DATAMONGO-1325 - Add $sample aggregation stage.

We now support the $sample aggregation pipeline stage via Aggregation to select a random subset of result documents.

TypedAggregation<Employee> agg = Aggregation.newAggregation(Employee.class,
		sample(5));

Original pull request: #452.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/Aggregation.java

@@ -51,6 +51,7 @@ import org.springframework.util.Assert;
  * @author Alessio Fachechi
  * @author Christoph Strobl
  * @author Nikolay Bogdanov
+ * @author Gustavo de Geus
  * @since 1.3
  */
 public class Aggregation {
@@ -382,6 +383,16 @@ public class Aggregation {
 		return new LimitOperation(maxElements);
 	}
 
+	/**
+	 * Creates a new {@link SampleOperation} to selects the specified number of documents from its input randomly.
+	 *
+	 * @param sampleSize must not be less than zero.
+	 * @return
+	 */
+	public static SampleOperation sample(long sampleSize) {
+		return new SampleOperation(sampleSize);
+	}
+	
 	/**
 	 * Creates a new {@link MatchOperation} using the given {@link Criteria}.
 	 *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SampleOperation.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2013-2017 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import org.bson.Document;
+import org.springframework.util.Assert;
+
+/**
+ * Encapsulates the {@code $sample}-operation.
+ * <p>
+ * We recommend to use the static factory method {@link Aggregation#sample(long)} instead of creating instances of this
+ * class directly.
+ *
+ * @author Gustavo de Geus
+ * @since 2.0
+ * @see <a href="https://docs.mongodb.com/master/reference/operator/aggregation/sample/">MongoDB Aggregation Framework: $sample</a>
+ */
+public class SampleOperation implements AggregationOperation {
+
+	private final long sampleSize;
+
+	/**
+	 * @param sampleSize number of documents to be randomly selected from its input.
+	 */
+	public SampleOperation(long sampleSize) {
+
+		Assert.isTrue(sampleSize > 0, "Sample size must be greater than zero!");
+		this.sampleSize = sampleSize;
+	}
+
+	/*
+	  (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.BucketOperationSupport#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+	 */
+	@Override
+	public Document toDocument(AggregationOperationContext context) {
+		return new Document("$sample", new Document("size", this.sampleSize));
+	}
+}

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

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2017 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import static org.hamcrest.CoreMatchers.*;
+import static org.junit.Assert.*;
+
+import org.bson.Document;
+import org.junit.Test;
+
+/**
+ * Unit tests for {@link SampleOperation}.
+ *
+ * @author Gustavo de Geus
+ */
+public class SampleOperationUnitTests {
+
+	private static final String SIZE = "size";
+	private static final String OP = "$sample";
+
+	@Test(expected = IllegalArgumentException.class)
+	public void rejectsNegativeSample() {
+		new SampleOperation(-1L);
+	}
+
+	@Test(expected = IllegalArgumentException.class)
+	public void rejectsZeroSample() {
+		new SampleOperation(0L);
+	}
+
+	@Test
+	public void rendersSampleOperation() {
+		long sampleSize = 5L;
+		SampleOperation sampleOperation = Aggregation.sample(sampleSize);
+		Document sampleOperationDocument = sampleOperation.toDocument(Aggregation.DEFAULT_CONTEXT);
+		assertNotNull(sampleOperationDocument.get(OP));
+		assertThat(sampleOperationDocument.get(OP), is(instanceOf(Document.class)));
+		Document sampleSizeDocument =  sampleOperationDocument.get(OP, Document.class);
+		assertEquals(sampleSize, sampleSizeDocument.get(SIZE));
+	}
+}