DATAMONGO-861 - Add support for $cond and $ifNull operators in aggregation operations.

We now support $cond and $ifNull operators for projection and grouping operations. ConditionalOperator and IfNullOperators are AggregationExpressions that can be applied to transform or generate values during aggregation.

TypedAggregation<InventoryItem> agg = newAggregation(InventoryItem.class,
  project().and("discount")
    .transform(ConditionalOperator.newBuilder().when(Criteria.where("qty").gte(250))
      .then(30)
      .otherwise(20))
    .and(ifNull("description", "Unspecified")).as("description")
);

corresponds to

{ "$project": { "discount": { "$cond": { "if": { "$gte": [ "$qty", 250 ] },
        "then": 30, "else": 20 } },
    "description": { "$ifNull": [ "$description", "Unspecified"] }
  }
}

Original Pull Request: #385

src/main/asciidoc/new-features.adoc

@@ -4,6 +4,7 @@
 [[new-features.1-10-0]]
 == What's new in Spring Data MongoDB 1.10
 * Support for `$min`, `$max` and `$slice` operators via `Update`.
+* Support for `$cond` and `$ifNull` operators via `Aggregation`.
 
 [[new-features.1-9-0]]
 == What's new in Spring Data MongoDB 1.9

src/main/asciidoc/reference/mongodb.adoc

@@ -1688,6 +1688,9 @@ At the time of this writing we provide support for the following Aggregation Ope
 | Array Aggregation Operators
 | size, slice
 
+| Conditional Aggregation Operators
+| cond, ifNull
+
 |===
 
 Note that the aggregation operations not listed here are currently not supported by Spring Data MongoDB. Comparison aggregation operators are expressed as `Criteria` expressions.
@@ -1985,6 +1988,50 @@ List<DBObject> resultList = result.getMappedResults();
 
 Note that we can also refer to other fields of the document within the SpEL expression.
 
+[[mongo.aggregation.examples.example6]]
+.Aggregation Framework Example 6
+
+This example uses conditional projection. It's derived from the https://docs.mongodb.com/manual/reference/operator/aggregation/cond/[$cond reference documentation].
+
+[source,java]
+----
+public class InventoryItem {
+
+  @Id int id;
+  String item;
+  String description;
+  int qty;
+}
+
+public class InventoryItemProjection {
+
+  @Id int id;
+  String item;
+  String description;
+  int qty;
+  int discount
+}
+----
+
+[source,java]
+----
+import static org.springframework.data.mongodb.core.aggregation.Aggregation.*;
+
+TypedAggregation<InventoryItem> agg = newAggregation(InventoryItem.class,
+  project("item").and("discount")
+    .transform(ConditionalOperator.newBuilder().when(Criteria.where("qty").gte(250))
+      .then(30)
+      .otherwise(20))
+    .and(ifNull("description", "Unspecified")).as("description")
+);
+
+AggregationResults<InventoryItemProjection> result = mongoTemplate.aggregate(agg, "inventory", InventoryItemProjection.class);
+List<InventoryItemProjection> stateStatsList = result.getMappedResults();
+----
+
+* This one-step aggregation uses a projection operation with the `inventory` collection. We project the `discount` field using a conditional transformation for all inventory items that have a `qty` greater or equal to `250`. An second conditional projection is performed for the `description` field. We apply the description `Unspecified` to all items that either do not have a `description` field of items that have a `null` description.
+
+
 [[mongo.custom-converters]]
 == Overriding default mapping with custom converters