Improve error message when extracting a sometimes absent sub-section

Fixes gh-715

spring-restdocs-core/src/main/java/org/springframework/restdocs/payload/FieldPathPayloadSubsectionExtractor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2019 the original author or authors.
+ * Copyright 2014-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -21,6 +21,7 @@ import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.TreeSet;
 import java.util.stream.Collectors;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
@@ -93,7 +94,9 @@ public class FieldPathPayloadSubsectionExtractor
 				List<?> extractedList = (List<?>) value;
 				JsonContentHandler contentHandler = new JsonContentHandler(payload, descriptorsByPath.values());
 				Set<JsonFieldPath> uncommonPaths = JsonFieldPaths.from(extractedList).getUncommon().stream()
-						.map((path) -> JsonFieldPath.compile(this.fieldPath + "." + path)).filter((path) -> {
+						.map((path) -> JsonFieldPath
+								.compile((path.equals("")) ? this.fieldPath : this.fieldPath + "." + path))
+						.filter((path) -> {
 							FieldDescriptor descriptorForPath = descriptorsByPath.getOrDefault(path,
 									new FieldDescriptor(path.toString()));
 							return contentHandler.isMissing(descriptorForPath);
@@ -105,7 +108,8 @@ public class FieldPathPayloadSubsectionExtractor
 					String message = this.fieldPath + " identifies multiple sections of "
 							+ "the payload and they do not have a common structure. The "
 							+ "following non-optional uncommon paths were found: ";
-					message += uncommonPaths;
+					message += uncommonPaths.stream().map(JsonFieldPath::toString)
+							.collect(Collectors.toCollection(TreeSet::new));
 					throw new PayloadHandlingException(message);
 				}
 			}

spring-restdocs-core/src/main/java/org/springframework/restdocs/payload/JsonFieldPaths.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2018 the original author or authors.
+ * Copyright 2014-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -24,6 +24,8 @@ import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Set;
 
+import org.springframework.restdocs.payload.JsonFieldProcessor.ExtractedField;
+
 /**
  * {@code JsonFieldPaths} provides support for extracting fields paths from JSON
  * structures and identifying uncommon paths.
@@ -69,6 +71,9 @@ final class JsonFieldPaths {
 		else if (object instanceof Map) {
 			from(paths, parent, (Map<?, ?>) object);
 		}
+		else if (object.equals(ExtractedField.ABSENT)) {
+			paths.add(parent);
+		}
 	}
 
 	private static void from(Set<String> paths, String parent, List<?> items) {