Introduction to MongoDB $size Operator

In MongoDB, the $size operator can be used to get the number of elements in an array field. This operator takes the array field as input and returns the number of elements in the array.

Syntax

The syntax of the $size operator is as follows:

{ $size: <array> }

Here, <array> is the array field whose length is to be calculated.

Use Cases

The $size operator can be used in aggregation pipelines to calculate the number of elements in an array field. A common use case is to return only documents containing a specific number of elements when filtering query results.

Example

The following example shows how to use the $size operator to find documents in an array field with a specific number of elements.

Assume we have a collection of orders as follows:

{ "_id" : 1, "customer" : "A", "items" : [ "apple", "banana", "orange" ] }
{ "_id" : 2, "customer" : "B", "items" : [ "banana", "orange" ] }
{ "_id" : 3, "customer" : "C", "items" : [ "apple", "orange" ] }
{ "_id" : 4, "customer" : "D", "items" : [ "banana", "orange" ] }
{ "_id" : 5, "customer" : "E", "items" : [ "apple", "banana" ] }

Now, suppose we want to query for documents that contain two elements in the items array. We can use the following aggregation pipeline:

db.orders.aggregate([
  {
    $project: {
      customer: 1,
      numItems: { $size: "$items" }
    }
  },
  {
    $match: {
      numItems: 2
    }
  }
])

In the above pipeline, the $project stage uses the $size operator to calculate the number of elements in the items array field and adds it as the numItems field to the output document. The $match stage filters out documents that do not have two elements in the items array field. After running the above pipeline, we get the following results:

{ "_id" : 1, "customer" : "A", "numItems" : 3 }
{ "_id" : 5, "customer" : "E", "numItems" : 2 }

This indicates that order 1 has 3 elements, which does not meet our requirement, while order 5 has two elements, which is the document we were looking for.

Conclusion

The $size operator can be a very useful tool for calculating the number of elements in an array field and for filtering operations in aggregation pipelines where such calculations are required.