How to create relationships between objects in Spring JDBC?

Question

I want to implement a relationship from JPA to Spring JDBC. For example, suppose I have Account and Ads objects. The relationship between account and advertising is @OneToMany according to JPA.

Account Class:

public class Account {

private Long id;
private String username;
private Set<Advert> adverts = new HashSet<Advert>();

// getters + setters
}

Ad class

public class Advert {

private Long id;
private String text;
private Account account;

// getters + setters
}

AccountMapper:

public class AccountMapper implements RowMapper<Account> {

public Account mapRow(ResultSet rs, int rowNum) throws SQLException {
    Account account = new Account();
    account.setId(rs.getLong("id"));
    account.setUsername(rs.getString("username"));
    return account;
}
}

Now I am trying to create a Mapper class for a class. How can I map an account variable from an advertising class to a string? Many thanks

Answer 1

Well, if you do not use ORM ... you do not have an object relationship mapping! After that, ORMs were created for this reason :-)

, ORM . JDBC JPA - . , , . :

• , ORM (: , ACL spring).

, JPA , DAO. JDBC.

, DAO JDBC, .

: ORM , JDBC SQL-.

Answer 2

Hibernate, , Hibernate tutorial , .

JDBC, :

• , .

• AccountMapper:

  public class AdvertMapper implements RowMapper<Advert> {
  
      public Advert mapRow(ResultSet rs, int rowNum) throws SQLException {
          Advert advert = new Advert();
          advert.setId(rs.getLong("advert_id"));
          advert.setText(rs.getString("advert_text"));
          return advert;      
      }
  }
  
  public class AccountMapper implements RowMapper<Account> {
  
      private final AdvertMapper advertMapper;
  
      public AccountMapper(AdvertMapper advertMapper) {
          this.advertMapper = advertMapper;
      }
  
      public Account mapRow(ResultSet rs, int rowNum) throws SQLException {
          Account account = new Account();
          account.setId(rs.getLong("account_id"));
          account.setUsername(rs.getString("account_username"));
  
          Advert advert = this.advertMapper.mapRow(rs, rowNum);
          advert.setAccount(account);
          account.getAdverts().add(advert);
  
          return account;
      }
  }
  

AccountMapper AdvertMapper .

Hibernate, .

Answer 3

- , : " , - , RowMapper, ResultSet , .

http://simpleflatmapper.org, ResultSet POJO . , , , , .

Checkout http://simpleflatmapper.org/0104-getting-started-springjdbc.html as well as https://arnaudroger.imtqy.com/blog/2017/02/27/jooq-one-to-many.html

you will need to get a ResutlSetExtractor - it is thread safe, so only one instance is needed -,

    private final ResultSetExtractor<List<Account>> mapper = 
        JdbcTemplateMapperFactory
            .newInstance()
            .addKeys("id") // assuming the account id will be on that column 
            .newResultSetExtractor(Account.class);

    // in the method
    String query = 
        "SELECT ac.id as id, ac.username, ad.id as adverts_id, ad.text as adverts_text"
        + "FROM account ac LEFT OUTER JOIN advert ad ON ad.account_id = ac.id order by id " 
        // the order by id is important here as it uses the break on id on the root object 
        // to detect new root object creation

    List<Account> results = template.query(query, mapper);

so that you should get a list of accounts in the list of filled ads. but the ad does not have an account.