Relating objects with mapped statements in Java

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Relating objects with mapped statements
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however, we can do it with a single database query, but we get all 25,000,000 rows in one big chunk. To decide whether to use complex properties, you need to understand your database and how your application will use it. If you use the techniques described in this section, you can save yourself a good deal of programming effort, but if you misuse it, you can create a big mess. In the next two sections, we look at how to decide which strategy to use, depending on your goals. Let s start by asking this question: is the example of relating accounts to orders to order items a good example of when to relate your data this way Actually, no the order-to-order item relationship is solid, but the account-to-order relationship is not a requirement. Our reasoning is that the order items are not complete objects without the order that owns them, while the account is. Think about it in terms of how you would use them. Generally, you would not be able to do much with an order without its order items, and conversely, the order items without the order are somewhat meaningless. An account, on the other hand, is something that could be thought of as a complete object. For the purposes of our example, however, the relationship shows the technique well using concepts that are familiar and recognizable (Accounts have Orders, Orders have Order Items), and that is what we are trying to accomplish here, so we will stick with it for a while longer.
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6.2.2 Lazy loading
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The first of the options we will look at is lazy loading. Lazy loading is useful if all of the related data will not be needed immediately. For example, if our application called for a web page to show all accounts, then a sales representative (our user) could click on an account to view all of the orders for that account, and then click on an order to view all of the details for that order. All we need in this case is a single list at any time. This is a reasonable use of lazy loading. To use lazy loading, we need to edit the SqlMapConfig.xml file to enable it by changing the lazyLoadingEnabled attribute to true in the <setting> element. If you want to use the cglib enhanced version of lazy loading, you will want to download it, add it to your application s classpath, and change the enhancementEnabled attribute of the <setting> element to true as well. One thing to note is that this is a global setting, so all of the mapped statements in the SQL map will use lazy loading if these properties are enabled. Once we have enabled lazy loading, we can get to more reasonable numbers for object creation and database I/O. For one user to get down to the order
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Using advanced query techniques
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detail level, we have three queries (one for the accounts, one for the orders, and one for the order details), and the application would only create 2,025 objects (1,000 accounts, 1,000 orders, and 25 order details). This is all done without any changes to the application code and only minor changes to the XML configuration of iBATIS. In one totally nonscientific test that we performed, using a non-lazy loading relationship to load the data took over three times as long to get the first list than the lazy version. However, to get all of the data, the lazy version took about 20 percent longer than the non-lazy loader. Obviously, this will depend greatly on the amount of data being loaded, and as with most things, your mileage may vary. Experience is the best guide. There are times when you do not want to defer the loading of all the data but want it all loaded when the data is first requested. In those cases, you can use the technique described in the next section, which will do just that, and do it all in one single query instead of several. This next approach avoids the N+1 Selects problem.
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