Question

Let’s say I have an object with two different one-to-many relations. Much like:

Customer 1<->M Brands and Customer 1<->M Orders

And let’s say that the my object Customer has two lists related to those two objects.

I’ve read this example:
http://forum.springsource.org/showthread.php?50617-rowmapper-with-one-to-many-query
which explains how to do it with a single one-to-many relationship. For your convenience here’s the ResultSetExtractor override:

private class MyObjectExtractor implements ResultSetExtractor{

    public Object extractData(ResultSet rs) throws SQLException, DataAccessException {
        Map<Integer, MyObject> map = new HashMap<Integer, MyObject>();
        MyObject myObject = null;
        while (rs.next()) {
            Integer id = rs.getInt("ID);
            myObject = map.get(id);
          if(myObject == null){
              String description = rs,getString("Description");
              myObject = new MyObject(id, description);
              map.put(id, myObject);
          }
      MyFoo foo = new MyFoo(rs.getString("Foo"), rs.getString("Bar"));
      myObject.add(myFoo);
        }
        return new ArrayList<MyObject>(map.values());;
    }
}

I don’t think it covers how to work with both. What would be the cleanest approach? Is there a simpler way than to iterate with conditions? Would sets be better off than lists in this case?

Answer 1

From your question, I assume that you have three tables; Customer, Brands, Orders. If you want to fetch the Brands and Orders properties of the Customer to your customer object, where there is no relationship between Brands and Orders, what I suggest is to use a UNION query. Something like this:

TBL_CUSTOMER
------------
CUSTOMER_ID
CUSTOMER_ACCOUNT_NO
CUSTOMER_NAME

TBL_CUSTOMER_BRANDS
-------------------
CUSTOMER_BRAND_ID            - UK
BRAND_NAME
CUSTOMER_ID                  - FK

TBL_ORDERS
-------------------
ORDER_ID                     - UK
CUSTOMER_ID                  - FK

Query:

SELECT CUS.*, BRANDS.CUSTOMER_BRAND_ID COL_A, BRANDS.BRAND_NAME COL_B, 1 IS_BRAND FROM TBL_CUSTOMER CUS JOIN TBL_CUSTOMER_BRANDS BRANDS ON (CUS.CUSTOMER_ID = BRANDS.CUSTOMER_ID)
UNION ALL
SELECT CUS.*, ORDERS.ORDER_ID, '', 0 IS_BRAND FROM TBL_CUSTOMER CUS JOIN TBL_ORDERS ORDERS ON (CUS.CUSTOMER_ID = ORDERS.CUSTOMER_ID)

Your ResultSetExtractor will become:

private class MyObjectExtractor implements ResultSetExtractor{

    public Object extractData(ResultSet rs) throws SQLException, DataAccessException {
            Map<Long, Customer> map = new HashMap<Long, Customer>();

        while (rs.next()) {
            Long id = rs.getLong("CUSTOMER_ID");
            Customer customer = map.get(id);
            if(customer == null){
                customer = new Customer();
                customer.setId(id);
                customer.setName(rs.getString("CUSTOMER_NAME"));
                customer.setAccountNumber(rs.getLong("CUSTOMER_ACCOUNT_NO"));
                map.put(id, customer);
                    }

            int type = rs.getInt("IS_BRAND");
            if(type == 1) {
                List brandList = customer.getBrands();
                if(brandsList == null) {
                    brandsList = new ArrayList<Brand>();
                    customer.setBrands(brandsList);
                }
                Brand brand = new Brand();
                brand.setId(rs.getLong("COL_A"));
                brand.setName(rs.getString("COL_B"));
                brandsList.add(brand);
            } else if(type == 0) {
                List ordersList = customer.getOrders();
                if(ordersList == null) {
                    ordersList = new ArrayList<Order>();
                    customer.setOrders(ordersList);
                }
                Order order = new Order();
                order.setId(rs.getLong("COL_A"));
                ordersList.add(order);
            }
        }
        return new ArrayList<Customer>(map.values());
    }
}