how can i insert a composite component (menu of menus) in my database

问题

I am implementing the Composite Design pattern and I need to insert a menu in the database but the menu may consist of other menus and menu items so when I tried to insert them recursively, I got an error because the submenu and sub items need to know the parent ID which isn't created yet.

public boolean insertMenu(Menu menu) {
    try {
            PreparedStatement statement = connection.prepareStatement("INSERT INTO `menu` VALUES (NULL, ?, ?, ?)");
            statement.setString(1, menu.getName());
            statement.setDate(2, java.sql.Date.valueOf(menu.getLocalDate(menu.getDate())));
            statement.setInt(3, menu.getParent_id()); // problem that this value always is null because it isn't created yet
            for (MenuList child : menu.getChildren()) {
                int x = child.getType();
                if (x == 0) {
                    insertMenu((Menu) child);
                } else {
                    MySqlMenuItemDAO a = new MySqlMenuItemDAO();
                    a.insertMenuItem((MenuItem) child);
                }
            }

            int res = statement.executeUpdate();
            if (res == 1) {
                System.out.println("menu "+menu.getName()+" inserted");
                return true;
            }

    } catch (SQLException e) {
        e.printStackTrace();
    }
    return false;
}

回答1:

menu.getParent_id() will always return null for the top-most Menu because by implication it doesn't have a parent. Since menu traversal begins with the top-most Menu you need to handle this specific case.

Firstly, you need a way to represent a Menu without a parent in your database table. For example, you may choose NULL to represent this. Then handle the case:

if(menu.getParent_id() == null) {    
    statement.setNull(3, java.sql.Types.INTEGER);
else {
    statement.setInt(3, menu.getParent_id());
}

You also need to create the menu in the database before calling the method recursively. Otherwise menu.getParent_id() will return null for every Menu and not just the top-most. So this needs to happen before creating the submenus: int res = statement.executeUpdate();

来源：https://stackoverflow.com/questions/53892501/how-can-i-insert-a-composite-component-menu-of-menus-in-my-database

标签

java

mysql

database

recursion

composite