# How to model Hibernate multi-table inheritance with ActiveRe

**URL:** <https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371>\
**Category:** Ruby\
**Created:** [June 28, 2006, 1:22pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371 "2006-06-28T13:22:29Z")\
**Posts on this page:** 16\
**Page:** 1

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 1:22pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/1 "2006-06-28T13:22:29Z")

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Hi

I have this “old” database mainly modeled for Hibernate. The java model  
has something called Asset. Asset is inherited by SomeAsset and  
AnotherAsset and the similar stuff is in the asset table and the  
specific data for SomeAsset is in table some\_asset and for AnotherAsset  
in another\_asset. This is then reflected in hibernate using a  
 element.

What would be the best way to model this in ActiveRecord? I cannot  
change the exisiting database structure beyond adding new columns (ie I  
cannot move values from the join tables back to the main table and use  
STI).

Could I implement method\_missing in Asset to just delegate to methods  
that might exist in SomeAsset or AnotherAsset? I only use duck typing so  
from a type perspective it would be fine. This would make it hard to  
override behaviour from Asset in subclasses though (since they aren’t  
subclasses…).

Is there another better solution?

Best Regards,

Marcus

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 2:53pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/2 "2006-06-28T14:53:48Z")

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On 6/28/06, Marcus A. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I have this “old” database mainly modeled for Hibernate…  
> [snip]

Depending on how your model looks, one idea would be to use RBatis  
which is a more flexible O/R mapper. If SQL doesn’t scare you that is.  
See [http://jutopia.tirsen.com/articles/2006/05/23](http://jutopia.tirsen.com/articles/2006/05/23) for more details.

/Peter

---

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 3:06pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/3 "2006-06-28T15:06:04Z")

</div>

Marcus A. wrote:

> change the exisiting database structure beyond adding new columns (ie I  
> cannot move values from the join tables back to the main table and use  
> STI).
> 
> Could I implement method\_missing in Asset to just delegate to methods  
> that might exist in SomeAsset or AnotherAsset? I only use duck typing so  
> from a type perspective it would be fine. This would make it hard to  
> override behaviour from Asset in subclasses though (since they aren’t  
> subclasses…).
> 
> Is there another better solution?  
> I’d have SomeAsset and AnotherAsset instances each hold a delegate Asset  
> object that they proxy any calls they can’t answer themselves to. I’ve  
> played around a little with trying to implement a full class table  
> inheritance scheme for AR on that basis, but never really had enough  
> need for it myself to polish it up.

---

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 4:31pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/4 "2006-06-28T16:31:52Z")

</div>

Marcus A. schrieb:

> STI).  
> I don’t know ActiveRecord, but are you allowed to add a (SQL) view to  
> your database schema? This way you might be able to use single table  
> inheritance. Some databases allow even DML Operations on views.

Regards,  
Pit

---

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 7:21pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/5 "2006-06-28T19:21:31Z")

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Alex Y. skrev:

> I’d have SomeAsset and AnotherAsset instances each hold a delegate  
> Asset object that they proxy any calls they can’t answer themselves  
> to. I’ve played around a little with trying to implement a full class  
> table inheritance scheme for AR on that basis, but never really had  
> enough need for it myself to polish it up.

If I do like that I have to search in several different tables when I  
want to search for assets when I don’t care about the type? Or did I  
misunderstand?

/Marcus

---

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 7:56pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/6 "2006-06-28T19:56:21Z")

</div>

On 6/28/06, Alex Y. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I’d have SomeAsset and AnotherAsset instances each hold a delegate Asset  
> object that they proxy any calls they can’t answer themselves to. I’ve  
> played around a little with trying to implement a full class table  
> inheritance scheme for AR on that basis, but never really had enough  
> need for it myself to polish it up.

I agree with Alex here. While inheritance was what the tables were  
originally trying to emulate, composition was the resulting design  
anyways. With Ruby and it’s duck typing, inheritance isn’t required  
for the ability to pass a SomeAsset where an Asset is expected. Java’s  
static typing would require SomeAsset to “be a” Asset, but Ruby only  
needs SomeAsset to “quack like” an asset, which can be covered with  
judicious use of composition and method\_missing. I’d actually provide  
a mixin that sets up this method\_missing. Example:

class Asset \< ActiveRecord::Base  
# we’re leaving out the reverse relationship to the “child” classes  
# – just as a parent class implementation wouldn’t really know  
about  
# the classes descended from it

```
def xyzzy
  # model specific functionality
end

def hidden
  # model specific functionality that's not delegatable
end

module Delegator
  AssetDelegates = [:prop1, :prop1=, :xyzzy]
  def self.included(other)
    other.module_eval <<-END
      # we use belongs_to rather than has_one because the foreign 

```

key  
# is in this table  
belongs\_to :asset

```
      alias :__asset_delegator_old_mm :method_missing
      def method_missing(method, *args, &block)
        if AssetDelegates.include?(method)
          self.asset.send(method, *args, &block)
        else
          __asset_delegator_old_mm(method, *args, &block)
        end
      end
    END
  end
end

```

end

class SomeAsset \< ActiveRecord::Base  
include Asset::Delegator

```
def xyzzy
  # override Asset's implementation of xyzzy, feels just like
  # inheritance, except that if we want to call up to the "parent"
  # version, we need to use asset.xyzzy rather than super
end

```

end

class AnotherAsset \< ActiveRecord::Base  
include Asset::Delegator

```
# make the hidden method, which wasn't explicitly delegated, 

```

available  
# from this “subclass”  
def hidden  
asset.hidden  
end  
end

Jacob F.

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 8:22pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/7 "2006-06-28T20:22:47Z")

</div>

delegation is powerful, no question…

> but if SomeAsset and AnotherAsset instances each hold a delegate Asset

why not simply use inheritance?

it’s not that bad 🙂

class SomeAsset \< Asset  
class AnotherAsset \< Asset

waaay tooo much java engineers here 🙂

-------- Original-Nachricht --------  
Datum: Thu, 29 Jun 2006 02:17:24 +0900  
Von: Jacob F. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
An: removed\_email\_address@domain.invalid  
Betreff: Re: How to model Hibernate multi-table inheritance with  
ActiveRecord?

---

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 8:14pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/8 "2006-06-28T20:14:37Z")

</div>

Read about polymorphic associations…

here’s an example  
[http://www.fromdelhi.com/2006/06/15/polymorphic-association-in-rails/](http://www.fromdelhi.com/2006/06/15/polymorphic-association-in-rails/)

-------- Original-Nachricht --------  
Datum: Thu, 29 Jun 2006 02:11:05 +0900  
Von: Marcus A. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
An: removed\_email\_address@domain.invalid  
Betreff: Re: How to model Hibernate multi-table inheritance with  
ActiveRecord?

---

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 10:08pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/9 "2006-06-28T22:08:28Z")

</div>

> Particularly because a model class can only map to one table.

Why should that be the case?

Did you try ActiveRecord::Base.set\_table\_name ?

class SomeAsset \< Asset  
set\_table\_name ‘some\_asset’  
end

Another approach would be to include the Asset base functionality using  
‘include’

class SomeAsset \< ActiveRecord::Base  
include Asset  
end

It should be more efficient than delegating methods…

Jacob F. schrieb:

---

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 10:25pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/10 "2006-06-28T22:25:38Z")

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Pete skrev:

> > Particularly because a model class can only map to one table.
> 
> Why should that be the case?
> 
> Did you try ActiveRecord::Base.set\_table\_name ?
> 
> class SomeAsset \< Asset  
> set\_table\_name ‘some\_asset’  
> end

But still, even if you override the default name of the table you map to  
you can only map to one table.

> Another approach would be to include the Asset base functionality  
> using ‘include’
> 
> class SomeAsset \< ActiveRecord::Base  
> include Asset  
> end
> 
> It should be more efficient than delegating methods…  
> Huh? I can only include modules as far as I know. And I can’t map a  
> module to a table using ActiveRecord so I think that would be hard. No?

/Marcus

---

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 8:30pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/11 "2006-06-28T20:30:58Z")

</div>

On 6/28/06, Peter E. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> delegation is powerful, no question…
> 
> > but if SomeAsset and AnotherAsset instances each hold a delegate Asset
> 
> why not simply use inheritance?
> 
> it’s not that bad 🙂

Woah… no one said inheritance was bad. Indeed, inheritance is what  
is desired here. But due to constraints from both ActiveRecord and the  
existing DB schema, we’re just trying to find any way that works. And  
inheritance won’t work here. Particularly because a model class can  
only map to one table. If you subclass an ActiveRecord model, the  
subclass is assumed to live in the same table (Single Table  
Inheritance). ActiveRecord doesn’t currently support Multiple Table  
Inheritance (as far as I know). Thus inheritance, sadly, doesn’t work  
for us here.

> waaay tooo much java engineers here 🙂

Nope. Not the case. 🙂 Just trying to solve a problem under  
constraint…

Jacob F.

---

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 10:54pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/12 "2006-06-28T22:54:59Z")

</div>

Hello Jacob,

I sort of like this solution but I have one problem. Querying. Say I  
want to query the complete set of Assets and I don’t care what  
underlying [other]Asset they have. Then I have to do:

assets = Asset.find(:all, :condition =\> “somecondition”)

But, I always need to get the delegate from the assets in order to work  
with them. Thus the above expression changes to:

assets = Asset.find(:all, :condition =\>  
“somecondition”).map(&:delegate\_asset)

Now I have the correct asset instances to work with. The problem is that  
I have to do that extra map() all over the place. 1) It violates DRY and  
2) someone will forget it

Is there a solution to this (apart from decreing project wide that you  
may never query Asset outside of Asset itself and thereby encapsulate  
all queries)?

/Marcus

---

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 10:38pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/13 "2006-06-28T22:38:21Z")

</div>

On 6/28/06, Pete [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > Particularly because a model class can only map to one table.
> 
> Why should that be the case?
> 
> Did you try ActiveRecord::Base.set\_table\_name ?
> 
> class SomeAsset \< Asset  
> set\_table\_name ‘some\_asset’  
> end

As Marcus mentioned, this lets you _change_ the table for the model,  
but not _aggregate_ multiple tables for the same model. The case  
Marcus proposed in his original post requires an aggregation of  
multiple tables to define one model.

> Another approach would be to include the Asset base functionality using  
> ‘include’
> 
> class SomeAsset \< ActiveRecord::Base  
> include Asset  
> end
> 
> It should be more efficient than delegating methods…

That’s actually pretty much what I’m trying to achieve with my  
Asset::Delegator. The reason I can’t just include Asset is 1) only  
modules can be included, as Marcus pointed out and 2) importing (or  
delegating) _all_ of the methods from Asset can conflict with the  
already existing method\_missing functionality for SomeAsset provided  
by ActiveRecord. So I threw together a quick example of a module that  
_can_ be mixed in that feels like you’re mixing in Asset itself, but  
with a limited method set. True, the number of methods that need to be  
enumerated in Asset::Delegator::AssetDelegates will probably be high,  
depending on usage, but I can’t see anyway around that without  
polluting method\_missing.

Jacob F.

---

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 11:11pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/14 "2006-06-28T23:11:43Z")

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On 6/28/06, Marcus A. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I sort of like this solution but I have one problem. Querying.

This is a good point. I actually noticed it in your earlier reply to  
Alex, but couldn’t think of anything to add. As far as I can tell, the  
situation is indeed as you tell it. ☹

Anyone else have any ideas on this?

Jacob F.

---

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 11:28pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/15 "2006-06-28T23:28:33Z")

</div>

So far I have two solutions:

1. implement method\_missing in Asset and delegate to [other]Asset where  
Asset cannot handle the requested method

2. implement method\_missing in [other]Asset and delegate back to Asset  
where [other]Asset cannot handle the request (or a variant thereof)

3. gives easy querying (I can just query Asset for all objects) but I  
cannot override methods from Asset in [other]Asset. Not really good.  
This solution could be good if I could somehow make Asset ask  
[other]Asset if they have the requested method before calling the method  
in itself instead? Sort of like a default before handler on every  
method.

4. In order for this to work I always need to work on instances of  
[other]Asset. That in turns mean that I either have to make the query on  
[other]Asset class to get instances of [other]Asset or make the query on  
Asset and then start off by getting the [other]Asset instance from it  
and work with that (ie using Asset.find(:all).map(&:delegate\_asset)).  
Neither of them feels really polymorphic. A solution here would be if  
one could bend ActiveRecord to return [other]Asset instead of Asset  
instances when querying the Asset class. I suppose this would be the  
solution to multi table inheritance though if I could solve it…

What I’m after is a solution to make it transparet for the clients of  
the Asset class (and associates). I don’t care how much/ugly code I have  
to hide in Asset to just make it transparent to the clients (since I  
have control over Asset, not over the clients).

Have I missed anything? 2) sounds easy and good but I can’t grip the  
querying situation (beyond what I described above). Have I missed  
something there?

Thanks for the answers.

/Marcus

---

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**Author:** ![marcan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/6bbea6/32.png) [@marcan](https://www.ruby-forum.com/u/marcan)\
**Post date:** [June 28, 2006, 11:21pm UTC](https://www.ruby-forum.com/t/how-to-model-hibernate-multi-table-inheritance-with-activere/96371/16 "2006-06-28T23:21:49Z")

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Peter K. skrev:

> On 6/28/06, Marcus A. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:
> 
> > I have this “old” database mainly modeled for Hibernate…  
> > [snip]
> 
> Depending on how your model looks, one idea would be to use RBatis  
> which is a more flexible O/R mapper. If SQL doesn’t scare you that is.  
> See [http://jutopia.tirsen.com/articles/2006/05/23](http://jutopia.tirsen.com/articles/2006/05/23) for more details.

Heh, I’m not scared of SQL. I’ve actually used iBatis (I used to be a  
hardcore java junkie…) in the past so I’m quite familiar with its  
concepts. It’s actually possible to use pure SQL queries with  
ActiveRecord as well.

The problem is that I don’t know how to solve this problem using  
straight SQL either without making the lives of the Asset clients a lot  
harder.

/Marcus