Understanding $sort: Ordering Documents

The $sort aggregation pipeline stage in MongoDB is used to order the documents in a pipeline based on one or more fields. It allows you to specify the sorting order for each field, either in ascending (1) or descending (-1) order. This is crucial for presenting data in a meaningful way to users or for subsequent processing steps that require ordered data. Without $sort, the order of documents returned from a MongoDB query is generally undefined.

The $sort stage does *not* change the document structure, only the order in which the documents are returned.

Using $sort to Order Documents Based on Single or Multiple Fields

The $sort stage takes a document as an argument. Each field in this document specifies a field from the input documents to sort by, and the value indicates the sort order.

Sorting by a Single Field

To sort by a single field, specify the field name and the sort order (1 for ascending, -1 for descending).

Example: Sort documents by age in ascending order.

 db.collection.aggregate([
  {
    $sort: {
      age: 1
    }
  }
]) 

This pipeline stage will sort the documents in the collection collection based on the age field in ascending order.

Example: Sort documents by date in descending order (most recent first).

 db.collection.aggregate([
  {
    $sort: {
      date: -1
    }
  }
]) 

This sorts the documents based on the date field, with the most recent dates appearing first.

Sorting by Multiple Fields

You can sort by multiple fields by including multiple fields in the $sort document. The documents will be sorted by the first field specified, and then within each group of documents with the same value for the first field, they will be sorted by the second field, and so on.

Example: Sort documents first by city in ascending order, and then by age in descending order within each city.

 db.collection.aggregate([
  {
    $sort: {
      city: 1,
      age: -1
    }
  }
]) 

This pipeline stage will first sort the documents alphabetically by city. Then, within each city, the documents will be sorted by age, with the oldest individuals appearing first.

Important Considerations

• Index Usage: MongoDB can use indexes to improve the performance of $sort operations, especially when sorting on fields that are indexed. Consider creating indexes on fields you frequently sort on.
• Memory Limits: For large collections, $sort might require significant memory. If the data to be sorted exceeds the memory limit (100MB by default), MongoDB will write temporary files to disk, which can impact performance. The allowDiskUse: true option can be added to the aggregate command to allow writing to disk. However, using indexes is generally preferable to avoid exceeding memory limits.
• Collation: When sorting strings, be aware of collation. Collation allows you to specify language-specific rules for sorting and comparing strings. If you need to sort strings in a specific language order, use collation.