MongoDB Aggregation

Aggregation is the process toward combining things. That is, assembling those things so we can refer to them altogether. Data aggregation is often used to give statistical analysis for groups of information and to create useful summary data for business analysis. Aggregation is done on a large scale for a major software application.

When MongoDB users need to accumulate metrics from a MongoDB database, aggregation of MongoDB is the best tool for this. In MongoDB, the Aggregation process returns an average of all values seen during the group and project. It gathers values from different documents and groups them together and then performs various kinds of operations on that grouped data like sum, average, minimum, maximum, etc to return a computed result.

MongoDB provides three ways to perform aggregation -

• Aggregation pipeline
• Map-reduce function
• Single-purpose aggregation

MongoDB aggregate() Method

MongoDB provides aggregate() method to perform the aggregation.

Syntax of aggregate() Method

db.collection_name.aggregate(aggregate_operation)

It returns the collective average of all numeric values that result from applying a specified expression to each document in a group of documents. It is available in the group and project stages.

MongoDB Aggregate Example

Suppose we have a collection named product, we want to get the average product amount and average quantity.

db.product.insert([
{'item': 'iron', 'price': 120, 'quantity': 2}, 
{'item':'alloy', 'price':90, 'quantity': 1}, 
{'item':'steel', 'price':47, 'quantity': 3}, 
{'item':'alloy', 'price':70, 'quantity': 6}, 
{'item':'iron', 'price':56, 'quantity':4}
]);

The following statement returns the average product amount and average quantity.

db.product.aggregate(
[{
	"$group" : {
		"_id": "$item",
		avgAmt: { $avg: {$multiply: ["$price", "$quantity"]}},
		avgQty: { $avg: "$quantity"}
	}
}]
) 

In the above example, we have grouped documents by the field item.

Different Aggregation Expressions

These are the following a list of available aggregation expressions.

Expression	Description
$sum	Adds up the defined value from all documents in the collection.
$avg	Calculates the average of all given values from every document in the collection.
$min	Gets the minimum of all values from within a collection.
$max	Gets the maximum of all values from within a collection.
$push	Inserts the value to an array in the associated document.
$first	Gets the first document from the source documents according to the grouping.
$last	Gets the last document from the source documents according to the grouping.