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Mapping flat table to object model...

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nw60312

Post subject: Mapping flat table to object model...

Posted: Thu Jul 06, 2006 12:31 pm

Newbie

Joined: Wed Jun 08, 2005 11:42 pm
Posts: 4

I have a single flat table that needs need to get mapped into a small object model, ~6 levels. The mapping only needs to be read only and I have no choice in the underlying data structure, I can't change it. I would like to use hibernate but I am not sure if/how to do it. Basically it's a heirarchical datamodel that has been flattened down to a single table. Obviously this will requried many distinct queries to get it to work. I have A mapped with a composite ID and I can get the it queried properly. The problem crops up with B, which should be related to A. With the mapping of B below I can load them properly by themselves, the problem comes up when I try to map A's list of Bs, and the parent reference back to A in B. I can't get this to work, see exception below. Any suggestions as to how to make this work? Since my data is flat I also need the collection to be distinct and I can't seem to find anyway to do that? I would really prefer to use Hibernate however I am wondering if due to the data model if I would be better off just doing this in JDBC?

Thanks!

-Nate

Hibernate version:3.1.3

Mapping documents:

Table structure (1 flat table - file_level_metadata):

Code:

date | time | validTime | ........(~3 more levels)

Code:

<hibernate-mapping>

    <class name="test.A" table="file_level_metadata" >
       <composite-id>
          <key-property name="date" />
          <key-property name="time" />
       </composite-id>
       
       <bag  name="bList" cascade="none"  lazy="false" >
           <key>
              <column name="date" />
              <column name="time" />
           </key>
         <one-to-many class="test.B" />
      </bag>
    </class>
    
    <class name="test.B" table="file_level_metadata" >
       <composite-id>
          <key-property name="date" />
          <key-property name="time" />
          <key-property name="validTime" />
       </composite-id>
        <!-- Would like to have a reference back to the parent A... -->
    </class>

</hibernate-mapping>

Code:

public class A implements Serializable{
   
   private static final long serialVersionUID = 1L;
   
   private Date date;
   private Date time;
   
   private List<B> bList;

        // Mutators removed for brevity...

   @Override
   public int hashCode() {
      final int PRIME = 31;
      int result = 1;
      result = PRIME * result + ((date == null) ? 0 : date.hashCode());
      result = PRIME * result + ((time == null) ? 0 : time.hashCode());
      return result;
   }

   @Override
   public boolean equals(Object obj) {
      if (this == obj)
         return true;
      if (obj == null)
         return false;
      if (getClass() != obj.getClass())
         return false;
      final A other = (A) obj;
      if (date == null) {
         if (other.date != null)
            return false;
      } else if (!date.equals(other.date))
         return false;
      if (time == null) {
         if (other.time != null)
            return false;
      } else if (!time.equals(other.time))
         return false;
      return true;
   }
   
   
}

Code:

public class B implements Serializable {
   
   private static final long serialVersionUID = 1L;

   private A parentA;
   
   private Date date;
   private Date time;
   private short validTime;
   
        // Mutators removed for brevity...

   @Override
   public int hashCode() {
      final int PRIME = 31;
      int result = 1;
      result = PRIME * result + ((date == null) ? 0 : date.hashCode());
      result = PRIME * result + ((time == null) ? 0 : time.hashCode());
      result = PRIME * result + validTime;
      return result;
   }

   @Override
   public boolean equals(Object obj) {
      if (this == obj)
         return true;
      if (obj == null)
         return false;
      if (getClass() != obj.getClass())
         return false;
      final B other = (B) obj;
      if (date == null) {
         if (other.date != null)
            return false;
      } else if (!date.equals(other.date))
         return false;
      if (time == null) {
         if (other.time != null)
            return false;
      } else if (!time.equals(other.time))
         return false;
      if (validTime != other.validTime)
         return false;
      return true;
   }
   
}

Code between sessionFactory.openSession() and session.close():

Full stack trace of any exception that occurs:
Foreign key (FK974916ADCC1CB060:file_level_metadata [date,time])) must have same number of columns as the referenced primary key (file_level_metadata [date,time,validTime])
]

Code:

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mojarrell

Post subject:

Posted: Mon Jul 10, 2006 12:08 pm

Beginner

Joined: Mon Jul 26, 2004 4:29 pm
Posts: 45
Location: TX, USA

Quote:

I have a single flat table that needs need to get mapped into a small object model, ~6 levels.

I guess my first question is: Does it really need to to be mapped hierarchically? What if you mapped all your A's and then created a lot of functions to give you lists of the A's that you needed? Take an example of a table of locations in the world. It could look like this:

Code:

-- drop the tables first
drop table location;

-- create the tables
create table locations (
   location_id identity
   continent varchar(30)
  ,country varchar(30)
  ,region varchar(30)
  ,province varchar(30)
  ,city varchar(30)
  ,street varchar(30)
  ,street_number integer);

insert into locations values (null, 'North America', 'USA', 'North', 'Michigan', 'Detroit', 'Elm', 1033);
insert into locations values (null, 'North America', 'USA', 'North', 'Michigan', 'Detroit', 'Elm', 1034);
insert into locations values (null, 'North America', 'USA', 'North', 'Michigan', 'Detroit', 'Elm', 1035);
insert into locations values (null, 'North America', 'USA', 'North', 'Michigan', 'Detroit', 'Elm', 1036);
insert into locations values (null, 'North America', 'USA', 'North', 'Michigan', 'Detroit', 'Elm', 1037);
insert into locations values (null, 'North America', 'USA', 'North', 'Michigan', 'Detroit', 'Maple', 4);
insert into locations values (null, 'North America', 'USA', 'North', 'Michigan', 'Detroit', 'Maple', 5);
insert into locations values (null, 'North America', 'USA', 'North', 'Michigan', 'Detroit', 'Maple', 6);
insert into locations values (null, 'North America', 'USA', 'North', 'Michigan', 'Detroit', 'Maple', 7);
insert into locations values (null, 'North America', 'USA', 'North', 'Michigan', 'Detroit', 'Maple', 8);
insert into locations values (null, 'North America', 'USA', 'North', 'Michigan', 'Detroit', 'Maple', 9);
insert into locations values (null, 'North America', 'USA', 'North', 'Michigan', 'Detroit', 'Maple', 10);

commit;

You could create functions to get lists of location objects based on criteria:

Code:

public interface LocationDao {
    public List locationsInContinent(String continent);
}

You could also pass in a list and have a function that would filter based on a criteria.

I know it's not true object orientation, but it would avoid the multi-level mapping that just seems like a nightmare to me...

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