Spring boot 1.5.10
MongoDB 3.4.3
Java 1.8

public class Comment {

@Id
private String id;
private String author;
private String text;

... constructor, getter and setter

public class Post {

@Id
private String        id;
private String        author;
private String        title;
private String        text;
private long          like;
private List<Comment> comments;

... constructor, getter and setter

public interface PostRepository extends MongoRepository<Post, String> {

}

@SpringBootApplication
@EnableWebMvc
public class PostRestApplication implements CommandLineRunner{

@Autowired
private PostRepository postRepository;

public static void main(String[] args) {
    SpringApplication.run(PostRestApplication.class, args);
}

@Override
public void run(String... arg0) throws Exception {

    postRepository.deleteAll(); //works

    List<Comment> comments = new ArrayList<>();
    comments.add(new Comment("1", "Tom", "test"));

    postRepository.save(new Post("2", "Clark", "title",
                                "test", 2, comments)); //works, write on db

    for(Post post:postRepository.findAll()){ //get error on findAll()
        System.out.println(post.toString());
    }

 }
}

at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.data.repository.core.support.SurroundingTransactionDetectorMethodInterceptor.invoke(SurroundingTransactionDetectorMethodInterceptor.java:57) ~[spring-data-commons-1.13.10.RELEASE.jar:na]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:213) ~[spring-aop-4.3.14.RELEASE.jar:4.3.14.RELEASE]
at com.sun.proxy.$Proxy62.findAll(Unknown Source) ~[na:na]
at com.example.postRest.repository.impl.PostRepositoryImpl.findAll(PostRepositoryImpl.java:24) ~[classes/:na]
... (continue)