#523 - Add example for declarative MongoDB aggregations.

Original pull request: #526.
closes: #523

mongodb/aggregation/README.md

@@ -0,0 +1,53 @@
+# Spring Data MongoDB - Aggregations
+
+This project contains usage samples for using MongoDB [Aggregations](https://docs.mongodb.com/manual/aggregation/) 
+showing both the programmatic and declarative approach for integrating an Aggregation Pipeline into the repository lay.
+
+## Programmatic
+
+The programmatic approach uses a [custom repository](https://docs.spring.io/spring-data/mongodb/docs/current/reference/html/#repositories.custom-implementations) implementation along with the [Aggregation Framework](https://docs.spring.io/spring-data/mongodb/docs/current/reference/html/#mongo.aggregation).
+
+```java
+class OrderRepositoryImpl implements OrderRepositoryCustom {
+
+	private MongoOperations operations;
+	
+	// ...
+
+	@Override
+	public Invoice getInvoiceFor(Order order) {
+
+		AggregationResults<Invoice> results = operations.aggregate(newAggregation(Order.class, 
+			match(where("id").is(order.getId())), 
+			unwind("items"), 
+			project("id", "customerId", "items") 
+				.andExpression("'$items.price' * '$items.quantity'").as("lineTotal"), 
+			group("id") 
+				.sum("lineTotal").as("netAmount") 
+				.addToSet("items").as("items"), 
+			project("id", "items", "netAmount") 
+				.and("orderId").previousOperation() 
+				.andExpression("netAmount * [0]", taxRate).as("taxAmount") 
+				.andExpression("netAmount * (1 + [0])", taxRate).as("totalAmount") 
+		), Invoice.class);
+
+		return results.getUniqueMappedResult();
+	}
+}
+```
+
+## Declarative
+
+The [declarative approach](https://docs.spring.io/spring-data/mongodb/docs/2.2.0.RC2/reference/html/#mongodb.repositories.queries.aggregation) allows to define an Aggregation Pipeline via the `@Aggregation` annotation.
+
+```java
+public interface OrderRepository extends CrudRepository<Order, String>, OrderRepositoryCustom {
+
+	@Aggregation("{ $group : { _id : $customerId, total : { $sum : 1 } } }")
+	List<OrdersPerCustomer> totalOrdersPerCustomer(Sort sort);
+
+	@Aggregation(pipeline = { "{ $match : { customerId : ?0 } }", "{ $count : total }" })
+	Long totalOrdersForCustomer(String customerId);
+}
+```
+