Spring JDBC RowMapper usage for eager fetches

Question

The question is about the best practice usage for RowMapper in master/detail scenarios where we want to eagerly fetch details using spring jdbc.

Assume that we have both Invoice and InvoiceLine classes.

public class Invoice{
    private BigDecimal invId;
    private Date invDate;
    private List<InvoiceLine> lines;
}
public class InvoiceLine{
    private int order;
    private BigDecimal price;
    private BigDecimal quantity;
}

When using Spring Jdbc with a row mapper we usually have a

public class InvoiceMapper implements RowMapper<Invoice>{
    public Invoice mapRow(ResultSet rs, int rowNum) throws SQLException {
         Invoice invoice = new Invoice();
         invoice.setInvId(rs.getBigDecimal("INVID"));
         invoice.setInvDate(rs.getDate("INVDATE"));
         return invoice;
    }
}

Now the problem is I want to eagerly fetch InvoiceLine's related with this invoice instance. Would it be OK if I query database in the rowmapper class? Or anyone prefers another way? I use the pattern below but not happy with that.

public class InvoiceMapper implements RowMapper<Invoice>{
    private JdbcTemplate jdbcTemplate;
    private static final String SQLINVLINE=
            "SELECT * FROM INVOICELINES WHERE INVID = ?";

    public Invoice mapRow(ResultSet rs, int rowNum) throws SQLException {
         Invoice invoice = new Invoice();
         invoice.setInvId(rs.getBigDecimal("INVID"));
         invoice.setInvDate(rs.getDate("INVDATE"));
         invoice.setLines(jdbcTemplate.query(SQLINVLINE, 
                          new Object[]{invoice.getInvId},new InvLineMapper());

         return invoice;
    }
}

I sense that something is wrong with this approach but could not get a better way. I would be more than glad if someone can show me why is this a bad design and if so what would be the correct usage.

Answer 1

The ResultSetExtractor is a better option for doing this. Execute one query that joins both the tables and then iterate through the result set. You will need to have some logic to aggregate multiple rows belonging to the same invoice - either by ordering by invoice id and checking when the id changes or using a map like shown in the example below.

jdbcTemplate.query("SELECT * FROM INVOICE inv JOIN INVOICE_LINE line " +
   + " on inv.id = line.invoice_id", new ResultSetExtractor<List<Invoice>>() {

    public List<Invoice> extractData(ResultSet rs) {
        Map<Integer,Invoice> invoices = new HashMap<Integer,Invoice>();
        while(rs.hasNext()) {
            rs.next();
            Integer invoiceId = rs.getInt("inv.id");
            Invoice invoice = invoces.get(invoiceId);
            if (invoice == null) {
               invoice = invoiceRowMapper.mapRow(rs);
               invoices.put(invoiceId,invoice);
            }
            InvoiceItem item = invLineMapper.mapRow(rs);
            invoice.addItem(item);  
        }
        return invoices.values();
    }


});