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First of all, I haven't written any SQL statements to create a table. I try to use Hibernate/jpa only without writing SQL.

My relation is in the following: A user can have many task, a task only has one user.

I created my models as this:

User Table:

@Entity
@Table(name="T_USER")
@EntityListeners(AuditingEntityListener.class)
public class User 

 @Id
 @GeneratedValue(strategy = GenerationType.AUTO)
 @Column(name = "uid")
 private Long uid;
...

Task Table:

@Entity
@Table(name = "T_TASK")
@EntityListeners(AuditingEntityListener.class)
public class TASK

 @Id
 @GeneratedValue(strategy = GenerationType.AUTO)
 private Long tid;

 @ManyToOne
 @JoinColumn(name ="oid")
 private User owner;

 public User getOwner() 
 return owner;
 

 public void setOwner(User owner) 
 this.owner = owner;
 
...

The relation is task's ownerid(oid) is user's uid.

To save a user to my database, I'm using postman with the following parameters:

 "username": "firstuser",
 "email": "firstuser@email.com"

To save a task to my database I'm using this:

 "description": "some description",
 "oid": "12" // I also can state username of the user rather than ID

However, as I execute save a task, the oid of task is NULL. In data access objects, I have:

@PostMapping("/save")
public QR createTask(@Valid @RequestBody Task task)

 return taskDAO.save(task);

1-What am I doing wrong? I just want to add a task with owner id to database, however it returns null as ownerid.

2-Should I create a table first with SQL using

 Create table task(
 tid BIGINT,
 description VARCHAR(255),
 oid BIGINT,
 PRIMARY KEY(tid), FOREIGN KEY(oid) REFERENCES (user.uid))

3-Should I change my save method in TaskDAO?

 public Task save(Task task)
 
 return taskRepository.save(task);
 

4- Should I change my controller method(createTask method using RESTcall)

5- Assume that all of the problems above is fixed. How can I fetch all task that a user has?

6- How can I delete a task when a user is deleted(cascase in SQL, but is there any method in Hibernate)

I hope I explained my problem. Any feedback will be appreciated.

1-What am I doing wrong? I just want to add a task with owner id to database, however it returns null as ownerid

First of all, I would make sure that the owner is being persisted in the db, just to be sure that you have a value to be referencing

2-Should I create a table first with SQL using

Since you're using ORM, writing an SQL query would defeat the purpose of that, you could, but it's not all that necessary, since the relationships are specified already

3-Should I change my save method in TaskDAO?

4- Should I change my controller method(createTask method using RESTcall)

I think it would be best to change your createTask method, you could include the user's id as a pathvariable or a queryparameter and in that method you find the user using their id and set the user field in the task before passing it to the dto to save the value.
The reason the oid is null is because you do not have such a field in there.

5- Assume that all of the problems above is fixed. How can I fetch all task that a user has?

In your task repository, you can create a method like

Collection<Task> findAllTasksByOwnerId(Long id);

6- How can I delete a task when a user is deleted(cascase in SQL, but is there any method in Hibernate)

You can specify the cascade type where you have specified the relationship between the task and the user

You can check this link for a simple tutorial on how to cascade in spring

The oid is null because there is no such field in Task. I think you are mixing two concepts here. The first one is the data transfer object that represents your REST data structure. This one should have an oid field. The second one is the persisting entity. This one you have, it's the Task class.

I would implement a TaskDTO, use Hibernate's session to load the User by its id, then build a Task from the User and the other fields from TaskDTO, then save the Task like you do.

Regarding your other questions

2 - With a create or updatevalue for hibernate.hbm2ddl.auto, Hibernate can generate or update the tables when you start the application.

5 - You could have this interface

@GetMapping("/user/id")
public List<TaskDTO> getTasks(@PathVariable Long id)

Then I think you can't escape coding a criteria query of some sort.

6 - This is done with configuring the relation with cascade = CascadeType.ALL

The hibernate builds table and foreign keys automatically.Complex queries we can write in repo/controller in hibernate syntax.
With Crud repository we can delete , create update and read data easily.

 for example we have student to course one to many relation.

@OneToMany
@JoinColumn(name = "studentId", referencedColumnName = "studentId", insertable = false, updatable = false)
private List<StudentCourses> studentCourses;

in StudentController I will write

 @CrossOrigin(origins = "http://localhost:8090")
 @RequestMapping(value = "/registerStudentCourse", method = RequestMethod.POST, consumes = MediaType.APPLICATION_JSON_VALUE, produces = 
 MediaType.APPLICATION_JSON_VALUE )
 public StudentCourses registerStudentCourse(@RequestBody StudentCourses studentCourse) 
 if (studentCourse != null) 
 studentCourse.setLastupdated(new SimpleDateFormat("yyyy-MM-dd HH:mm").format(new Date()));
 studentCourse = studentCourseRepository.save(studentCourse);
 
 return studentCourse;
 

 @CrossOrigin(origins = "http://localhost:8090")
 @RequestMapping(value = "/findStudentCourses", method = RequestMethod.POST, consumes = MediaType.APPLICATION_JSON_VALUE, produces = 
 MediaType.APPLICATION_JSON_VALUE )
 public List<StudentCourses> findStudentCourses(@RequestBody String studentId) 
 List<StudentCourses> courses = new ArrayList<>();
 JSONObject requestedJSONObject;
 try 
 requestedJSONObject = new JSONObject(studentId);
 String student = requestedJSONObject.getString("studentId");
 courses = 
 studentCourseRepository.findCoursesByStudent(Long.parseLong(student));
 catch (JSONException e) 
 // TODO Auto-generated catch block
 
 return courses;