Add support for $integral aggregation operator.

Closes: #3721

Author: christophstrobl

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

@@ -208,6 +208,27 @@ public Floor floor() {
 			return usesFieldRef() ? Floor.floorValueOf(fieldReference) : Floor.floorValueOf(expression);
 		}
 
+		/**
+		 * Creates new {@link AggregationExpression} that calculates the approximation for the mathematical integral value.
+		 *
+		 * @return new instance of {@link Integral}.
+		 * @since 3.3
+		 */
+		public Integral integral() {
+			return usesFieldRef() ? Integral.integralOf(fieldReference) : Integral.integralOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that calculates the approximation for the mathematical integral value.
+		 *
+		 * @param unit the unit of measure.
+		 * @return new instance of {@link Integral}.
+		 * @since 3.3
+		 */
+		public Integral integral(String unit) {
+			return integral().unit(unit);
+		}
+
 		/**
 		 * Creates new {@link AggregationExpression} that calculates the natural logarithm ln (i.e loge) of the assoicated
 		 * number.
@@ -1665,4 +1686,52 @@ protected String getMongoMethod() {
 			return "$round";
 		}
 	}
+
+	/**
+	 * Value object to represent an {@link AggregationExpression expression} that calculates the approximation for the mathematical integral value.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class Integral extends AbstractAggregationExpression {
+
+		private Integral(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Create a new instance of {@link Integral} for the value stored at the given field holding a numeric value.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link Integral}.
+		 */
+		public static Integral integralOf(String fieldReference) {
+			return new Integral(Collections.singletonMap("input", Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Create a new instance of {@link Integral} for the value provided by the given expression that resolves to a numeric value.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Integral}.
+		 */
+		public static Integral integralOf(AggregationExpression expression) {
+			return new Integral(Collections.singletonMap("input", expression));
+		}
+
+		/**
+		 * Set the unit of measure.
+		 *
+		 * @param unit the unit of measure.
+		 * @return new instance of {@link Integral}.
+		 */
+		public Integral unit(String unit) {
+			return new Integral(append("unit", unit));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$integral";
+		}
+	}
 }

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

@@ -85,6 +85,7 @@ public class MethodReferenceNode extends ExpressionNode {
 		map.put("subtract", arrayArgRef().forOperator("$subtract"));
 		map.put("trunc", singleArgRef().forOperator("$trunc"));
 		map.put("round", arrayArgRef().forOperator("$round"));
+		map.put("integral", mapArgRef().forOperator("$integral").mappingParametersTo("input", "unit"));
 
 		// STRING OPERATORS
 		map.put("concat", arrayArgRef().forOperator("$concat"));

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

@@ -59,4 +59,14 @@ void roundShouldWithPlaceFromExpression() {
 				.toDocument(Aggregation.DEFAULT_CONTEXT))
 						.isEqualTo(new Document("$round", Arrays.asList("$field", new Document("$first", "$source"))));
 	}
+
+	@Test // GH-3721
+	void rendersIntegral() {
+		assertThat(valueOf("kilowatts").integral().toDocument(Aggregation.DEFAULT_CONTEXT)).isEqualTo(Document.parse("{ $integral : { input : \"$kilowatts\" } }"));
+	}
+
+	@Test // GH-3721
+	void rendersIntegralWithUnit() {
+		assertThat(valueOf("kilowatts").integral("hour").toDocument(Aggregation.DEFAULT_CONTEXT)).isEqualTo(Document.parse("{ $integral : { input : \"$kilowatts\", unit : \"hour\" } }"));
+	}
 }

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

@@ -946,6 +946,16 @@ public void shouldRenderRoundWithPlace() {
 		assertThat(transform("round(field, 2)")).isEqualTo(Document.parse("{ \"$round\" : [\"$field\", 2]}"));
 	}
 
+	@Test // GH-3721
+	public void shouldIntegral() {
+		assertThat(transform("integral(field)")).isEqualTo(Document.parse("{ \"$integral\" : { \"input\" : \"$field\" }}"));
+	}
+
+	@Test // GH-3721
+	public void shouldIntegralWithUnit() {
+		assertThat(transform("integral(field, 'hour')")).isEqualTo(Document.parse("{ \"$integral\" : { \"input\" : \"$field\", \"unit\" : \"hour\" }}"));
+	}
+
 	private Object transform(String expression, Object... params) {
 		Object result = transformer.transform(expression, Aggregation.DEFAULT_CONTEXT, params);
 		return result == null ? null : (!(result instanceof org.bson.Document) ? result.toString() : result);