.student.find().sort({name:1})

Output:

[
  {
    _id: ObjectId('600f1abb923681e7681ebdce'),
    name: 'Akshay',
    age: 19
  },
  { _id: ObjectId('600f1abb923681e7681ebdcf'), name: 'Bablu', age: 18 },
  {
    _id: ObjectId('600f1abb923681e7681ebdd1'),
    name: 'Gourav',
    age: 20
  },
  {
    _id: ObjectId('600f1abb923681e7681ebdd0'),
    name: 'Rakesh',
    age: 21
  }
]

Explanation:

• The documents are sorted alphabetically by name in ascending order.
• The sorting is case-sensitive, so it follows the lexicographical order, with "Akshay" first, followed by "Bablu", "Gourav", and "Rakesh".

Example 4: Descending order of student name::

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

Output:

[
  {
    _id: ObjectId('600f1abb923681e7681ebdd0'),
    name: 'Rakesh',
    age: 21
  },
  {
    _id: ObjectId('600f1abb923681e7681ebdd1'),
    name: 'Gourav',
    age: 20
  },
  { _id: ObjectId('600f1abb923681e7681ebdcf'), name: 'Bablu', age: 18 },
  {
    _id: ObjectId('600f1abb923681e7681ebdce'),
    name: 'Akshay',
    age: 19
  }
]

Explanation:

• The documents are now sorted in descending order of name.
• "Rakesh" appears first, followed by "Gourav", "Bablu", and "Akshay".

Performance Considerations

Sorting operations can be slow on large datasets so it’s important to optimize them for better performance:

• Indexes: Create indexes on the fields that we frequently sort by. MongoDB can use indexes to improve the sorting performance.
• allowDiskUse: When sorting large result sets, enable allowDiskUse to perform the sort operation on disk rather than in memory.
• Limit Results: Use the limit() method in combination with sort() to reduce the number of documents processed.