Overview of persisting a domain model - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 143

:Pending
Order

All requests

Update payment information

PendingOrder-Restaurant

PendingOrder-PendingOrderLineItem

: PendingOrder
LineItem

:Restaurant

Update
quantities

PendingOrderLineItem-MenuItem

Restaurant-MenuItem

:MenuItem

All requests

Figure 4.4 The relationships traversed by each request in the Place Order use case

The application only traverses the Restaurant - MenuItem relationship when han-
dling the update quantities requests and only traverses PendingOrder - Pending-
OrderLineItem when handling some requests, such as updating payment
information.
Once we have determined when each relationship is traversed, we can then
configure eager loading for some of them. We can statically configure eager load-
ing for those relationships that are always traversed and dynamically configure
eager loading for the other relationships. However, we do not want to blindly use
eager loading because that can sometimes reduce performance. For example,
eagerly loading two one-to-many relationships will create a result set containing
the Cartesian product of both relationships. If the result set is extremely large,
then it is usually better to load one of the relationships lazily. It's important to use
eager loading carefully and do a lot of performance testing.
Let's see how to configure eager loading for the Place Order use case.

Configuring eager loading statically
If we statically configure eager loading for the PendingOrder - Restaurant and Pend-
ingOrderLineItem - MenuItem relationships, then the persistence framework will always
load the related objects. It will always load the pending order's restaurant with the