DATAMONGO-2023 - Polishing.

Add tests verifying the behavior when using both typed and untyped aggregation.

Original Pull Request: #585

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationUnitTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013-2017 the original author or authors.
+ * Copyright 2013-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -591,6 +591,22 @@ public class AggregationUnitTests {
 		assertThat($project.containsField("plts.ests"), is(true));
 	}
 
+	@Test // DATAMONGO-2023
+	public void mapsNativeKeywordsCorrectly() {
+
+		Aggregation agg = Aggregation.newAggregation(new AggregationOperation() {
+			@Override
+			public DBObject toDBObject(AggregationOperationContext context) {
+				return new BasicDBObject("$sample", new BasicDBObject("size", "my-custom-value"));
+			}
+		});
+
+		DBObject $sample = extractPipelineElement(agg.toDbObject("collection-1", Aggregation.DEFAULT_CONTEXT), 0,
+				"$sample");
+
+		assertThat($sample, is(equalTo(BasicDBObjectBuilder.start("size", "my-custom-value").get())));
+	}
+
 	private DBObject extractPipelineElement(DBObject agg, int index, String operation) {
 
 		List<DBObject> pipeline = (List<DBObject>) agg.get("pipeline");

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/TypeBasedAggregationOperationContextUnitTests.java

@@ -52,6 +52,7 @@ import org.springframework.data.mongodb.core.query.Criteria;
 import org.springframework.data.mongodb.test.util.BasicDbListBuilder;
 
 import com.mongodb.BasicDBObject;
+import com.mongodb.BasicDBObjectBuilder;
 import com.mongodb.DBObject;
 import com.mongodb.util.JSON;
 
@@ -367,6 +368,23 @@ public class TypeBasedAggregationOperationContextUnitTests {
 				new BasicDbListBuilder().add("$nested1.value1").add("$field2.nestedValue2").get())))));
 	}
 
+	@Test // DATAMONGO-2023
+	public void mapsNativeKeywordsForTypedAggregationCorrectly() {
+
+		AggregationOperationContext context = getContext(FooPerson.class);
+
+		DBObject agg = Aggregation.newAggregation(new AggregationOperation() {
+
+			@Override
+			public DBObject toDBObject(AggregationOperationContext context) {
+				return new BasicDBObject("$sample", new BasicDBObject("name", "foo"));
+			}
+		}).toDbObject("collection", context);
+
+		BasicDBObject $sample = (BasicDBObject) getPipelineElementFromAggregationAt(agg, 0).get("$sample");
+		assertThat($sample, is(equalTo(BasicDBObjectBuilder.start("name", "foo").get())));
+	}
+
 	@Document(collection = "person")
 	@AllArgsConstructor
 	public static class FooPerson {