Issue in persisting nested comments using Spring Hibernate - spring

Question

I am trying to create a simple CRUD application using Spring Boot with User, UserEntity, Post, Comment entities.
-> UserEntity is super class of Comment and Post.
-> Each comment has a ManyToOne relationship to a UserEntity (which can be a Post or another Comment)
UserEntity
   |
   #ManyToOne
   createdBy - refers to user table (id)
   |
--------------------
|        |
|        |
Post    Comment
        |
        #ManytoOne
          UserEntity - refers to PK(entity_id) of user_entity table as comment can be on post or reply to another comment
On trying to save a comment on post from the CommentService class,
//Controller
#PostMapping(path = "api/v1/addComment")
public void addComment(#RequestBody Comment comment){ commentService.addCommentOnPost(comment); }
//Service
public void addCommentOnEntity(Comment comment){ commentRepos.save(comment); }
the foreign key in comment table (parent_entity_id) referring to entity_id in user_entity table is not getting updated. The value is blank.
On the other hand UserEntity has a manytoone relationship with User -- createdBy -- which is updating foriegn key user_id in user_entity table properly
Can someone guide me what could be wrong, I have been trying since yesterday night but no luck. Have checked some other answers but could not get an answer for this case.
User.java
#Entity
#Table(name="[user]")
public class User {
#Id
#SequenceGenerator(name="student_sequence",
sequenceName = "student_sequence",
allocationSize = 1)
#GeneratedValue(strategy = GenerationType.SEQUENCE,
generator = "student_sequence")
private long id;
private String name;
private String email;
private int age;
private LocalDate DOB;
//Setters and Getters and default constructor
}
UserEntity.java
#Entity
#Inheritance(strategy = InheritanceType.JOINED)
public abstract class UserEntity {
#Id
#SequenceGenerator(sequenceName = "entity_sequence", name="entity_sequence", allocationSize = 1)
#GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "post_sequence")
private long entityId;
private char entityType;
private LocalDate createdOn;
private LocalDate modifiedOn;
#ManyToOne
#JoinColumn(name = "user_id", referencedColumnName = "id")
private User createdBy;
//Setters and Getters and default constructor
}
Post.java
#Entity
public class Post extends UserEntity{
private String postHeading;
private String postBody;
//Setters and Getters and default constructor
}
Comment.java
#Entity
public class Comment extends UserEntity{
private String comment;
#ManyToOne
#JoinColumn(name="parent_entity_id", referencedColumnName = "entityId")
private UserEntity parentEntity;
//Setters and Getters and default constructor
}
and their repositories
#NoRepositoryBean
public interface UserEntityBaseRepos<T extends UserEntity> extends JpaRepository<T, Long>{
Optional<List<T>> findByCreatedBy_Id(Long user_id);
Optional<List<T>> findByEntityId(Long entity_id);
}
#Repository
public interface UserRespository extends JpaRepository<User, Long> {
Optional<User> findUserByEmail(String email);
Optional<User> findUserByName(String name);
}
#Repository
public interface PostRepos extends UserEntityBaseRepos<Post>, JpaRepository<Post, Long> {
}
#Repository
public interface CommentRepos extends UserEntityBaseRepos<Comment>, JpaRepository<Comment, Long> {
}
Json for postComment service
{
"entityType" : "C",
"createdOn" : "2020-02-05",
"createdBy" : {
"id" : 1
},
"comment": "I am the comment",
"parentEntity" : {
"entityId" : 1
}
}
//User with id = 1 and UserEntity(Post) with entityId = 1 available in database.
Here createdBy.id (user id) is getting updated in the user_entity table, but userEntity.entityId is not getting updated in the comment table

Answer 1

You have very complex entity relationships, it seems to me...
Anyway, I found that you added a generator property to the UserEntity entity with a post_sequence value, but I can't find any relationship to the Post entity in your database. This is probably the reason of the breakdown. You have to connect UserEntity to Post as shown on your diagram or change the generator value.

Answer 2

I was able to solve the problem. The issue was in the following piece of code in Comment concrete class
#ManyToOne
#JoinColumn(name="parent_entity_id", referencedColumnName = "entityId")
private UserEntity parentEntity;
and this Json input
"parentEntity" : {
"entityId" : 1
}
It seems the parentEntity in json input was not being parsed. This was solved on placing JsonProperty("parentEntity") above parentEntity in the Json input was being parsed correctly.
However there was another issue. The parentEntity was not being deserialized to UserEntity as UserEntity is an abstract class. I had to use JacksonPolymorphicDeserialization by introducing a new field parentType("P" for post, "C" for comment) along with some Annotations like below to deserialize parentEntity to corresponding concrete class object.
public class Comment extends UserEntity{
private String comment;
#Transient
#JsonProperty("parentType")
private char parentType;
#ManyToOne
#JoinColumn(name="parent_entity_id", referencedColumnName = "entity_id", foreignKey = #ForeignKey(value=ConstraintMode.CONSTRAINT))
#JsonProperty("parentEntity")
#JsonTypeInfo(use = JsonTypeInfo.Id.NAME , property = "parentType", include = JsonTypeInfo.As.EXTERNAL_PROPERTY)
#JsonSubTypes(value = {
#JsonSubTypes.Type(value = Comment.class, name = "C"),
#JsonSubTypes.Type(value = Post.class, name = "P")
})
private UserEntity parentEntity;
reference - Jackson Polymorphic Deserialization via field. I am not really sure how this works. Will try to make sense of it and update the answer.
If anyone knows a better way to deserialize json, do mention it in the comments or as a new answer.