Controller and hibernate integration. Not saved entity

I have problem with JPA provided by Hibernate. Here are my entities:

@Entity
public class Action {
    @Id
    @GeneratedValue
    private Integer id;

    @Column
    private String name;

    @JoinColumn
    @ManyToOne
    private User user;

    public Integer getId() { return this.id; }
    protected void setId(Integer id) { this.id = id; }

    public String getName() { return this.name; }
    public void setName(String name) { this.name = name; }

    public void getUser() { return this.user; }
    protected void setUser(User user) { this.user = user; }
}
@Entity
public class User {
    @Id
    @GeneratedValue
    private Integer id;

    @Column
    private String name;

    @OneTomany(mappedBy = "user", cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    private List<Action> actions = new ArrayList<>();

    public Integer getId() { return this.id; }
    protected void setId(Integer id) { this.id = id; }

    public String getName() { return this.name; }
    public void setName(String name) { this.name = name; }

    public void addAction(Action action) {
        action.setUser(this);
        actions.add(action);
    }
    public void removeAction(Action action) {
        actions.remove(action);
        action.setUser(null);
    }
    public List<Action> getActions() { return Collections.unmodifiableList(actions); }
}

Here is my controller code:

@Controller
@RequestMapping("/user/{username}")
@SessionAttributes("user")
public class ActionController {
    @Autowired
    private TestService service;

    @ModelAttribute("user")
    public User contextRegistration(@PathVariable("username") String username) {
        return this.service.getByName(username);
    }

    @RequestMapping("")
    public String userView() {
        return "user";
    }

    @RequestMapping(value = "/add", method = RequestMethod.GET)
    public String addAction(@ModelAttribute Action action) {
        return "addAction";
    }

    @RequestMapping(value = "/add", method = RequestMethod.POST)
    public String postAddAction(@Valid Action action, BindingResult result, User user, SessionStatus sessionStatus) {
        if(result.hasErrors()) {
            return "addAction";
        }

        user.addAction(action);
        this.service.saveAction(action);
        sessionStatus.setComplete();
        return "redirect:/user/{username}";
    }
}

When method postAddAction is executed I got the following error:

org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing : pl.zaprogramowany.test.model.Action.user -> pl.zaprogramowany.test.model.User

in line where method saveAction is executed.

My code is modeled on this code https://github.com/spring-projects/spring-petclinic/blob/master/src/main/java/org/springframework/samples/petclinic/web/PetController.java#L79 but I don't know why I have this error because my code is pretty similar to this.

Can anybody help?

@Edit

I'm using spring Jpa repositories and code in service layer is very simple.

@Transactional(readyOnly = true)
public User getByName(String name) {
    return this.userRepository.getByName(name);
}

@Transactional
public void saveAction(Action action) {
    this.actionRepository.save(action);
}