Add support for $dateDiff aggregation operator.

Closes: #3713
Original pull request: #3748.

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

@@ -340,6 +340,42 @@ public class DateOperators {
 			return applyTimezone(DayOfWeek.dayOfWeek(dateReference()), timezone);
 		}
 
+		/**
+		 * Creates new {@link AggregationExpression} that calculates the difference (in {@literal units) to the date
+		 * computed by the given {@link AggregationExpression expression}. @param expression must not be {@literal null}.
+		 * 
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 * @since 3.3
+		 */
+		public DateDiff diffValueOf(AggregationExpression expression, String unit) {
+			return applyTimezone(DateDiff.diffValueOf(expression, unit).toDate(dateReference()), timezone);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that calculates the difference (in {@literal units) to the date stored
+		 * at the given {@literal field}. @param expression must not be {@literal null}.
+		 * 
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 * @since 3.3
+		 */
+		public DateDiff diffValueOf(String fieldReference, String unit) {
+			return applyTimezone(DateDiff.diffValueOf(fieldReference, unit).toDate(dateReference()), timezone);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that calculates the difference (in {@literal units) to the date given
+		 * {@literal value}. @param value anything the resolves to a valid date. Must not be {@literal null}.
+		 * 
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 * @since 3.3
+		 */
+		public DateDiff diff(Object value, String unit) {
+			return applyTimezone(DateDiff.diffValue(value, unit).toDate(dateReference()), timezone);
+		}
+
 		/**
 		 * Creates new {@link AggregationExpression} that returns the year portion of a date.
 		 *
@@ -2550,6 +2586,114 @@ public class DateOperators {
 		}
 	}
 
+	/**
+	 * {@link AggregationExpression} for {@code $dateDiff}.<br />
+	 * <strong>NOTE:</strong> Requires MongoDB 5.0 or later.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class DateDiff extends TimezonedDateAggregationExpression {
+
+		private DateDiff(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Add the number of {@literal units} of the result of the given {@link AggregationExpression expression} to a
+		 * {@link #toDate(Object) start date}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @param unit must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public static DateDiff diffValueOf(AggregationExpression expression, String unit) {
+			return diffValue(expression, unit);
+		}
+
+		/**
+		 * Add the number of {@literal units} from a {@literal field} to a {@link #toDate(Object) start date}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @param unit must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public static DateDiff diffValueOf(String fieldReference, String unit) {
+			return diffValue(Fields.field(fieldReference), unit);
+		}
+
+		/**
+		 * Add the number of {@literal units} to a {@link #toDate(Object) start date}.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @param unit must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public static DateDiff diffValue(Object value, String unit) {
+
+			Map<String, Object> args = new HashMap<>();
+			args.put("unit", unit);
+			args.put("endDate", value);
+			return new DateDiff(args);
+		}
+
+		/**
+		 * Define the start date, in UTC, for the addition operation.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public DateDiff toDateOf(AggregationExpression expression) {
+			return toDate(expression);
+		}
+
+		/**
+		 * Define the start date, in UTC, for the addition operation.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public DateDiff toDateOf(String fieldReference) {
+			return toDate(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Define the start date, in UTC, for the addition operation.
+		 *
+		 * @param dateExpression anything that evaluates to a valid date. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public DateDiff toDate(Object dateExpression) {
+			return new DateDiff(append("startDate", dateExpression));
+		}
+
+		/**
+		 * Optionally set the {@link Timezone} to use. If not specified {@literal UTC} is used.
+		 *
+		 * @param timezone must not be {@literal null}. Consider {@link Timezone#none()} instead.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public DateDiff withTimezone(Timezone timezone) {
+			return new DateDiff(appendTimezone(argumentMap(), timezone));
+		}
+
+		/**
+		 * Set the start day of the week if the unit if measure is set to {@literal week}. Uses {@literal Sunday} by
+		 * default.
+		 *
+		 * @param day must not be {@literal null}.
+		 * @return new instance of {@link DateDiff}.
+		 */
+		public DateDiff startOfWeek(Object day) {
+			return new DateDiff(append("startOfWeek", day));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$dateDiff";
+		}
+	}
+
 	@SuppressWarnings("unchecked")
 	private static <T extends TimezonedDateAggregationExpression> T applyTimezone(T instance, Timezone timezone) {
 		return !ObjectUtils.nullSafeEquals(Timezone.none(), timezone) && !instance.hasTimezone()

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

@@ -145,6 +145,7 @@ public class MethodReferenceNode extends ExpressionNode {
 
 		// DATE OPERATORS
 		map.put("dateAdd", mapArgRef().forOperator("$dateAdd").mappingParametersTo("startDate", "unit", "amount", "timezone"));
+		map.put("dateDiff", mapArgRef().forOperator("$dateDiff").mappingParametersTo("startDate", "endDate", "unit","timezone", "startOfWeek"));
 		map.put("dayOfYear", singleArgRef().forOperator("$dayOfYear"));
 		map.put("dayOfMonth", singleArgRef().forOperator("$dayOfMonth"));
 		map.put("dayOfWeek", singleArgRef().forOperator("$dayOfWeek"));

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

@@ -40,5 +40,21 @@ class DateOperatorsUnitTests {
 				.toDocument(Aggregation.DEFAULT_CONTEXT)).isEqualTo(Document.parse(
 						"{ $dateAdd: { startDate: \"$purchaseDate\", unit: \"day\", amount: 3, timezone : \"America/Chicago\" } }"));
 	}
+
+	@Test // GH-3713
+	void rendersDateDiff() {
+
+		assertThat(
+				DateOperators.dateOf("purchaseDate").diffValueOf("delivered", "day").toDocument(Aggregation.DEFAULT_CONTEXT))
+						.isEqualTo(Document
+								.parse("{ $dateDiff: { startDate: \"$purchaseDate\", endDate: \"$delivered\", unit: \"day\" } }"));
+	}
+
+	@Test // GH-3713
+	void rendersDateDiffWithTimezone() {
+
+		assertThat(DateOperators.dateOf("purchaseDate").withTimezone(Timezone.valueOf("America/Chicago"))
+				.diffValueOf("delivered", "day").toDocument(Aggregation.DEFAULT_CONTEXT)).isEqualTo(Document.parse(
+						"{ $dateDiff: { startDate: \"$purchaseDate\", endDate: \"$delivered\", unit: \"day\", timezone : \"America/Chicago\" } }"));
 	}
 }

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

@@ -1044,6 +1044,11 @@ public class SpelExpressionTransformerUnitTests {
 		assertThat(transform("dateAdd(purchaseDate, 'day', 3)")).isEqualTo(Document.parse("{ $dateAdd: { startDate: \"$purchaseDate\", unit: \"day\", amount: 3 } }"));
 	}
 
+	@Test // GH-3713
+	void shouldRenderDateDiff() {
+		assertThat(transform("dateDiff(purchaseDate, delivered, 'day')")).isEqualTo(Document.parse("{ $dateDiff: { startDate: \"$purchaseDate\", endDate: \"$delivered\", unit: \"day\" } }"));
+	}
+
 	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);