Fix field resolution for `ExposedFieldsAggregationContext`.

This commit fixes an issue where the context is not relaxed and errors on unknown fields if multiple stages of nesting contexts happen.

Closes #3917
Original pull request: #4328

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

@@ -96,6 +96,25 @@ class ExposedFieldsAggregationOperationContext implements AggregationOperationCo
 			return exposedField;
 		}
 
+		if(rootContext instanceof RelaxedTypeBasedAggregationOperationContext) {
+			return new DirectFieldReference(new ExposedField(new Field() {
+				@Override
+				public String getName() {
+					return name;
+				}
+
+				@Override
+				public String getTarget() {
+					return field != null ? field.getTarget() : name;
+				}
+
+				@Override
+				public boolean isAliased() {
+					return true;
+				}
+			}, true));
+		}
+
 		throw new IllegalArgumentException(String.format("Invalid reference '%s'", name));
 	}
 

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

@@ -29,6 +29,7 @@ import java.util.Map;
 import org.bson.Document;
 import org.junit.jupiter.api.Test;
 import org.springframework.data.annotation.Id;
+import org.springframework.data.domain.Sort;
 import org.springframework.data.domain.Sort.Direction;
 import org.springframework.data.mongodb.core.aggregation.ConditionalOperators.Cond;
 import org.springframework.data.mongodb.core.aggregation.ProjectionOperationUnitTests.BookWithFieldAnnotation;
@@ -629,6 +630,28 @@ public class AggregationUnitTests {
 		Document target = newAggregation(stage).toDocument("col-1", DEFAULT_CONTEXT);
 		assertThat(extractPipelineElement(target, 0, "$project")).containsKey("name");
 	}
+	
+	@Test // GH-3917
+	void inheritedFieldsExposingContextShouldNotFailOnUnknownFieldReferenceForRelaxedRootContext() {
+
+		List<AggregationOperation> aggregationOperations = new ArrayList<>();
+
+		GroupOperation groupOperation = Aggregation.group("_id", "label_name");
+		aggregationOperations.add(groupOperation);
+
+		ProjectionOperation projectionOperation = Aggregation.project("label_name").andExclude("_id");
+		aggregationOperations.add(projectionOperation);
+
+		Sort sort = Sort.by(Sort.Direction.DESC, "serial_number");
+		SortOperation sortOperation = new SortOperation(sort).and(Sort.Direction.DESC, "label_name");
+		aggregationOperations.add(sortOperation);
+
+		MongoMappingContext mappingContext = new MongoMappingContext();
+		QueryMapper queryMapper = new QueryMapper(new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext));
+		
+		List<Document> documents = newAggregation(City.class, aggregationOperations).toPipeline(new RelaxedTypeBasedAggregationOperationContext(City.class, mappingContext, queryMapper));
+		assertThat(documents.get(2)).isEqualTo("{ $sort : { 'serial_number' : -1, 'label_name' : -1 } }");
+	}
 
 	private Document extractPipelineElement(Document agg, int index, String operation) {