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Retrieving objects efficiently
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would be thrown away immediately (Hibernate flattens the tabular result set to build the object graph). You may fetch as many one-to-one or many-toone associations as you like.
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If you fetch a collection, Hibernate doesn t return a distinct result list. For example, an individual Item might appear several times in the result List, if you outer-join fetch the bids. You ll probably need to make the results distinct yourself using, for example: distinctResults = new HashSet(resultList);. A Set doesn t allow duplicate elements.
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This is how Hibernate implements what we call runtime association fetching strategies, a powerful feature that is essential for achieving high performance in ORM. Let s continue with the other join operations.
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7.3.3 Using aliases with joins
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We ve already discussed the role of the where clause in expressing restriction. Often, you ll need to apply restriction criteria to multiple associated classes (joined tables). If we want to do this using an HQL from clause join, we need to assign an alias to the joined class:
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from Item item join item.bids bid where item.description like '%gc%' and bid.amount > 100
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This query assigns the alias item to the class Item and the alias bid to the joined Item s bids. We then use both aliases to express our restriction criteria in the where clause. The resulting SQL is as follows:
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select I.DESCRIPTION, I.CREATED, I.SUCCESSFUL_BID, B.BID_ID, B.AMOUNT, B.ITEM_ID, B.CREATED from ITEM I inner join BID B on I.ITEM_ID = B.ITEM_ID where I.DESCRIPTION like '%gc%' and B.AMOUNT > 100
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The query returns all combinations of associated Bids and Items. But unlike a fetch join, the bids collection of the Item isn t initialized by the query! So what do we mean by a combination here We mean an ordered pair: (bid, item). In the query result, Hibernate represents an ordered pair as an array. Let s discuss a full code example with the result of such a query:
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Query q = session.createQuery("from Item item join item.bids bid"); Iterator pairs = q.list().iterator();
Joining associations
while ( pairs.hasNext() ) { Object[] pair = (Object[]) pairs.next(); Item item = (Item) pair[0]; Bid bid = (Bid) pair[1]; }
Instead of a List of Items, this query returns a List of Object[] arrays. At index 0 is the Item, and at index 1 is the Bid. A particular Item may appear multiple times, once for each associated Bid. This is all different from the case of a query with an eager fetch join. The query with the fetch join returned a List of Items, with initialized bids collections. If we don t want the Bids in the query result, we can specify a select clause in HQL. This clause is optional (it isn t in SQL), so we only have to use it when we aren t satisfied with the result returned by default. We use the alias in a select clause to retrieve only the selected objects:
select item from Item item join item.bids bid where item.description like '%gc%' and bid.amount > 100
Now the generated SQL looks like this:
select I.DESCRIPTION, I.CREATED, I.SUCCESSFUL_BID, from ITEM I inner join BID B on I.ITEM_ID = B.ITEM_ID where I.DESCRIPTION like '%gc%' and B.AMOUNT > 100
The query result contains just Items, and because it s an inner join, only Items that have Bids:
Query q = session.createQuery("select i from Item i join i.bids b"); Iterator items = q.list().iterator(); while ( items.hasNext() ) { Item item = (Item) items.next(); }
As you can see, using aliases in HQL is the same for both direct classes and joined associations. We assign aliases in the from clause and use them in the where and in the optional select clause. The select clause in HQL is much more powerful; we discuss it in detail later in this chapter. There are two ways to express a join in the Criteria API; hence there are two ways to use aliases for restriction. The first is the createCriteria() method of the Criteria interface. It means that you can nest calls to createCriteria():
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