java qr code generator library free Mapping collections and entity associations in Java

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Mapping collections and entity associations
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Making the association bidirectional
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You want to be able to easily fetch all the bids for a particular item without an explicit query, by navigating and iterating through the network of persistent objects. The most convenient way to do this is with a collection property on Item: anItem.getBids().iterator(). (Note that there are other good reasons to map a collection of entity references, but not many. Always try to think of these kinds of collection mappings as a feature, not a requirement. If it gets too difficult, don t do it.) You now map a collection of entity references by making the relationship between Item and Bid bidirectional. First add the property and scaffolding code to the Item class:
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public class Item { ... private Set bids = new HashSet(); public void setBids(Set bids) { this.bids = bids; } public Set getBids() { return bids; } public void addBid(Bid bid) { bid.setItem(this); bids.add(bid); } ... }
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You can think of the code in addBid() (a convenience method) as implementing a managed association in the object model! (We had more to say about these methods in chapter 3, section 3.2, Implementing the domain model. You may want to review the code examples there.) A basic mapping for this one-to-many association looks like this:
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<class name="Item" table="ITEM"> ... <set name="bids"> <key column="ITEM_ID"/>
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<one-to-many class="Bid"/> </set> </class>
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If you compare this with the collection mappings earlier in this chapter, you see that you map the content of the collection with a different element, <one-tomany>. This indicates that the collection contains not value type instances, but references to entity instances. Hibernate now knows how to treat shared references and the lifecycle of the associated objects (it disables all the implicit dependent lifecycle of value type instances). Hibernate also knows that the table used for the collection is the same table the target entity class is mapped to the <set> mapping needs no table attribute. The column mapping defined by the <key> element is the foreign key column ITEM_ID of the BID table, the same column you already mapped on the other side of the relationship. Note that the table schema didn t change; it s the same as it was before you mapped the many side of the association. There is, however, one difference: The not null="true" attribute is missing. The problem is that you now have two different unidirectional associations mapped to the same foreign key column. What side controls that column At runtime, there are two different in-memory representations of the same foreign key value: the item property of Bid and an element of the bids collection held by an Item. Suppose the application modifies the association, by, for example, adding a bid to an item in this fragment of the addBid() method:
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bid.setItem(item); bids.add(bid);
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This code is fine, but in this situation, Hibernate detects two changes to the inmemory persistent instances. From the point of view of the database, only one value has to be updated to reflect these changes: the ITEM_ID column of the BID table. Hibernate doesn t transparently detect the fact that the two changes refer to the same database column, because at this point you ve done nothing to indicate that this is a bidirectional association. In other words, you ve mapped the same column twice (it doesn t matter that you did this in two mapping files), and Hibernate always needs to know about this because it can t detect this duplicate automatically (there is no reasonable default way it could be handled).
Mapping collections and entity associations
You need one more thing in the association mapping to make this a real bidirectional association mapping. The inverse attribute tells Hibernate that the collection is a mirror image of the <many-to-one> association on the other side:
<class name="Item" table="ITEM"> ... <set name="bids" inverse="true"> <key column="ITEM_ID"/> <one-to-many class="Bid"/> </set> </class>
Without the inverse attribute, Hibernate tries to execute two different SQL statements, both updating the same foreign key column, when you manipulate the link between two instances. By specifying inverse="true", you explicitly tell Hibernate which end of the link it should not synchronize with the database. In this example, you tell Hibernate that it should propagate changes made at the Bid end of the association to the database, ignoring changes made only to the bids collection. If you only call anItem.getBids().add(bid), no changes are made persistent! You get what you want only if the other side, aBid.setItem(anItem), is set correctly. This is consistent with the behavior in Java without Hibernate: If an association is bidirectional, you have to create the link with pointers on two sides, not just one. It s the primary reason why we recommend convenience methods such as addBid() they take care of the bidirectional references in a system without container-managed relationships. Note that an inverse side of an association mapping is always ignored for the generation of SQL DDL by the Hibernate schema export tools. In this case, the ITEM_ID foreign key column in the BID table gets a NOT NULL constraint, because you ve declared it as such in the noninverse <many-to-one> mapping. (Can you switch the inverse side The <many-to-one> element doesn t have an inverse attribute, but you can map it with update="false" and insert="false" to effectively ignore it for any UPDATE or INSERT statements. The collection side is then noninverse and considered for insertion or updating of the foreign key column. We ll do this in the next chapter.)
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