add child entity to an exsisting parent hibernate onetomany

I am new to hibernate
i want to add child to an exsisting parent entity

@Entity
@Table(name="kanban_col")
public class kanban_columns {
    @Id
    @GeneratedValue(generator="system-uuid")
    @GenericGenerator(name="system-uuid", strategy = "uuid")
    private String id;

    @Column(name="col_name")
    private String colName;

    @Column(name="kbid")
    private String boardId;

    @OneToMany(mappedBy = "kanban_col_id",cascade = CascadeType.PERSIST,orphanRemoval = true,fetch=FetchType.LAZY)
    private List<Tasks> tasks;

    Getter and setters....
}

@Entity
@Table(name="kanban_tasks")
public class Tasks {
    @Id
    @GeneratedValue(generator="system-uuid")
    @GenericGenerator(name="system-uuid", strategy = "uuid")
    private String id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JsonBackReference
    @JoinColumn(name = "cid", updatable = true, insertable = true,referencedColumnName = "id")
    private kanban_columns kanban_col_id;

    @Column(name="work_id")
    private String workId;

Getter and setters....
}

service

    @Transactional
    public kanban_columns createTask(final kanban_columns source) {
        final kanban_columns destination = new kanban_columns();

        copyTask(source, destination);

        repository.save(destination);

        return destination;
    }

    @Transactional
    public void save(final kanban_columns attachment)
    {
        repository.save(attachment);
    }
 public void copyTask(final kanban_columns source, final kanban_columns destination){
        destination.setTasks(source.getTasks());
    }

{
    "kanban_col_id":"ff80808174f95b8c0174f95ba1e30000",
        "tasks":[{
        "workId":"4028ab37735ae04e01735b4833ba0000"
    },
{
        "workId":"4028ab37735ae04e01735b4833ba0001"
    },
]
    
}

I am trying to do but a new row in kanban_col is created but i need to insert into existing one
please tell me how to add new row in child entity for existing parent in @onetomany and also how to update,delete child row for existing parent
thank you..!!

You create the new instance of kanban_columns in the createTask method, so the new row is added in the database. You should find the existing kanban_columns with the associated tasks and then add a new task to list.

You need a new method in the repository, that loads and returns the kanban_columns instance with the already associated tasks:

@Query("select kc from kanban_columns kc left join fetch kc.tasks where kc.id = :id")
kanban_columns findByIdWithTasks(@Param("id") String id);

Define in the kanban_columns a helper method:

public void addTasks(Collection<Task> tasks) {
    tasks.forEach(t -> t.setKanban_column_id(this));
    this.tasks.addAll(tasks);
}

And your createTask and copyTask methods should be changed to:

@Transactional
public kanban_columns createTask(final kanban_columns source) {
    final kanban_columns destination = repository.findByIdWithTasks(souce.getId());

    copyTask(source, destination);

    repository.save(destination);

    return destination;
}

public void copyTask(final kanban_columns source, final kanban_columns destination) {
    destination.addTasks(source.getTasks());
}

To delete a task you will need to change cascade from PERSIST to ALL and define a similar method in the kanban_columns:

public void deleteTask(Task task) {
    this.tasks.remove(task);
}