Package-level declarations

Aggregation pipelines are powerful ways to query and update MongoDB documents. Compared to regular queries, aggregation pipelines can perform more complex operations, can compare fields from the same document together, or combine data from other sources.

Aggregation pipelines are declared as a Sequence-like chain of stages. Each stage is responsible for transforming documents in a certain way. MongoDB is able to parallelize work from different stages. To see which stages are available, see Pipeline.

users.aggregate()
	.match {
		// Similar to List.filter
		User::age gt 18
	}
	.project {
		// Select which fields we are interested in.
		// All other fields are ignored.
		include(User::age)
		include(User::name)
	}
	.toList()

Additionally, many stages allow building complex expressions based on different fields. For example, within $set and $project, we can use the $cond operator to conditionally set the value of a field based on other fields:

users.aggregate()
	.set {
		User::risk set cond(
			condition = of(User::isAdult),
			ifTrue = of(User::age) * of(5),
			iFalse = of(18) - of(User::age),
		)
	}
	.toList()

To learn more about aggregation operators and their syntax, see opensavvy.ktmongo.dsl.aggregation.ValueDsl.

You may also be interested in reading the official documentation on aggregations.