从数据库获取对象列表

Question

I have an object product in my application. This object has a list of items inside it (ArrayList). In the database I have 2 tables: Product and Item. Items have ids of products connected with foreign keys to product id.

It is quite simple to get the Product from database and then get a list of items for it.

How to get a list of products from the database, which will contain all the relevant items inside?

As far as I know, it is not efficient to call dao for every product in the list. How can I manage the query, or object creation to make it more efficient?

I am using Spring JdbcTemplate to get data from Database. Currently I am getting list of products in DAO class and straight from there I call ItemDAO for every item. The database is MySQL.

Update Okay, probably I was unclear in my first approach. The goal is to get nested lists from Database and how to make it performance-wise.

public class Product {
    private Integer id;
    private String name;
    private String description;
    private ArrayList<Item> items;
    private Timestamp timeAdded;
}

public class Item {
    private String name;
    private Float price;
    private String type;
    private Integer productId;
}

public ArrayList getProducts(String searchName) throws SQLException {
    ArrayList<Product> products = new ArrayList<Product>();
    String sql = "SELECT * FROM product WHERE name LIKE '%" + searchName + "%'";
    List<Map> rows = getJdbcTemplate().queryForList(sql);
    for (Map row : rows) {
        Product product = setNewProduct(row);
        products.add(product);
    }
    return products;
}

private Product setNewProduct(Map row) {
    Product product = new Product();
    product.setId((Integer) row.get("id"));
    product.setName((String) row.get("name"));
    product.setDescription((String) row.get("description"));
    ItemsDao itemsDao = (ItemsDao) appContext.getBean("itemsDao");
    ArrayList<Item> items = itemsDao.getItems(product.getId());
    if (items.size() > 0) {
        product.setItems(items);
    }
    product.setTimeAdded(new java.util.Date(((Timestamp) row.get("time_added")).getTime()));
}

In ItemsDao class I just get a list of items for the product, based on the ID sent.

So the question is how to call list of products and get list of items inside every product? (maybe in one complex SQL statement?). Now the performance is horrible, because when there are 30 products with 10 items each, to get the whole list of products it will have to call database so many times.

Answer 1

The answer is that you can't when using an ORM, they all perform like crap when you have any amount of data. OK if you lazy-load the related collections and only use a few, but if you need to display field from the child objects then you can get 100,000+ queries for a page load. Believe me I just ahd to optimise a page using Doctrine that ran 200,000 queries.

What you have to do is either to paginate the results, or to write a custom query that joins the data and then process the results with a loop control: eg

$sql = "SELECT * FROM table1 t1 JOIN table2 t2 ON t1.id = t2.fk_id";
// run query and get resdults
$cur_prod = '';
While( // fetch a row ){
  if($row['prod'] != $cur_prod){
    $cur_prod = $row['prod'];
    // start the new record in your layout
  }
  echo $row['t1.f1'];
  echo // blah blah blah the related data
}

What I am getting at here is that you have to track the parent product and manage the output when it changes.