Add support for $derivative aggregation operator.

Closes: #3716

Author: christophstrobl

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

@@ -24,7 +24,9 @@
 import org.springframework.data.mongodb.core.aggregation.AccumulatorOperators.StdDevPop;
 import org.springframework.data.mongodb.core.aggregation.AccumulatorOperators.StdDevSamp;
 import org.springframework.data.mongodb.core.aggregation.AccumulatorOperators.Sum;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.StringUtils;
 
 /**
  * Gateway to {@literal Arithmetic} aggregation operations that perform math operations on numbers.
@@ -532,6 +534,31 @@ public Round roundToPlace(int place) {
 			return round().place(place);
 		}
 
+		/**
+		 * Creates new {@link AggregationExpression} that calculates the mathematical derivative value.
+		 *
+		 * @return new instance of {@link Derivative}.
+		 * @since 3.3
+		 */
+		public Derivative derivative() {
+			return derivative(null);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that calculates the mathematical derivative value.
+		 *
+		 * @param unit The time unit ({@literal week, day, hour, minute, second, millisecond}) to apply can be
+		 *          {@literal null}.
+		 * @return new instance of {@link Derivative}.
+		 * @since 3.3
+		 */
+		public Derivative derivative(@Nullable String unit) {
+
+			Derivative derivative = usesFieldRef() ? Derivative.derivativeOf(fieldReference)
+					: Derivative.derivativeOf(expression);
+			return StringUtils.hasText(unit) ? derivative.unit(unit) : derivative;
+		}
+
 		private boolean usesFieldRef() {
 			return fieldReference != null;
 		}
@@ -1665,4 +1692,32 @@ protected String getMongoMethod() {
 			return "$round";
 		}
 	}
+
+	public static class Derivative extends AbstractAggregationExpression {
+
+		private Derivative(Object value) {
+			super(value);
+		}
+
+		public static Derivative derivativeOf(String fieldReference) {
+			return new Derivative(Collections.singletonMap("input", Fields.field(fieldReference)));
+		}
+
+		public static Derivative derivativeOf(AggregationExpression expression) {
+			return new Derivative(Collections.singletonMap("input", expression));
+		}
+
+		public static Derivative derivativeOfValue(Number value) {
+			return new Derivative(Collections.singletonMap("input", value));
+		}
+
+		public Derivative unit(String unit) {
+			return new Derivative(append("unit", unit));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$derivative";
+		}
+	}
 }

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("$round"));
+		map.put("derivative", mapArgRef().forOperator("$derivative").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 +59,11 @@ void roundShouldWithPlaceFromExpression() {
 				.toDocument(Aggregation.DEFAULT_CONTEXT))
 						.isEqualTo(new Document("$round", Arrays.asList("$field", new Document("$first", "$source"))));
 	}
+
+	@Test // GH-3716
+	void rendersDerivativeCorrectly() {
+
+		assertThat(valueOf("miles").derivative("hour").toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo(Document.parse("{ $derivative: { input: \"$miles\", unit: \"hour\" } }"));
+	}
 }

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

@@ -946,6 +946,11 @@ public void shouldRenderRoundWithPlace() {
 		assertThat(transform("round(field, 2)")).isEqualTo(Document.parse("{ \"$round\" : [\"$field\", 2]}"));
 	}
 
+	@Test // GH-3716
+	void shouldRenderDerivative() {
+		assertThat(transform("derivative(miles, 'hour')")).isEqualTo(Document.parse("{ \"$derivative\" : { input : '$miles', 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);