Add support for $maxN aggregation operator.

See #4139
Original pull request: #4182.

Author: christophstrobl

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AbstractAggregationExpression.java

@@ -19,6 +19,7 @@
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
@@ -76,11 +77,11 @@ private Object unpack(Object value, AggregationOperationContext context) {
 			return context.getReference(field).toString();
 		}
 
-		if(value instanceof Fields fields) {
+		if (value instanceof Fields fields) {
 			return fields.asList().stream().map(it -> unpack(it, context)).collect(Collectors.toList());
 		}
 
-		if(value instanceof Sort sort) {
+		if (value instanceof Sort sort) {
 
 			Document sortDoc = new Document();
 			for (Order order : sort) {
@@ -154,9 +155,40 @@ protected Map<String, Object> append(String key, Object value) {
 
 		Assert.isInstanceOf(Map.class, this.value, "Value must be a type of Map");
 
-		Map<String, Object> clone = new LinkedHashMap<>((java.util.Map) this.value);
+		return append((Map<String, Object>) this.value, key, value);
+	}
+
+	private Map<String, Object> append(Map<String, Object> existing, String key, Object value) {
+
+		Map<String, Object> clone = new LinkedHashMap<>(existing);
 		clone.put(key, value);
 		return clone;
+	}
+
+	protected Map<String, Object> appendTo(String key, Object value) {
+
+		Assert.isInstanceOf(Map.class, this.value, "Value must be a type of Map");
+
+		if (this.value instanceof Map map) {
+
+			Map<String, Object> target = new HashMap<>(map);
+			if (!target.containsKey(key)) {
+				target.put(key, value);
+				return target;
+			}
+			target.computeIfPresent(key, (k, v) -> {
+
+				if (v instanceof List<?> list) {
+					List<Object> targetList = new ArrayList<>(list);
+					targetList.add(value);
+					return targetList;
+				}
+				return Arrays.asList(v, value);
+			});
+			return target;
+		}
+		throw new IllegalStateException(
+				String.format("Cannot append value to %s type", ObjectUtils.nullSafeClassName(this.value)));
 
 	}
 
@@ -247,6 +279,10 @@ protected <T> T get(Object key) {
 		return (T) ((Map<String, Object>) this.value).get(key);
 	}
 
+	protected boolean isArgumentMap() {
+		return this.value instanceof Map;
+	}
+
 	/**
 	 * Get the argument map.
 	 *

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AccumulatorOperators.java

@@ -112,6 +112,17 @@ public Max max() {
 			return usesFieldRef() ? Max.maxOf(fieldReference) : Max.maxOf(expression);
 		}
 
+		/**
+		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and returns the
+		 * requested number of maximum values.
+		 *
+		 * @return new instance of {@link Max}.
+		 * @since 4.0
+		 */
+		public Max max(int numberOfResults) {
+			return max().limit(numberOfResults);
+		}
+
 		/**
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and returns the
 		 * minimum value.
@@ -441,7 +452,7 @@ private Max(Object value) {
 
 		@Override
 		protected String getMongoMethod() {
-			return "$max";
+			return contains("n") ? "$maxN" : "$max";
 		}
 
 		/**
@@ -453,7 +464,7 @@ protected String getMongoMethod() {
 		public static Max maxOf(String fieldReference) {
 
 			Assert.notNull(fieldReference, "FieldReference must not be null");
-			return new Max(asFields(fieldReference));
+			return new Max(Collections.singletonMap("input", Fields.field(fieldReference)));
 		}
 
 		/**
@@ -465,7 +476,7 @@ public static Max maxOf(String fieldReference) {
 		public static Max maxOf(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null");
-			return new Max(Collections.singletonList(expression));
+			return new Max(Collections.singletonMap("input", expression));
 		}
 
 		/**
@@ -478,7 +489,7 @@ public static Max maxOf(AggregationExpression expression) {
 		public Max and(String fieldReference) {
 
 			Assert.notNull(fieldReference, "FieldReference must not be null");
-			return new Max(append(Fields.field(fieldReference)));
+			return new Max(appendTo("input", Fields.field(fieldReference)));
 		}
 
 		/**
@@ -491,7 +502,26 @@ public Max and(String fieldReference) {
 		public Max and(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null");
-			return new Max(append(expression));
+			return new Max(appendTo("input", expression));
+		}
+
+		/**
+		 * Creates new {@link Max} that returns the given number of maxmimum values ({@literal $maxN}).
+		 * <strong>NOTE</strong>: Cannot be used with more than one {@literal input} value.
+		 *
+		 * @param numberOfResults
+		 * @return new instance of {@link Max}.
+		 */
+		public Max limit(int numberOfResults) {
+			return new Max(append("n", numberOfResults));
+		}
+
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+			if (get("n") == null) {
+				return toDocument(get("input"), context);
+			}
+			return super.toDocument(context);
 		}
 
 		@Override

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/spel/MethodReferenceNode.java

@@ -224,6 +224,8 @@ public class MethodReferenceNode extends ExpressionNode {
 				.mappingParametersTo("output", "sortBy"));
 		map.put("topN", mapArgRef().forOperator("$topN") //
 				.mappingParametersTo("n", "output", "sortBy"));
+		map.put("maxN", mapArgRef().forOperator("$maxN") //
+				.mappingParametersTo("n", "input"));
 
 		// CONVERT OPERATORS
 		map.put("convert", mapArgRef().forOperator("$convert") //

Diff for: spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AccumulatorOperatorsUnitTests.java

@@ -80,6 +80,20 @@ void rendersExpMovingAvgWithAlpha() {
 				.isEqualTo(Document.parse("{ $expMovingAvg: { input: \"$price\", alpha: 0.75 } }"));
 	}
 
+	@Test // GH-4139
+	void rendersMax() {
+
+		assertThat(valueOf("price").max().toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo(Document.parse("{ $max: \"$price\" }"));
+	}
+
+	@Test // GH-4139
+	void rendersMaxN() {
+
+		assertThat(valueOf("price").max(3).toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo(Document.parse("{ $maxN: { n: 3, input : \"$price\" } }"));
+	}
+
 	static class Jedi {
 
 		String name;

Diff for: spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/SpelExpressionTransformerUnitTests.java

@@ -1204,6 +1204,11 @@ void shouldRenderLastN() {
 		assertThat(transform("lastN(3, \"$score\")")).isEqualTo("{ $lastN : { n : 3, input : \"$score\" }}");
 	}
 
+	@Test // GH-4139
+	void shouldRenderMaxN() {
+		assertThat(transform("maxN(3, \"$score\")")).isEqualTo("{ $maxN : { n : 3, input : \"$score\" }}");
+	}
+
 	private Document transform(String expression, Object... params) {
 		return (Document) transformer.transform(expression, Aggregation.DEFAULT_CONTEXT, params);
 	}