Add support for $expMovingAvg aggregation operator.

The SpEL support for this one is missing due to the differing argument map (N, alpha).

Closes: #3718
Original pull request: #3744.

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

@@ -199,11 +199,61 @@ public class AccumulatorOperators {
 					: CovarianceSamp.covarianceSampOf(expression);
 		}
 
+		/**
+		 * Creates new {@link ExpMovingAvgBuilder} that to build {@link AggregationExpression expMovingAvg} that calculates
+		 * the exponential moving average of numeric values
+		 *
+		 * @return new instance of {@link ExpMovingAvg}.
+		 * @since 3.3
+		 */
+		public ExpMovingAvgBuilder expMovingAvg() {
+
+			ExpMovingAvg expMovingAvg = usesFieldRef() ? ExpMovingAvg.expMovingAvgOf(fieldReference)
+					: ExpMovingAvg.expMovingAvgOf(expression);
+			return new ExpMovingAvgBuilder() {
+
+				@Override
+				public ExpMovingAvg historicalDocuments(int numberOfHistoricalDocuments) {
+					return expMovingAvg.n(numberOfHistoricalDocuments);
+				}
+
+				@Override
+				public ExpMovingAvg alpha(double exponentialDecayValue) {
+					return expMovingAvg.alpha(exponentialDecayValue);
+				}
+			};
+		}
+
 		private boolean usesFieldRef() {
 			return fieldReference != null;
 		}
 	}
 
+	/**
+	 * Builder for {@link ExpMovingAvg}.
+	 *
+	 * @since 3.3
+	 */
+	public interface ExpMovingAvgBuilder {
+
+		/**
+		 * Define the number of historical documents with significant mathematical weight.
+		 *
+		 * @param numberOfHistoricalDocuments
+		 * @return new instance of {@link ExpMovingAvg}.
+		 */
+		ExpMovingAvg historicalDocuments(int numberOfHistoricalDocuments);
+
+		/**
+		 * Define the exponential decay value.
+		 *
+		 * @param exponentialDecayValue
+		 * @return new instance of {@link ExpMovingAvg}.
+		 */
+		ExpMovingAvg alpha(double exponentialDecayValue);
+
+	}
+
 	/**
 	 * {@link AggregationExpression} for {@code $sum}.
 	 *
@@ -835,4 +885,65 @@ public class AccumulatorOperators {
 			return "$covarianceSamp";
 		}
 	}
+
+	/**
+	 * {@link ExpMovingAvg} calculates the exponential moving average of numeric values.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class ExpMovingAvg extends AbstractAggregationExpression {
+
+		private ExpMovingAvg(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Create a new {@link ExpMovingAvg} by defining the field holding the value to be used as input.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link ExpMovingAvg}.
+		 */
+		public static ExpMovingAvg expMovingAvgOf(String fieldReference) {
+			return new ExpMovingAvg(Collections.singletonMap("input", Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Create a new {@link ExpMovingAvg} by defining the {@link AggregationExpression expression} to compute the value
+		 * to be used as input.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link ExpMovingAvg}.
+		 */
+		public static ExpMovingAvg expMovingAvgOf(AggregationExpression expression) {
+			return new ExpMovingAvg(Collections.singletonMap("input", expression));
+		}
+
+		/**
+		 * Define the number of historical documents with significant mathematical weight. <br />
+		 * Specify either {@link #n(int) N} or {@link #alpha(double) aplha}. Not both!
+		 *
+		 * @param numberOfHistoricalDocuments
+		 * @return new instance of {@link ExpMovingAvg}.
+		 */
+		public ExpMovingAvg n/*umber of historical documents*/(int numberOfHistoricalDocuments) {
+			return new ExpMovingAvg(append("N", numberOfHistoricalDocuments));
+		}
+
+		/**
+		 * Define the exponential decay value. <br />
+		 * Specify either {@link #alpha(double) aplha} or {@link #n(int) N}. Not both!
+		 *
+		 * @param exponentialDecayValue
+		 * @return new instance of {@link ExpMovingAvg}.
+		 */
+		public ExpMovingAvg alpha(double exponentialDecayValue) {
+			return new ExpMovingAvg(append("alpha", exponentialDecayValue));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$expMovingAvg";
+		}
+	}
 }

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

@@ -16,6 +16,7 @@
 package org.springframework.data.mongodb.core.aggregation;
 
 import static org.assertj.core.api.Assertions.*;
+import static org.springframework.data.mongodb.core.aggregation.AccumulatorOperators.*;
 
 import java.util.Arrays;
 import java.util.Date;
@@ -46,7 +47,7 @@ class AccumulatorOperatorsUnitTests {
 
 		assertThat(AccumulatorOperators.valueOf(Year.yearOf("birthdate")).covariancePop("midichlorianCount")
 				.toDocument(TestAggregationContext.contextFor(Jedi.class)))
-				.isEqualTo(new Document("$covariancePop", Arrays.asList(new Document("$year", "$birthdate"), "$force")));
+						.isEqualTo(new Document("$covariancePop", Arrays.asList(new Document("$year", "$birthdate"), "$force")));
 	}
 
 	@Test // GH-3712
@@ -54,7 +55,7 @@ class AccumulatorOperatorsUnitTests {
 
 		assertThat(AccumulatorOperators.valueOf("balance").covarianceSamp("midichlorianCount")
 				.toDocument(TestAggregationContext.contextFor(Jedi.class)))
-				.isEqualTo(new Document("$covarianceSamp", Arrays.asList("$balance", "$force")));
+						.isEqualTo(new Document("$covarianceSamp", Arrays.asList("$balance", "$force")));
 	}
 
 	@Test // GH-3712
@@ -62,7 +63,21 @@ class AccumulatorOperatorsUnitTests {
 
 		assertThat(AccumulatorOperators.valueOf(Year.yearOf("birthdate")).covarianceSamp("midichlorianCount")
 				.toDocument(TestAggregationContext.contextFor(Jedi.class)))
-				.isEqualTo(new Document("$covarianceSamp", Arrays.asList(new Document("$year", "$birthdate"), "$force")));
+						.isEqualTo(new Document("$covarianceSamp", Arrays.asList(new Document("$year", "$birthdate"), "$force")));
+	}
+
+	@Test // GH-3718
+	void rendersExpMovingAvgWithNumberOfHistoricDocuments() {
+
+		assertThat(valueOf("price").expMovingAvg().historicalDocuments(2).toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo(Document.parse("{ $expMovingAvg: { input: \"$price\", N: 2 } }"));
+	}
+
+	@Test // GH-3718
+	void rendersExpMovingAvgWithAlpha() {
+
+		assertThat(valueOf("price").expMovingAvg().alpha(0.75).toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo(Document.parse("{ $expMovingAvg: { input: \"$price\", alpha: 0.75 } }"));
 	}
 
 	static class Jedi {
@@ -71,8 +86,7 @@ class AccumulatorOperatorsUnitTests {
 
 		Date birthdate;
 
-		@Field("force")
-		Integer midichlorianCount;
+		@Field("force") Integer midichlorianCount;
 
 		Integer balance;
 	}

src/main/asciidoc/reference/mongodb.adoc

@@ -2503,7 +2503,7 @@ At the time of this writing, we provide support for the following Aggregation Op
 | `setEquals`, `setIntersection`, `setUnion`, `setDifference`, `setIsSubset`, `anyElementTrue`, `allElementsTrue`
 
 | Group/Accumulator Aggregation Operators
-| `addToSet`, `covariancePop`, `covarianceSamp`, `first`, `last`, `max`, `min`, `avg`, `push`, `sum`, `(*count)`, `stdDevPop`, `stdDevSamp`
+| `addToSet`, `covariancePop`, `covarianceSamp`, `expMovingAvg`, `first`, `last`, `max`, `min`, `avg`, `push`, `sum`, `(*count)`, `stdDevPop`, `stdDevSamp`
 
 | Arithmetic Aggregation Operators
 | `abs`, `add` (*via `plus`), `ceil`, `divide`, `exp`, `floor`, `ln`, `log`, `log10`, `mod`, `multiply`, `pow`, `round`, `sqrt`, `subtract` (*via `minus`), `trunc`