Add support for $rand aggregation operator.

Closes #3724
Original pull request: #3759

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

@@ -19,6 +19,7 @@ import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
 
+import org.bson.Document;
 import org.springframework.data.mongodb.core.aggregation.AccumulatorOperators.Avg;
 import org.springframework.data.mongodb.core.aggregation.AccumulatorOperators.CovariancePop;
 import org.springframework.data.mongodb.core.aggregation.AccumulatorOperators.CovarianceSamp;
@@ -63,6 +64,16 @@ public class ArithmeticOperators {
 		return new ArithmeticOperatorFactory(expression);
 	}
 
+	/**
+	 * Creates new {@link AggregationExpression} that returns a random float between 0 and 1 each time it is called.
+	 *
+	 * @return new instance of {@link Rand}.
+	 * @since 3.3
+	 */
+	public static Rand rand() {
+		return new Rand();
+	}
+
 	/**
 	 * @author Christoph Strobl
 	 */
@@ -2671,4 +2682,18 @@ public class ArithmeticOperators {
 			return "$tanh";
 		}
 	}
+
+	/**
+	 * {@link Rand} returns a floating value between 0 and 1.
+	 *
+	 * @author Mushtaq Ahmed
+	 * @since 3.3
+	 */
+	public static class Rand implements AggregationExpression {
+
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+			return new Document("$rand", new Document());
+		}
+	}
 }

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

@@ -99,6 +99,7 @@ public class MethodReferenceNode extends ExpressionNode {
 		map.put("cosh", singleArgRef().forOperator("$cosh"));
 		map.put("tan", singleArgRef().forOperator("$tan"));
 		map.put("tanh", singleArgRef().forOperator("$tanh"));
+		map.put("rand", emptyRef().forOperator("$rand"));
 
 		// STRING OPERATORS
 		map.put("concat", arrayArgRef().forOperator("$concat"));

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

@@ -166,4 +166,9 @@ class ArithmeticOperatorsUnitTests {
 				.isEqualTo("{ $tanh : { $degreesToRadians : \"$angle\" } }");
 	}
 
+
+	@Test // GH-3724
+	void rendersRank() {
+		assertThat(rand().toDocument(Aggregation.DEFAULT_CONTEXT)).isEqualTo(new Document("$rand", new Document()));
+	}
 }

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

@@ -1044,6 +1044,11 @@ public class SpelExpressionTransformerUnitTests {
 		assertThat(transform("dateDiff(purchaseDate, delivered, 'day')")).isEqualTo(Document.parse("{ $dateDiff: { startDate: \"$purchaseDate\", endDate: \"$delivered\", unit: \"day\" } }"));
 	}
 
+	@Test // GH-3724
+	void shouldRenderRand() {
+		assertThat(transform("rand()")).isEqualTo(Document.parse("{ $rand : {} }"));
+	}
+
 	private Document transform(String expression, Object... params) {
 		return (Document) transformer.transform(expression, Aggregation.DEFAULT_CONTEXT, params);
 	}