DATAMONGO-1457 - Add support for $slice in aggregation.

We now support $slice in aggregation projections via the ProjectionOperationBuilder.

    Aggregation.project().and("field").slice(10, 20)

Original pull request: #372.

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013-2015 the original author or authors.
+ * Copyright 2013-2016 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.
@@ -352,6 +352,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 	 * 
 	 * @author Oliver Gierke
 	 * @author Thomas Darimont
+	 * @author Christoph Strobl
 	 */
 	public static class ProjectionOperationBuilder extends AbstractProjectionOperationBuilder {
 
@@ -566,6 +567,31 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 			return project("size");
 		}
 
+		/**
+		 * Generates a {@code $slice} expression that returns a subset of the array held by the given field. <br />
+		 * If {@literal n} is positive, $slice returns up to the first n elements in the array. <br />
+		 * If {@literal n} is negative, $slice returns up to the last n elements in the array.
+		 *
+		 * @param count max number of elements.
+		 * @return never {@literal null}.
+		 * @since 1.10
+		 */
+		public ProjectionOperationBuilder slice(int count) {
+			return project("slice", count);
+		}
+
+		/**
+		 * Generates a {@code $slice} expression that returns a subset of the array held by the given field. <br />
+		 *
+		 * @param count max number of elements. Must not be negative.
+		 * @param offset the offset within the array to start from.
+		 * @return never {@literal null}.
+		 * @since 1.10
+		 */
+		public ProjectionOperationBuilder slice(int count, int offset) {
+			return project("slice", offset, count);
+		}
+
 		/* 
 		 * (non-Javadoc)
 		 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#toDBObject(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)

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

@@ -1228,6 +1228,27 @@ public class AggregationTests {
 				skip(100));
 	}
 
+	/**
+	 * @see DATAMONGO-1457
+	 */
+	@Test
+	public void sliceShouldBeAppliedCorrectly() {
+
+		assumeTrue(mongoVersion.isGreaterThanOrEqualTo(THREE_DOT_TWO));
+
+		createUserWithLikesDocuments();
+
+		TypedAggregation<UserWithLikes> agg = newAggregation(UserWithLikes.class, match(new Criteria()),
+				project().and("likes").slice(2));
+
+		AggregationResults<UserWithLikes> result = mongoTemplate.aggregate(agg, UserWithLikes.class);
+
+		assertThat(result.getMappedResults(), hasSize(9));
+		for (UserWithLikes user : result) {
+			assertThat(user.likes.size() <= 2, is(true));
+		}
+	}
+
 	private void createUsersWithReferencedPersons() {
 
 		mongoTemplate.dropCollection(User.class);

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013-2015 the original author or authors.
+ * Copyright 2013-2016 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.
@@ -36,6 +36,7 @@ import com.mongodb.DBObject;
  * 
  * @author Oliver Gierke
  * @author Thomas Darimont
+ * @author Christoph Strobl
  */
 public class ProjectionOperationUnitTests {
 
@@ -371,6 +372,36 @@ public class ProjectionOperationUnitTests {
 		assertThat(projected.get("tags_count"), is((Object) new BasicDBObject("$size", Arrays.asList("$tags"))));
 	}
 
+	/**
+	 * @see DATAMONGO-1457
+	 */
+	@Test
+	public void shouldRenderSliceCorrectly() throws Exception {
+
+		ProjectionOperation operation = Aggregation.project().and("field").slice(10).as("renamed");
+
+		DBObject dbObject = operation.toDBObject(Aggregation.DEFAULT_CONTEXT);
+		DBObject projected = exctractOperation("$project", dbObject);
+
+		assertThat(projected.get("renamed"),
+				is((Object) new BasicDBObject("$slice", Arrays.<Object> asList("$field", 10))));
+	}
+
+	/**
+	 * @see DATAMONGO-1457
+	 */
+	@Test
+	public void shouldRenderSliceWithPositionCorrectly() throws Exception {
+
+		ProjectionOperation operation = Aggregation.project().and("field").slice(10, 5).as("renamed");
+
+		DBObject dbObject = operation.toDBObject(Aggregation.DEFAULT_CONTEXT);
+		DBObject projected = exctractOperation("$project", dbObject);
+
+		assertThat(projected.get("renamed"),
+				is((Object) new BasicDBObject("$slice", Arrays.<Object> asList("$field", 5, 10))));
+	}
+
 	private static DBObject exctractOperation(String field, DBObject fromProjectClause) {
 		return (DBObject) fromProjectClause.get(field);
 	}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013 the original author or authors.
+ * Copyright 2013-2016 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.
@@ -20,9 +20,15 @@ import java.util.Date;
 import java.util.HashSet;
 import java.util.Set;
 
+import lombok.Data;
+import lombok.NoArgsConstructor;
+
 /**
  * @author Thomas Darimont
+ * @author Christoph Strobl
  */
+@Data
+@NoArgsConstructor
 public class UserWithLikes {
 
 	String id;
@@ -30,6 +36,7 @@ public class UserWithLikes {
 	Set<String> likes = new HashSet<String>();
 
 	public UserWithLikes(String id, Date joined, String... likes) {
+
 		this.id = id;
 		this.joined = joined;
 		this.likes = new HashSet<String>(Arrays.asList(likes));