@Entity(name = "account")
public class Account {

    @Id
    @GeneratedValue
    private long id;

    @OneToOne(mappedBy = "account", fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    private User user;
....

@Entity(name = "user")
public class User {

    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "account_id")
    private Account account;

            Account account = new Account();
            User newUser = User.builder().userName("admin").email("admin@ca.com").password("admin").account(account).status(Status.ENABLED).lastLogin(LocalDateTime.now()).build();

            account.setUser(newUser);
            accountRepository.save(account);

Account account = user.getAccount();

Account account = new Account();
            Purchase purchase = new Purchase();
            purchase.setAccount(account);
            account.setPurchases(ImmutableList.of(purchase));
            User newUser = User.builder().userName("admin").email("admin@ca.com").password("admin").account(account).status(Status.ENABLED).lastLogin(LocalDateTime.now()).build();

            account.setUser(newUser);
            accountRepository.save(account);