Fix NPE when rendering untyped aggregation operation as part of query.

pom.xml

@@ -5,7 +5,7 @@
 
 	<groupId>org.springframework.data</groupId>
 	<artifactId>spring-data-mongodb-parent</artifactId>
-	<version>4.3.0-SNAPSHOT</version>
+	<version>4.3.x-4687-SNAPSHOT</version>
 	<packaging>pom</packaging>
 
 	<name>Spring Data MongoDB</name>

spring-data-mongodb-benchmarks/pom.xml

@@ -7,7 +7,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>4.3.0-SNAPSHOT</version>
+		<version>4.3.x-4687-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb-distribution/pom.xml

@@ -15,7 +15,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>4.3.0-SNAPSHOT</version>
+		<version>4.3.x-4687-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb/pom.xml

@@ -13,7 +13,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>4.3.0-SNAPSHOT</version>
+		<version>4.3.x-4687-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/QueryMapper.java

@@ -548,8 +548,11 @@ protected Object convertSimpleOrDocument(Object source, @Nullable MongoPersisten
 		}
 
 		if (source instanceof AggregationExpression age) {
-			return age
-					.toDocument(new RelaxedTypeBasedAggregationOperationContext(entity.getType(), this.mappingContext, this));
+
+			if(entity == null) {
+				return age.toDocument();
+			}
+			return age.toDocument(new RelaxedTypeBasedAggregationOperationContext(entity.getType(), this.mappingContext, this));
 		}
 
 		if (source instanceof MongoExpression exr) {

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/QueryMapperUnitTests.java

@@ -1604,6 +1604,15 @@ void usageOfMongoExpressionOnCriteriaDoesNotUnwrapAnExprAggregationExpression()
 		assertThat(mappedObject).isEqualTo("{ $expr : { $expr : { $gt : [ '$foo', '$budget'] } } }");
 	}
 
+	@Test // GH-4687
+	void usageOfUntypedAggregationShouldRenderOperationsAsIs() {
+
+		Query query = query(expr(Expr.valueOf(ComparisonOperators.valueOf("field").greaterThan("budget"))));
+		org.bson.Document mappedObject = mapper.getMappedObject(query.getQueryObject(),
+			context.getPersistentEntity(Object.class));
+		assertThat(mappedObject).isEqualTo("{ $expr : { $expr : { $gt : [ '$field', '$budget'] } } }");
+	}
+
 	@Test // GH-2750
 	void usesMongoExpressionDocumentAsIsIfItIsNotAnAggregationExpression() {