Wrong type of node instantiated causing ClassCastException

[cberes]

SDN 6.2.4 may instantiate the wrong type of node under certain circumstances.

I have an example repo but here is a summary of the problem.

For example, say you have this weird type hierarchy

@Node
public abstract class Animal {
}

@Node
public interface Cat {
}

// No Node annotation!
public class DomesticCat extends Animal implements Cat {
}

@Node
public class Tiger extends Animal implements Cat {
}

If you save and query for a Tiger, then save and query for a DomesticCat, the second node will be a Tiger instead

Tiger tiger = new tiger();
animalRepository.save(tiger);

Tiger found1 = (Tiger) animalRepository.findById(tiger.getId()).get();

DomesticCat cat = new DomesticCat();
animalRepository.save(cat);

// ClassCastException: found2 is a Tiger
DomesticCat found2 = (DomesticCat) animalRepository.findById(cat.getId()).get();

If you add @Node to DomesticCat or save and query in a different order, there are no errors.

This example worked correctly in SDN 6.2.1 but it fails with SDN 6.2.4.

[meistermeier]

Good observation and thanks for pointing this out.
I won't consider this as a bug because you have hit a scenario that is the result of Spring Boot auto-configuration and SDN's lazy registration of eligible entities.
If you start your application with no manual bean configuration, Spring Boot will configure the initial entities based on the explicitly @Node annotated classes. Those will be collected and passed over to the configuration (initialEntitySet) of the Neo4jMappingContext. This is the basic knowledge SDN will start to work with.
Only the moment, you are saving the DomesticCat, SDN will register the non-annotated entity as an additional option. This is the reason why changing the order also makes your test run green.
I hope it makes sense for you, if you have any questions, feel free to let us dig deeper into the topic, otherwise also feel free to close this issue.

[cberes]

thanks for looking into this. If I understand correctly, my organization is already using manual bean configuration but we are seeing this bug. I wonder if there is a bug in Neo4jEntityScanner. It doesn't seem to find all available entities. For example I added this configuration to my example project

@Configuration
public class Neo4jConfig extends AbstractNeo4jConfig {

  @Override
  protected Collection<String> getMappingBasePackages() {
	return List.of("com.example.sdnbug");
  }

  @Override
  public Driver driver() {
	Config config = Config.builder().withoutEncryption().build();
	AuthToken authToken = AuthTokens.basic("neo4j", "neo4j");

	return GraphDatabase.driver("bolt://localhost:7687", authToken, config);
  }

  @Bean
  @Override
  protected DatabaseSelectionProvider databaseSelectionProvider() {
	return () -> DatabaseSelection.byName("db");
  }
}

When I debug the tests and put a breakpoint in Neo4jConfigurationSupport.getInitialEntitySet, I see that Neo4jEntityScanner.get().scan(getMappingBasePackages()) finds only 1 entity (Tiger), when there should be 2 (Tiger and DomesticCat).

[meistermeier]

The Neo4jEntityScanner does exactly what I have explained in my previous comment. For a bunch of classes it looks for the @Node annotated ones. Otherwise automatic scans, like the one that got triggered by Spring Boot, would discover all classes as -at least- candidates for entity registration.
If you are sure that all classes that you pass over should get scanned as entities, you could overwrite this method in your configuration:

spring-data-neo4j/src/main/java/org/springframework/data/neo4j/config/Neo4jConfigurationSupport.java

Lines 51 to 58 in 72869e7

	@Bean
	public Neo4jMappingContext neo4jMappingContext(Neo4jConversions neo4JConversions) throws ClassNotFoundException {
	Neo4jMappingContext mappingContext = new Neo4jMappingContext(neo4JConversions);
	mappingContext.setInitialEntitySet(getInitialEntitySet());
	return mappingContext;
	}

[cberes]

to confirm, the solution is to do something like this?

@Override
public Neo4jMappingContext neo4jMappingContext(Neo4jConversions neo4JConversions) throws ClassNotFoundException {
	Neo4jMappingContext mappingContext = new Neo4jMappingContext(neo4JConversions);
	Set<Class<?>> initialEntitySet = getInitialEntitySet();
	initialEntitySet.add(DomesticCat.class);
	mappingContext.setInitialEntitySet(initialEntitySet);
	return mappingContext;
}

I tried this in my example repo and the test in question passed.

I ran the test with a few recent SDN versions as well, and it appears this extra code is needed only in 6.2.3+