春季RowMapper OneToMany？

Question

我试图为两个对象调整两个RowMappers，它们之间存在@OneToMany关系。

假设，我有两种方法：

public Account findAccount(long id) {
    SQL = "SELECT * FROM accounts WHERE id = ?";
    Account account = template.queryForObject(SQL, new Object[] { id }, MAP_ACCOUNT);
    return account;
}


public Card findCard(String number) {
    SQL = "SELECT * FROM cards WHERE number = ?";
    Card card = template.queryForObject(SQL, new Object[] { number }, MAP_CARD);
    return card;
}

两排地图绘制者：

   private final RowMapper<Card> MAP_CARD = new RowMapper<Card>() {

    public Card mapRow(ResultSet rs, int rowNum) throws SQLException {
        Account account = findAccount(rs.getLong("account_id"));
        Card card = new DefaultCard(rs.getString("number"), account);
        return card;
    }

};

private final RowMapper<Account> MAP_ACCOUNT = new RowMapper<Account>() {

    public Account mapRow(ResultSet rs, int rowNum) throws SQLException {
        SQL = "SELECT * FROM cards where account_id = " + rs.getLong("id");
        List<Card> cards = template.query(SQL, MAP_CARD);
        Account account = new DefaultAccount(rs.getLong("id"), rs.getString("username"), cards);
        return account;
    }
};

运行findAccount或findCard方法将引发一个异常，说明“连接太多了！”。这是因为它们之间的行映射器具有循环依赖性。我知道我这样做是错误的，我想知道如何正确地重写行映射器。非常感谢。

Answer 1

首先，java对象构造函数是“递归”紧密耦合的。Account和Card构造函数相互期望对方作为参数。你可以有一个没有信用卡的账户，对吗？因此，删除帐户构造函数的卡片列表。

现在转到查询，当加载帐户卡时，有两种情况：

1-加载卡从帐户：您已经有帐户，这是不必要的查询帐户为每一张卡。因此，您可以有一个MAP_CARD_FROM_ACCOUNT行映射程序，它通过参数接收帐户。

2- 加载单张卡：在这个场景中，您只需要卡及其帐户，因此对于MAP_CARD映射程序，您可以查询返回卡和帐户信息：SELECT * FROM cards C, accounts a WHERE c.account_id=a.id and number = ?

下面是映射程序代码的外观示例：

 private final RowMapper<Card> MAP_CARD_FROM_ACCOUNT = new RowMapper<Card>() {

    public void setAccount(Account account){
       this.account = account;
    }

    public Card mapRow(ResultSet rs, int rowNum) throws SQLException {
        Card card = new DefaultCard(rs.getString("number"), account);
        return card;
    }

};

 private final RowMapper<Card> MAP_CARD = new RowMapper<Card>() {

    public Card mapRow(ResultSet rs, int rowNum) throws SQLException {
        Account account = new Account(rs.getLong("a.account_id"), rs.getString("a.username");
        Card card = new DefaultCard(rs.getString("c.number"), account);
        return card;
    }

};

private final RowMapper<Account> MAP_ACCOUNT = new RowMapper<Account>() {

    public Account mapRow(ResultSet rs, int rowNum) throws SQLException {
        SQL = "SELECT * FROM cards where account_id = " + rs.getLong("id");
        Account account = new DefaultAccount(rs.getLong("id"), rs.getString("username"));
        MAP_CARD_FROM_ACCOUNT.setAccount(account)
        List<Card> cards = template.query(SQL, MAP_CARD_FROM_ACCOUNT);
        account.setCards(cards);
        return account;
    }
};