Add support for $minN aggregation operator.

See #4139
Original pull request: #4182.

Author: christophstrobl

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

@@ -133,6 +133,17 @@ public Min min() {
 			return usesFieldRef() ? Min.minOf(fieldReference) : Min.minOf(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 Min min(int numberOfResults) {
+			return min().limit(numberOfResults);
+		}
+
 		/**
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and calculates the
 		 * population standard deviation of the input values.
@@ -551,7 +562,7 @@ private Min(Object value) {
 
 		@Override
 		protected String getMongoMethod() {
-			return "$min";
+			return contains("n") ? "$minN" : "$min";
 		}
 
 		/**
@@ -563,7 +574,7 @@ protected String getMongoMethod() {
 		public static Min minOf(String fieldReference) {
 
 			Assert.notNull(fieldReference, "FieldReference must not be null");
-			return new Min(asFields(fieldReference));
+			return new Min(Collections.singletonMap("input", Fields.field(fieldReference)));
 		}
 
 		/**
@@ -575,7 +586,7 @@ public static Min minOf(String fieldReference) {
 		public static Min minOf(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null");
-			return new Min(Collections.singletonList(expression));
+			return new Min(Collections.singletonMap("input", expression));
 		}
 
 		/**
@@ -588,7 +599,7 @@ public static Min minOf(AggregationExpression expression) {
 		public Min and(String fieldReference) {
 
 			Assert.notNull(fieldReference, "FieldReference must not be null");
-			return new Min(append(Fields.field(fieldReference)));
+			return new Min(appendTo("input", Fields.field(fieldReference)));
 		}
 
 		/**
@@ -601,7 +612,27 @@ public Min and(String fieldReference) {
 		public Min and(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null");
-			return new Min(append(expression));
+			return new Min(appendTo("input", expression));
+		}
+
+		/**
+		 * Creates new {@link Min} that returns the given number of minimum values ({@literal $minN}).
+		 * <strong>NOTE</strong>: Cannot be used with more than one {@literal input} value.
+		 *
+		 * @param numberOfResults
+		 * @return new instance of {@link Min}.
+		 */
+		public Min limit(int numberOfResults) {
+			return new Min(append("n", numberOfResults));
+		}
+
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+
+			if (get("n") == null) {
+				return toDocument(get("input"), context);
+			}
+			return super.toDocument(context);
 		}
 
 		@Override

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

@@ -226,6 +226,8 @@ public class MethodReferenceNode extends ExpressionNode {
 				.mappingParametersTo("n", "output", "sortBy"));
 		map.put("maxN", mapArgRef().forOperator("$maxN") //
 				.mappingParametersTo("n", "input"));
+		map.put("minN", mapArgRef().forOperator("$minN") //
+				.mappingParametersTo("n", "input"));
 
 		// CONVERT OPERATORS
 		map.put("convert", mapArgRef().forOperator("$convert") //

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

@@ -94,6 +94,20 @@ void rendersMaxN() {
 				.isEqualTo(Document.parse("{ $maxN: { n: 3, input : \"$price\" } }"));
 	}
 
+	@Test // GH-4139
+	void rendersMin() {
+
+		assertThat(valueOf("price").min().toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo(Document.parse("{ $min: \"$price\" }"));
+	}
+
+	@Test // GH-4139
+	void rendersMinN() {
+
+		assertThat(valueOf("price").min(3).toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo(Document.parse("{ $minN: { n: 3, input : \"$price\" } }"));
+	}
+
 	static class Jedi {
 
 		String name;

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

@@ -1209,6 +1209,11 @@ void shouldRenderMaxN() {
 		assertThat(transform("maxN(3, \"$score\")")).isEqualTo("{ $maxN : { n : 3, input : \"$score\" }}");
 	}
 
+	@Test // GH-4139
+	void shouldRenderMinN() {
+		assertThat(transform("minN(3, \"$score\")")).isEqualTo("{ $minN : { n : 3, input : \"$score\" }}");
+	}
+
 	private Document transform(String expression, Object... params) {
 		return (Document) transformer.transform(expression, Aggregation.DEFAULT_CONTEXT, params);
 	}