Polishing.

Extract query that yields no hits into constant. Guard Map-typed reference properties against empty $or.

See #3805
Original pull request: #3807.

Author: mp911de

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

@@ -62,6 +62,8 @@
  */
 public final class ReferenceLookupDelegate {
 
+	private static final Document NO_RESULTS_PREDICATE = new Document("_id", new Document("$exists", false));
+
 	private final MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext;
 	private final SpELContext spELContext;
 	private final ParameterBindingDocumentCodec codec;
@@ -262,25 +264,32 @@ DocumentReferenceQuery computeFilter(MongoPersistentProperty property, Object so
 						sort);
 			}
 
-			List<Document> ors = new ArrayList<>();
-			for (Object entry : (Collection<Object>) value) {
+			Collection<Object> objects = (Collection<Object>) value;
 
-				Document decoded = codec.decode(lookup, bindingContext(property, entry, spELContext));
-				ors.add(decoded);
+			if (objects.isEmpty()) {
+				return new ListDocumentReferenceQuery(NO_RESULTS_PREDICATE, sort);
 			}
 
-			if(ors.isEmpty()) {
-				return new ListDocumentReferenceQuery(new Document("_id", new Document("$exists", false)), sort);
+			List<Document> ors = new ArrayList<>(objects.size());
+			for (Object entry : objects) {
+
+				Document decoded = codec.decode(lookup, bindingContext(property, entry, spELContext));
+				ors.add(decoded);
 			}
 
 			return new ListDocumentReferenceQuery(new Document("$or", ors), sort);
 		}
 
 		if (property.isMap() && value instanceof Map) {
 
-			Map<Object, Document> filterMap = new LinkedHashMap<>();
+			Set<Entry<Object, Object>> entries = ((Map<Object, Object>) value).entrySet();
+			if (entries.isEmpty()) {
+				return new MapDocumentReferenceQuery(NO_RESULTS_PREDICATE, sort, Collections.emptyMap());
+			}
+
+			Map<Object, Document> filterMap = new LinkedHashMap<>(entries.size());
 
-			for (Entry<Object, Object> entry : ((Map<Object, Object>) value).entrySet()) {
+			for (Entry<Object, Object> entry : entries) {
 
 				Document decoded = codec.decode(lookup, bindingContext(property, entry.getValue(), spELContext));
 				filterMap.put(entry.getKey(), decoded);

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

@@ -698,6 +698,24 @@ void loadEmptyCollectionReference() {
 		assertThat(result.simplePreinitializedValueRef).isEmpty();
 	}
 
+	@Test // GH-3805
+	void loadEmptyMapReference() {
+
+		String rootCollectionName = template.getCollectionName(CollectionRefRoot.class);
+
+		// an empty reference array.
+		Document source = new Document("_id", "id-1").append("value", "v1").append("simplePreinitializedMapRef",
+				new Document());
+
+		template.execute(db -> {
+			db.getCollection(rootCollectionName).insertOne(source);
+			return null;
+		});
+
+		CollectionRefRoot result = template.findOne(query(where("id").is("id-1")), CollectionRefRoot.class);
+		assertThat(result.simplePreinitializedMapRef).isEmpty();
+	}
+
 	@Test // GH-3805
 	void loadNoExistingCollectionReference() {
 
@@ -1167,6 +1185,9 @@ static class CollectionRefRoot {
 		@DocumentReference(lookup = "{ '_id' : '?#{#target}' }") //
 		Map<String, SimpleObjectRef> mapValueRef;
 
+		@DocumentReference //
+		Map<String, SimpleObjectRef> simplePreinitializedMapRef = new LinkedHashMap<>();
+
 		@Field("simple-value-ref-annotated-field-name") //
 		@DocumentReference(lookup = "{ '_id' : '?#{#target}' }") //
 		List<SimpleObjectRef> simpleValueRefWithAnnotatedFieldName;