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Fine-grained models and mappings
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You can also control access strategies on the property level in Hibernate XML mappings with the access attribute:
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<property name="description" column="DESCR" access="field"/>
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Or, you can set the access strategy for all class mappings inside a root <hibernatemapping> element with the default-access attribute. Another strategy besides field and property access that can be useful is noop. It maps a property that doesn t exist in the Java persistent class. This sounds strange, but it lets you refer to this virtual property in HQL queries (in other words, to use the database column in HQL queries only). If none of the built-in access strategies are appropriate, you can define your own customized property-access strategy by implementing the interface org.hibernate.property.PropertyAccessor. Set the (fully qualified) class name on the access mapping attribute or @AccessType annotation. Have a look at the Hibernate source code for inspiration; it s a straightforward exercise. Some properties don t map to a column at all. In particular, a derived property takes its value from an SQL expression. Using derived properties The value of a derived property is calculated at runtime by evaluating an expression that you define using the formula attribute. For example, you may map a totalIncludingTax property to an SQL expression:
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<property name="totalIncludingTax" formula="TOTAL + TAX_RATE * TOTAL" type="big_decimal"/>
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The given SQL formula is evaluated every time the entity is retrieved from the database (and not at any other time, so the result may be outdated if other properties are modified). The property doesn t have a column attribute (or subelement) and never appears in an SQL INSERT or UPDATE, only in SELECTs. Formulas may refer to columns of the database table, they can call SQL functions, and they may even include SQL subselects. The SQL expression is passed to the underlying database as is; this is a good chance to bind your mapping file to a particular database product, if you aren t careful and rely on vendor-specific operators or keywords. Formulas are also available with a Hibernate annotation:
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@org.hibernate.annotations.Formula("TOTAL + TAX_RATE * TOTAL") public BigDecimal getTotalIncludingTax() {
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Mapping persistent classes
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return totalIncludingTax; }
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The following example uses a correlated subselect to calculate the average amount of all bids for an item:
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<property name="averageBidAmount" type="big_decimal" formula= "( select AVG(b.AMOUNT) from BID b where b.ITEM_ID = ITEM_ID )"/>
Notice that unqualified column names refer to columns of the table of the class to which the derived property belongs. Another special kind of property relies on database-generated values. Generated and default property values Imagine a particular property of a class has its value generated by the database, usually when the entity row is inserted for the first time. Typical database-generated values are timestamp of creation, a default price for an item, and a trigger that runs for every modification. Typically, Hibernate applications need to refresh objects that contain any properties for which the database generates values. Marking properties as generated, however, lets the application delegate this responsibility to Hibernate. Essentially, whenever Hibernate issues an SQL INSERT or UPDATE for an entity that has defined generated properties, it immediately does a SELECT afterwards to retrieve the generated values. Use the generated switch on a property mapping to enable this automatic refresh:
<property name="lastModified" column="LAST_MODIFIED" update="false" insert="false" generated="always"/>
Properties marked as database-generated must additionally be noninsertable and nonupdateable, which you control with the insert and update attributes. If both are set to false, the property s columns never appear in the INSERT or UPDATE statements the property value is read-only. Also, you usually don t add a public setter method in your class for an immutable property (and switch to field access). With annotations, declare immutability (and automatic refresh) with the @Generated Hibernate annotation:
Fine-grained models and mappings
@Column(updatable = false, insertable = false) @org.hibernate.annotations.Generated( org.hibernate.annotations.GenerationTime.ALWAYS ) private Date lastModified;
The settings available are GenerationTime.ALWAYS and GenerationTime.INSERT, and the equivalent options in XML mappings are generated="always" and generated="insert". A special case of database-generated property values are default values. For example, you may want to implement a rule that every auction item costs at least $1. First, you d add this to your database catalog as the default value for the INITIAL_PRICE column:
create table ITEM ( ... INITIAL_PRICE number(10,2) default '1', ... );
If you use Hibernate s schema export tool, hbm2ddl, you can enable this output by adding a default attribute to the property mapping:
<class name="Item" table="ITEM" dynamic-insert="true" dynamic-update="true"> ... <property name="initialPrice" type="big_decimal"> <column name="INITIAL_PRICE" default="'1'" generated="insert"/> </property> ... </class>
Note that you also have to enable dynamic insertion and update statement generation, so that the column with the default value isn t included in every statement if its value is null (otherwise a NULL would be inserted instead of the default value). Furthermore, an instance of Item that has been made persistent but not yet flushed to the database and not refreshed again won t have the default value set on the object property. In other words, you need to execute an explicit flush:
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