CHAPTER 8 Legacy databases (Best web hosting site) and custom SQL one-to-one,
CHAPTER 8 Legacy databases and custom SQL one-to-one, is needed to single out one particular Bid instance as the winning bid. You map the first association because you d like to be able to get all the bids for an auctioned item by calling anItem.getBids(). The second association allows you to call anItem.getSuccessfulBid(). Logically, one of the elements in the collection is also the successful bid object referenced by getSuccessfulBid(). The first association is clearly a bidirectional one-to-many/many-to-one association, with a foreign key ITEM_ID in the BID table. (If you haven t mapped this before, look at chapter 6, section 6.4, Mapping a parent/children relationship. ) The one-to-one association is more difficult; you can map it several ways. The most natural is a uniquely constrained foreign key in the ITEM table referencing a row in the BID table the winning row, for example a SUCCESSFUL_ BID_ID column. Legacy schemas often need a mapping that isn t a simple foreign key relationship. Mapping a formula join condition Imagine that each row in the BID table has a flag column to mark the winning bid, as shown in figure 8.3. One BID row has the flag set to true, and all other rows for this auction item are naturally false. Chances are good that you won t find a constraint or an integrity rule for this relationship in a legacy schema, but we ignore this for now and focus on the mapping to Java classes. To make this mapping even more interesting, assume that the legacy schema didn t use the SQL BOOLEAN datatype but a CHAR(1) field and the values T (for true) and F (for false) to simulate the boolean switch. Your goal is to map this flag column to a successfulBid property of the Item class. To map this as an object reference, you need a literal join condition, because there is no foreign key Hibernate can use for a join. In other words, for each ITEM row, you need to join a row from the BID table that has the SUCCESSFUL flag set to T. If there is no such row, the anItem.getSuccessfulBid() call returns null. Let s first map the Bid class and a successful boolean property to the SUCCESSFUL database column: Figure 8.3 The winning bid is marked with the SUCCESSFUL column flag.
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