Need .02 on modeling objects with BLOB attributes

devonl · **Posted:** Fri Jun 10, 2005 2:13 am

We're storing files in the database. Mapping files work fine and we can store and retrieve File Objects. However, if a User Object has a collection of files (File Objects) they own, providing them with a list of files causes the collection to be loaded with all of the data from the [image] column instantiated. Obviously this is unacceptable.

Any ideas on how to get around this? Perhaps a 1-1 relationship with a FileObject mapped with outer-join="false"? Not sure I like that, but I'd like to be able to load a collection of File Objects and only load the BLOB (content) when the user asks for it.

TIA,

-devon

sergey · **Posted:** Fri Jun 10, 2005 1:00 pm

Just using lazy="true" for FileObjects, and lazy="true" for User.Files isn't enough?

devonl · **Posted:** Sat Jun 11, 2005 1:46 am

thanks sergey. that will work. i just though ti might appear to be a hack that i was creating a new object (FileObject) to get around the problem of loading the many MB of data.

-devon

rolandz · **Posted:** Wed Jun 15, 2005 4:44 am

devonl wrote:

thanks sergey. that will work. i just though ti might appear to be a hack that i was creating a new object (FileObject) to get around the problem of loading the many MB of data.

This all looks fine, but... How do you realize loading (even if a collection is lazy) all files that belong to one user. This still means loading of tons of megabytes...

I am asking 'cause have same problem... And I did not find satisfying solution yet :-(

devonl · **Posted:** Wed Jun 15, 2005 11:57 am

The model makes a distinction between File Object and FileContent Object. The Member has a collection of File Objects that is essentially a collection meta data objects. Each File Object has a lazily-loaded FileContent Object that I'll only load a FileContent Object when it is requested specifically by a URL.

Hope that makes sense.

-devon

jnapier · **Joined:** Sat Sep 10, 2005 3:46 pm **Posts:** 178

This design seems decent when a file is navigated to for reading purposes, but what about when a file is overwritten. If you need to overwite the binary data, as soon as you access the file content object at the end of the one to one lazy load, the binary data will be pulled from the database and loaded into the object, but all you really wanted to do was overwite the existing data. Am I wrong on this? It seems like NHibernate should handle blobs differently since they are so different in nature.

I would like to see blobs as a proxy that were only retrieved when the get on the property was called.

s.rottem · **Posted:** Tue Nov 01, 2005 6:31 am

Hmmm...that's actually a pretty interesting idea. Does anyone know if this is possible given the current architecture?

Symon.

jnapier · **Joined:** Sat Sep 10, 2005 3:46 pm **Posts:** 178

I can not get the lazy loading of my one to one file object to work. The file objects are loaded when I try to get the collection.

Here is my mapping:

Code:

<class name="Document" table="Document" lazy="true">      
      <!--ID-->
      <id name="Id" column="DocumentId" unsaved-value="00000000-0000-0000-0000-000000000000" access="nosetter.camelcase-underscore">
         <generator class="guid" />
      </id>
            
      <!--PROPERTIES-->
      <property name="Name" column="Name" length="50" not-null="true" access="nosetter.camelcase-underscore" />
   
      <!--ASSOSCIATIONS-->
      <one-to-one name="DataFile" class="DataFile" cascade="all" access="field.camelcase-underscore"  outer-join="false" /> 
         
   </class>

Code:

<class name="DataFile" table="DataFile" lazy="true">      
      <!--ID-->
      <id name="Id" column="DocumentId" access="field.camelcase-underscore">
         <generator class="foreign">
            <param name="property">Document</param>
         </generator>
      </id>
            
      <!--PROPERTIES-->
      <property name="Contents" column="FileContent" type="BinaryBlob" />
   
      <!--ASSOSCIATIONS-->
      <one-to-one name="Document" class="Document" access="field.camelcase-underscore" constrained="true" /> 
         
   </class>

Is ther something wrong with my mapping or is lazy loading of the one-to-one just not working correctly?

s.rottem · **Posted:** Thu Nov 10, 2005 4:47 pm

Nope, nothing wrong - this is the expected behavior. Loading a one-to-one object always loads it's connected object.

The best way around this is actually to use a one-to-one mapping on one end and a many-to-one mapping with unique="true". The many-to-one end will be the "parent" and the one-to-one end will be the "child".

The child object's id needs to be set using the "foreign" generator.

Code:

   <class name="Parent" table="Parents" lazy="true">
      
      <id name="Id" column="ParentID" unsaved-value="0" >
         <generator class="native" />
      </id>
         
      <many-to-one name="MyChild" class="Child" column="ChildID" unique="true"/>
            
   </class>

Code:

   <class name="Child" table="Children" lazy="true">
      
      <id name="Id" column="ChildID" >
         <generator class="foreign">
            <param name="property">MyParent</param>
         </generator>
      </id>
            
      <one-to-one name="MyParent" class="Parent" />

   </class>

Note that this this also allows you to have a 1 -> 0 or 1 relationship.

Hope this helps.

Cheers,

Symon.

jnapier · **Joined:** Sat Sep 10, 2005 3:46 pm **Posts:** 178

Thanks Symon. This should work, the only thing that is lame is that I have to have the child Id column in my parent table as you have laid out, but I guess there is no other way.

I guess to get this really working well, we will have to wait for lazy loaded properties.

Thanks again.

s.rottem · **Posted:** Fri Nov 11, 2005 8:19 am

Actually, my mistake - I was typing code late at night and wasn't quite as thorough as I should have been... :oops:

What I should have given for the Parent mapping is:

Code:

<class name="Parent" table="Parents" lazy="true">
      
      <id name="Id" column="ParentID" unsaved-value="0" >
         <generator class="native" />
      </id>
         
      <many-to-one name="MyChild" class="Child" column="ParentID" unique="true"/>
            
   </class>

Note that the only difference is that the many-to-one in the parent uses it's own ParentID column as the column for the child. I actually got the information on how to do this from a posting in another thread from the Hibernate forums (http://forum.hibernate.org/viewtopic.php?t=935557&highlight=lazy+load+onetoone) which covered a whole lot of other stuff. This is thanks to Regis Pires Magalhaes who made a posting at the bottom of the above thread.

Good luck!

Cheers,

Symon.

P.S. There's some explanation of why lazy loading isn't supported for one-to-one here: http://www.hibernate.org/162.html. Regis replied to this wiki entry too.