@DatabaseTable(tableName = "Threads")
public class Thread {

    @DatabaseField(id = true)
    private String subject;

    @DatabaseField(foreign = true, foreignAutoRefresh = true)
    public User user = new User();

    public void setSubject(String subject) {
        this.subject = subject;
    }

    public String getSubject() {
        return subject;
    }
}

@DatabaseTable(tableName = "Users")
public class User {
    @DatabaseField(id = true)
    private String nickName;
    @DatabaseField
    private String password;
    @DatabaseField
    private String email;

    @ForeignCollectionField(eager = false)
    public Collection<Thread> threads = new ArrayList<Thread>();

    public void setNickName(String nickName) {
        this.nickName = nickName;
    }

    public String getNickName() {
        return nickName;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getPassword() {
        return password;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getEmail() {
        return email;
    }
}

Exception in thread "main" java.lang.IllegalArgumentException: No fields have a DatabaseField annotation in class java.lang.Thread

JdbcConnectionSource connectionSource = new JdbcConnectionSource(
                    databaseUrl);
            Dao<Board, String> boardDAO = DaoManager.createDao(
                    connectionSource, Board.class);
            Dao<Category, String> categoryDAO = DaoManager.createDao(
                    connectionSource, Category.class);

            Dao<Thread, String> threadDAO = DaoManager.createDao(
                    connectionSource, Thread.class);

            Dao<User, String> userDAO = DaoManager.createDao(
                    connectionSource, User.class);



            TableUtils.createTable(connectionSource, Board.class);
            TableUtils.createTable(connectionSource, Category.class);
            TableUtils.createTable(connectionSource, Thread.class);
            TableUtils.createTable(connectionSource, User.class);