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Naming entities for querying
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By default, all class names are automatically imported into the namespace of the Hibernate query language, HQL. In other words, you can use the short class names without a package prefix in HQL, which is convenient. However, this autoimport can be turned off if two classes with the same name exist for a given SessionFactory, maybe in different packages of the domain model. If such a conflict exists, and you don t change the default settings, Hibernate won t know which class you re referring to in HQL. You can turn off auto-import
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Mapping persistent classes
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of names into the HQL namespace for particular mapping files with the autoimport="false" setting on the <hibernate-mapping> root element. Entity names can also be imported explicitly into the HQL namespace. You can even import classes and interfaces that aren t explicitly mapped, so a short name can be used in polymorphic HQL queries:
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<hibernate-mapping> <import class="auction.model.Auditable" rename="IAuditable"/> </hibernate-mapping>
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You can now use an HQL query such as from IAuditable to retrieve all persistent instances of classes that implement the auction.model.Auditable interface. (Don t worry if you don t know whether this feature is relevant to you at this point; we ll get back to queries later in the book.) Note that the <import> element, like all other immediate child elements of <hibernate-mapping>, is an application-wide declaration, so you don t have to (and can t) duplicate this in other mapping files. With annotations, you can give an entity an explicit name, if the short name would result in a collision in the JPA QL or HQL namespace:
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@Entity(name="AuctionItem") public class Item { ... }
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Now let s consider another aspect of naming: the declaration of packages.
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Declaring a package name
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All the persistent classes of the CaveatEmptor application are declared in the Java package auction.model. However, you don t want to repeat the full package name whenever this or any other class is named in an association, subclass, or component mapping. Instead, specify a package attribute:
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<hibernate-mapping package="auction.model"> <classname="Item" table="ITEM"> ... </class> </hibernate-mapping>
Now all unqualified class names that appear in this mapping document will be prefixed with the declared package name. We assume this setting in all mapping examples in this book and use unqualified names for CaveatEmptor model classes. Names of classes and tables must be selected carefully. However, a name you ve chosen may be reserved by the SQL database system, so the name has to be quoted.
Class mapping options
Quoting SQL identifiers
By default, Hibernate doesn t quote table and column names in the generated SQL. This makes the SQL slightly more readable, and it also allows you to take advantage of the fact that most SQL databases are case insensitive when comparing unquoted identifiers. From time to time, especially in legacy databases, you encounter identifiers with strange characters or whitespace, or you wish to force case sensitivity. Or, if you rely on Hibernate s defaults, a class or property name in Java may be automatically translated to a table or column name that isn t allowed in your database management system. For example, the User class is mapped to a USER table, which is usually a reserved keyword in SQL databases. Hibernate doesn t know the SQL keywords of any DBMS product, so the database system throws an exception at startup or runtime. If you quote a table or column name with backticks in the mapping document, Hibernate always quotes this identifier in the generated SQL. The following property declaration forces Hibernate to generate SQL with the quoted column name "DESCRIPTION". Hibernate also knows that Microsoft SQL Server needs the variation [DESCRIPTION] and that MySQL requires `DESCRIPTION`.
<property name="description" column="`DESCRIPTION`"/>
There is no way, apart from quoting all table and column names in backticks, to force Hibernate to use quoted identifiers everywhere. You should consider renaming tables or columns with reserved keyword names whenever possible. Quoting with backticks works with annotation mappings, but it s an implementation detail of Hibernate and not part of the JPA specification.
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