Add support for $rand aggregation operator.

Original pull request spring-projects#3741
Closes spring-projects#3724

Author: Mushtaq Ahmed

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

@@ -18,6 +18,7 @@
 import java.util.Collections;
 import java.util.List;
 
+import org.bson.Document;
 import org.springframework.data.mongodb.core.aggregation.AccumulatorOperators.Avg;
 import org.springframework.data.mongodb.core.aggregation.AccumulatorOperators.Max;
 import org.springframework.data.mongodb.core.aggregation.AccumulatorOperators.Min;
@@ -54,6 +55,16 @@ public static ArithmeticOperatorFactory valueOf(AggregationExpression expression
 		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
 	 */
@@ -1665,4 +1676,18 @@ protected String getMongoMethod() {
 			return "$round";
 		}
 	}
+
+	/**
+	 * {@link Rand} returns a floating value between 0 and 1.
+	 *
+	 * @author Mushtaq Ahmed
+	 * @since 3.3
+	 */
+	public static class Rand implements AggregationExpression {
+
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+			return new Document("$rand", new Document());
+		}
+	}
 }

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

@@ -91,6 +91,7 @@ public class MethodReferenceNode extends ExpressionNode {
 		map.put("subtract", arrayArgRef().forOperator("$subtract"));
 		map.put("trunc", singleArgRef().forOperator("$trunc"));
 		map.put("round", arrayArgRef().forOperator("$round"));
+		map.put("rand", emptyRef().forOperator("$rand"));
 
 		// STRING OPERATORS
 		map.put("concat", arrayArgRef().forOperator("$concat"));

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

@@ -59,4 +59,9 @@ void roundShouldWithPlaceFromExpression() {
 				.toDocument(Aggregation.DEFAULT_CONTEXT))
 						.isEqualTo(new Document("$round", Arrays.asList("$field", new Document("$first", "$source"))));
 	}
+
+	@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

@@ -975,6 +975,11 @@ void rendersShiftWithDefault() {
 				.isEqualTo(Document.parse("{ $shift: { output: \"$quantity\", by: 1, default: \"Not available\" } }"));
 	}
 
+	@Test // GH-3724
+	void shouldRenderRand() {
+		assertThat(transform("rand()")).isEqualTo(Document.parse("{ $rand : {} }"));
+	}
+
 	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);