Do I need DTOs for my DAOs?

arfff · **Posted:** Sat Feb 21, 2004 4:54 pm

I'm building a data services layer for a multitier application. A DAO model will be used to map between domain objects and the database and hibernate is the engine.

I can't figure out how to get hibernate to retrieve the keys instead of the actual referenced objects. Perhaps if I want to return only references to the caller in the other tier I need to copy the objects into DTOs to map the references not actual referencees?

For example, here's a simple slice of the data model:

Code:

  public class Customer {
    public CustomerKey key; 
    public CustomerFamilyKey cfKey; 
    public String name; 
  }

  public class CustomerKey {
    public long id; 
    public int version; 
  } 

and customers have a many-to-one relationship with a CustomerFamily:

Code:

  public class CustomerFamily { 
    public CustomerFamilyKey key; 
    public String name; 
  }

  public class CustomerFamilyKey { 
    public long id; 
    public int version; 
  }

Note that both objects are versioned and have composite keys. Not sure if that matters.

In the object that I return to the calling tier (caller -> StatelessSessionBean -> DAO -> RDBMS) I am intending to use a style of interface that provides on the the primary key of the parent/family not a reference to the actual object:

Code:

  public class Customer {
    public CustomerFamilyKey key; 
       .
       .
  

Hibernate seems to want to make this more like:

Code:

 public class Customer { 
    public CustomerFamily family; 
      . 
      .

is there even a way to declare that what I want is the list of associated ids instead of the associated objects themselves?

Or, do I need to have the Hibernate managed objects give me back the connected objects and then I need to copy things over into more referenced-by-id DTOs?

Thanks in advance.

Here are the snippets of .hbm code I'm using if that helps:

Code:

  <hibernate-mapping package="foo">
    <class name="Customer" table="CUSTOMER">

      <composite-id name="primaryKey" class="Customer" unsaved-value="any">
        <key-property name="id" column="id"/>
        <key-property name="version" column="version"/>
      </composite-id>

      <property name="name"/>

       <many-to-one name="family" class="CustomerFamilyVO" cascade="all">
           <column name="family_id"/>
           <column name="family_version"/>
       </many-to-one>

    </class>
  </hibernate-mapping>

and

Code:

  <hibernate-mapping pacakge="foo">
    <class name="CustomerFamily" table="CUSTOMER_FAMILY">

      <composite-id name="primaryKey" unsaved-value="any">
        <key-property name="id" column="id"/>
        <key-property name="version" column="version"/>
      </composite-id>

      <property name="name"/>

    </class>
  </hibernate-mapping>

michael · **Posted:** Sat Feb 21, 2004 7:44 pm

You should not use "id-references" at all. Instead, you should rather use proxys and lazy loading collections to partition your object graph. Hibernate is really all about not having such crappy DTO patterns, which IMHO are a real ugly thing produced by the shortcomings of EJBs, and now people think having DTOs is somehow a "good thing".

emmanuel · **Posted:** Tue Feb 24, 2004 2:14 pm

Plus the O in ORM means Object. Hibernate is done not to bother with ids-references. Use it that way and you'll find it fantastic in your domain model.