Hibernate entities without underlying tables

Question

I have a really ugly legacy database system that I need to integrate with. Essentially I'm doing some read only reporting on the system, and I don't want to set up a thousand entities representing each of the tables that I'm working on. Instead, I'd like to just define an Entity for each of the report-types that I generate (essentially a union of a bunch of columns from different tables), and then let hibernate map from the nasty (many joined, many unioned) sql query to a list of such entities.

The question is: can I create an entity that doesn't have an underlying table, and use a sql statement to populate a list of said entities?

Thanks!

Answer 1

We do that sort of thing all the time - and here is how we do it:

1. Define a simple bean-like object to represent each row of output in your report:

   public class CityStateRevenueReport {
   
       private String mId;
       private String mState;
       private String mCity;
       private Double mRevenue;
   
       public String getId() { return mId; }
       public void setId(String i) { mId = i; }
       public String getState() { return mState; }
       public void setState(String s) { mState = s; }
       public String getCity() { return mCity; }
       public void setCity(String c) { mCity = c; }
       public Double getReveneue() { return mRevenue; }
       public void setRevneue(Double d) { mRevenue = d; }
   }
   

2. Define a hibernate mapping file, CityStateRevneueReport.hbm.xml:

   <?xml version="1.0" ?>
   <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd&quot;>
   <hibernate-mapping>
       <class entity-name="CityStateRevenueReport">
           <id name="Id" type="java.lang.String" column="report_id">
               <generator class="sequence" />
           </id>
           <property name="city" type="string" column="city" />
           <property name="state" type="string" column="state" />
           <property name="revenue" type="double" column="revenue" />
       </class>
       <sql-query name="runReport">
           <![CDATA[ 
           SELECT {r.*} FROM 
               (select some_id_value as report_id, 
                       state_abbreviation as state, 
                       city_name as city, 
                       dollar_amount as revenue 
                  from -- tables, joins, other SQL insanity 
                ) r
           ]]>
           <return alias="r" class="CityStateRevenueReport" />
       </sql-query>
   </hibernate-mapping>
   

3. Then run the query and populate instances:

   public List<CityStateRevenueReport> runReport() {
   
       List<CityStateRevenueReport> reports = 
                                new ArrayList<CityStateRevenueReport>();
       List<HashMap> maps = session.getNamedQuery("runReport").list()
       for ( HashMap map : results ) {
           CityStateRevenueReport report = new CityStateRevenueReport();
           report.setState(map.get("state"));
           report.setCity(map.get("city"));
           report.setRevenue(Double.parseDouble(map.get("revenue"));
           reports.add(report);
       }
       return reports;
   }
   

Answer 2

Use an Entity query:

http://docs.jboss.org/hibernate/core/3.3/reference/en/html/querysql.html#d0e13696

sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").addEntity(Cat.class);

Or create a view in the database and map against that.

Here's a longer tutorial using addEntity:

http://timezra.blogspot.com/2009/05/mapping-hibernate-entities-to-views.html

private Collection<AuthorAggregate> findByFirstName() {
    return sessionFactory.getCurrentSession() //
            .createSQLQuery(AUTHORS_BY_FIRST_NAME) // 
            .addEntity(AuthorAggregate.class) //
            .list();
}

Answer 3

If you're free to choose hibernate or not, I suggest you take a look at Spring JDBC. it's lighter than hibernate and does the job right. With your requirements it will suit nicely into the picture.