Add support for $degreesToRadians aggregation operator.

Closes: #3714
Original pull request: #3755.

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

@@ -231,6 +231,16 @@ public class ConvertOperators {
 			return ToString.toString(valueObject());
 		}
 
+		/**
+		 * {@link AggregationExpression} for {@code $degreesToRadians} that converts an input value measured in degrees to radians.\
+		 *
+		 * @return new instance of {@link DegreesToRadians}.
+		 * @since 3.3
+		 */
+		public DegreesToRadians convertDegreesToRadians() {
+			return DegreesToRadians.degreesToRadians(valueObject());
+		}
+
 		private Convert createConvert() {
 			return usesFieldRef() ? Convert.convertValueOf(fieldReference) : Convert.convertValueOf(expression);
 		}
@@ -692,4 +702,52 @@ public class ConvertOperators {
 			return "$toString";
 		}
 	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $degreesToRadians} that converts an input value measured in degrees to radians.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class DegreesToRadians extends AbstractAggregationExpression {
+
+		private DegreesToRadians(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Create a new instance of {@link DegreesToRadians} that converts the value of the given field, measured in degrees, to radians.
+		 *
+		 * @param fieldName must not be {@literal null}.
+		 * @return new instance of {@link DegreesToRadians}.
+		 */
+		public static DegreesToRadians degreesToRadiansOf(String fieldName) {
+			return degreesToRadians(Fields.field(fieldName));
+		}
+
+		/**
+		 * Create a new instance of {@link DegreesToRadians} that converts the result of the given {@link AggregationExpression expression}, measured in degrees, to radians.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link DegreesToRadians}.
+		 */
+		public static DegreesToRadians degreesToRadiansOf(AggregationExpression expression) {
+			return degreesToRadians(expression);
+		}
+
+		/**
+		 * Create a new instance of {@link DegreesToRadians} that converts the given value, measured in degrees, to radians.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link DegreesToRadians}.
+		 */
+		public static DegreesToRadians degreesToRadians(Object value) {
+			return new DegreesToRadians(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$degreesToRadians";
+		}
+	}
 }

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

@@ -199,6 +199,7 @@ public class MethodReferenceNode extends ExpressionNode {
 		map.put("toLong", singleArgRef().forOperator("$toLong"));
 		map.put("toObjectId", singleArgRef().forOperator("$toObjectId"));
 		map.put("toString", singleArgRef().forOperator("$toString"));
+		map.put("degreesToRadians", singleArgRef().forOperator("$degreesToRadians"));
 
 		FUNCTIONS = Collections.unmodifiableMap(map);
 	}

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

@@ -222,4 +222,11 @@ public class ConvertOperatorsUnitTests {
 		assertThat(ConvertOperators.valueOf(EXPRESSION).convertToString().toDocument(Aggregation.DEFAULT_CONTEXT))
 				.isEqualTo(Document.parse("{ $toString: " + EXPRESSION_STRING + " } "));
 	}
+
+	@Test // GH-3714
+	void degreesToRadiansUsingFieldReference() {
+
+		assertThat(ConvertOperators.valueOf("angle_a").convertDegreesToRadians().toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo(Document.parse("{ $degreesToRadians : \"$angle_a\"}"));
+	}
 }

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

@@ -941,6 +941,11 @@ public class SpelExpressionTransformerUnitTests {
 		assertThat(transform("round(field, 2)")).isEqualTo(Document.parse("{ \"$round\" : [\"$field\", 2]}"));
 	}
 
+	@Test // GH-3714
+	void shouldRenderDegreesToRadians() {
+		assertThat(transform("degreesToRadians(angle_a)")).isEqualTo(Document.parse("{ \"$degreesToRadians\" : \"$angle_a\"}"));
+	}
+
 	@Test // GH-3712
 	void shouldRenderCovariancePop() {
 		assertThat(transform("covariancePop(field1, field2)"))