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Chapter 18. Filtering data

Hibernate3 provides an innovative new approach to handling data with "visibility" rules. A Hibernate filter is a global, named, parameterized filter that can be enabled or disabled for a particular Hibernate session.

18.1. Hibernate filters
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Hibernate3 has the ability to pre-define filter criteria and attach those filters at both a class level and a collection level. A filter criteria allows you to define a restriction clause similar to the existing "where" attribute available on the class and various collection elements. These filter conditions, however, can be parameterized. The application can then decide at runtime whether certain filters should be enabled and what their parameter values should be. Filters can be used like database views, but they are parameterized inside the application.
In order to use filters, they must first be defined and then attached to the appropriate mapping elements. To define a filter, use the <filter-def/> element within a <hibernate-mapping/> element: 
<filter-def name="myFilter">
    <filter-param name="myFilterParam" type="string"/>
</filter-def>
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This filter can then be attached to a class: 
<class name="myClass" ...>
    ...
    <filter name="myFilter" condition=":myFilterParam = MY_FILTERED_COLUMN"/>
</class>
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Or, to a collection: 
<set ...>
    <filter name="myFilter" condition=":myFilterParam = MY_FILTERED_COLUMN"/>
</set>
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Or, to both or multiples of each at the same time.
The methods on Session are: enableFilter(String filterName), getEnabledFilter(String filterName), and disableFilter(String filterName). By default, filters are not enabled for a given session. Filters must be enabled through use of the Session.enableFilter() method, which returns an instance of the Filter interface. If you used the simple filter defined above, it would look like this: 
session.enableFilter("myFilter").setParameter("myFilterParam", "some-value");
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Methods on the org.hibernate.Filter interface do allow the method-chaining common to much of Hibernate.
The following is a full example, using temporal data with an effective record date pattern: 
<filter-def name="effectiveDate">
    <filter-param name="asOfDate" type="date"/>
</filter-def>

<class name="Employee" ...>
...
    <many-to-one name="department" column="dept_id" class="Department"/>
    <property name="effectiveStartDate" type="date" column="eff_start_dt"/>
    <property name="effectiveEndDate" type="date" column="eff_end_dt"/>
...
    <!--
        Note that this assumes non-terminal records have an eff_end_dt set to
        a max db date for simplicity-sake
    -->
    <filter name="effectiveDate"
            condition=":asOfDate BETWEEN eff_start_dt and eff_end_dt"/>
</class>

<class name="Department" ...>
...
    <set name="employees" lazy="true">
        <key column="dept_id"/>
        <one-to-many class="Employee"/>
        <filter name="effectiveDate"
                condition=":asOfDate BETWEEN eff_start_dt and eff_end_dt"/>
    </set>
</class>
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In order to ensure that you are provided with currently effective records, enable the filter on the session prior to retrieving employee data: 
Session session = ...;
session.enableFilter("effectiveDate").setParameter("asOfDate", new Date());
List results = session.createQuery("from Employee as e where e.salary > :targetSalary")
         .setLong("targetSalary", new Long(1000000))
         .list();
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Even though a salary constraint was mentioned explicitly on the results in the above HQL, because of the enabled filter, the query will return only currently active employees who have a salary greater than one million dollars.
If you want to use filters with outer joining, either through HQL or load fetching, be careful of the direction of the condition expression. It is safest to set this up for left outer joining. Place the parameter first followed by the column name(s) after the operator.
After being defined, a filter might be attached to multiple entities and/or collections each with its own condition. This can be problematic when the conditions are the same each time. Using <filter-def/> allows you to definine a default condition, either as an attribute or CDATA: 
<filter-def name="myFilter" condition="abc > xyz">...</filter-def>
<filter-def name="myOtherFilter">abc=xyz</filter-def>
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This default condition will be used whenever the filter is attached to something without specifying a condition. This means you can give a specific condition as part of the attachment of the filter that overrides the default condition in that particular case.
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