GH-2537 - Improve documentation.

# Conflicts:
#	src/test/java/org/springframework/data/neo4j/integration/issues/IssuesIT.java
#	src/test/java/org/springframework/data/neo4j/integration/issues/gh2323/PersonRepository.java
#	src/test/java/org/springframework/data/neo4j/integration/issues/gh2323/PersonService.java

src/main/asciidoc/appendix/custom-queries.adoc

@@ -357,7 +357,10 @@ All properties - and type of relationships - appear in those maps as they would
 have been written by SDN.
 Values will have the correct Cypher type and won't need further conversion.
 
-All relationships are lists of maps. Dynamic relationships will be resolved accordingly.
+TIP: All relationships are lists of maps. Dynamic relationships will be resolved accordingly.
+     One-to-one relationships will also be serialized as singleton lists. So to access a one-to-one mapping
+     between people, you would write this das `$person.\\__properties__.BEST_FRIEND[0].\\__target__.\\__id__`.
+
 If an entity has a relationship with the same type to different types of others nodes, they will all appear in the same list.
 If you need such a mapping and also have the need to work with those custom parameters, you have to unroll it accordingly.
 One way to do this are correlated subqueries (Neo4j 4.1+ required).

src/test/java/org/springframework/data/neo4j/integration/issues/gh2323/GH2323IT.java

@@ -18,6 +18,7 @@ package org.springframework.data.neo4j.integration.issues.gh2323;
 import static org.assertj.core.api.Assertions.assertThat;
 
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
 import java.util.stream.Collectors;
@@ -65,6 +66,9 @@ class GH2323IT {
 					.asString();
 			transaction.run("unwind ['German', 'English'] as name create (n:Language {name: name}) return name")
 					.consume();
+			personId = transaction.run("MATCH (l:Language {name: 'German'}) CREATE (n:Person {id: randomUUID(), name: 'Helge'}) -[:HAS_MOTHER_TONGUE]-> (l) return n.id").single()
+					.get(0)
+					.asString();
 			transaction.commit();
 			bookmarkCapture.seedWith(session.lastBookmark());
 		}
@@ -105,6 +109,20 @@ class GH2323IT {
 		});
 	}
 
+	@Test // "GH-2537"
+	void ensure1To1RelationshipsAreSerialized(@Autowired PersonService personService) {
+
+		Optional<Person> optionalPerson = personService.updateRel3(personId);
+		assertThat(optionalPerson).isPresent().hasValueSatisfying(person -> {
+
+			assertThat(person.getKnownLanguages()).hasSize(1);
+			assertThat(person.getKnownLanguages()).first().satisfies(knows -> {
+				assertThat(knows.getDescription()).isEqualTo("Whatever");
+				assertThat(knows.getLanguage()).extracting(Language::getName).isEqualTo("German");
+			});
+		});
+	}
+
 	@Repository
 	public interface PersonRepository extends Neo4jRepository<Person, String> {
 
@@ -126,6 +144,16 @@ class GH2323IT {
 			   + "CREATE (f) - [r:KNOWS {description: rel.__properties__.description}] -> (t) "
 			   + "RETURN f, collect(r), collect(t)")
 		Person updateRel2(@Param("person") Person person);
+
+		@Query(""
+			   + "MATCH (f:Person {id: $person.__id__}) "
+			   + "MATCH (mt:Language {name: $person.__properties__.HAS_MOTHER_TONGUE[0].__target__.__id__}) "
+			   + "MATCH (f)-[frl:HAS_MOTHER_TONGUE]->(mt) WITH f, frl, mt "
+			   + "UNWIND $person.__properties__.KNOWS As rel WITH f, frl, mt, rel "
+			   + "MATCH (t:Language {name: rel.__target__.__id__}) "
+			   + "MERGE (f)- [r:KNOWS {description: rel.__properties__.description}] -> (t) "
+			   + "RETURN f, frl, mt, collect(r), collect(t)")
+		Person updateRelWith11(@Param("person") Person person);
 	}
 
 	@Service
@@ -159,6 +187,17 @@ class GH2323IT {
 
 			return original;
 		}
+
+		public Optional<Person> updateRel3(String id) {
+			Optional<Person> original = personRepository.findById(id);
+			if (original.isPresent()) {
+				Person person = original.get();
+				person.setKnownLanguages(Collections.singletonList(new Knows("Whatever", new Language("German"))));
+				return Optional.of(personRepository.updateRelWith11(person));
+			}
+
+			return original;
+		}
 	}
 
 	@Configuration

src/test/java/org/springframework/data/neo4j/integration/issues/gh2323/Person.java

@@ -37,6 +37,9 @@ public class Person {
 	@Relationship("KNOWS")
 	private List<Knows> knownLanguages = new ArrayList<>();
 
+	@Relationship("HAS_MOTHER_TONGUE")
+	private Knows motherTongue;
+
 	public Person(String name) {
 		this.name = name;
 	}