# How to use two databases within one model

**URL:** <https://www.ruby-forum.com/t/how-to-use-two-databases-within-one-model/90463>\
**Category:** Rails\
**Created:** [March 29, 2007, 12:35am UTC](https://www.ruby-forum.com/t/how-to-use-two-databases-within-one-model/90463 "2007-03-29T00:35:46Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![sebastian](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/4da419/32.png) [@sebastian](https://www.ruby-forum.com/u/sebastian)\
**Post date:** [March 29, 2007, 12:35am UTC](https://www.ruby-forum.com/t/how-to-use-two-databases-within-one-model/90463/1 "2007-03-29T00:35:46Z")

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For an online shop I will have to work with a legacy data base for  
some of the models.  
Furthermore, within my application those models will need additional  
attributes that I will have to save in my own database.  
I was thinking of using aggregation, encapsulating the legacy data in  
their own ActiveRecords::Base objects, which I would configure to use  
the legacy database. Then I would wrap those objects into other  
ActiveRecords::Base objects that would contain the extra attributes  
and use my own database.  
Do you think this is a viable approach?  
In the docs I read that composed\_of is meant for value objects not  
business objects…  
Has anybody solved a similar situation?

Chances are I won’t even be able to access the legacy database  
directly, but will have to query it through WebServices.  
Does anybody have a suggestion how to go about if that were case?

Any feedback is highly appreciated!

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<div class="post-metadata">

**Author:** ![sebastian](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/4da419/32.png) [@sebastian](https://www.ruby-forum.com/u/sebastian)\
**Post date:** [April 4, 2007, 9:05am UTC](https://www.ruby-forum.com/t/how-to-use-two-databases-within-one-model/90463/2 "2007-04-04T09:05:50Z")

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I’d be interested in anything you or any one else has found out on this.  
Any one have any ideas?

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<div class="post-metadata">

**Author:** ![sebastian](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/4da419/32.png) [@sebastian](https://www.ruby-forum.com/u/sebastian)\
**Post date:** [April 4, 2007, 7:47pm UTC](https://www.ruby-forum.com/t/how-to-use-two-databases-within-one-model/90463/3 "2007-04-04T19:47:33Z")

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sebastian wrote:

> For an online shop I will have to work with a legacy data base for  
> some of the models.  
> Furthermore, within my application those models will need additional  
> attributes that I will have to save in my own database.  
> I was thinking of using aggregation, encapsulating the legacy data in  
> their own ActiveRecords::Base objects, which I would configure to use  
> the legacy database. Then I would wrap those objects into other  
> ActiveRecords::Base objects that would contain the extra attributes  
> and use my own database.  
> Do you think this is a viable approach?  
> In the docs I read that composed\_of is meant for value objects not  
> business objects…  
> Has anybody solved a similar situation?
> 
> Chances are I won’t even be able to access the legacy database  
> directly, but will have to query it through WebServices.  
> Does anybody have a suggestion how to go about if that were case?
> 
> Any feedback is highly appreciated!

I think I can get you started, but hopefully someone who has done this  
can give real advice.

First, you are going to need to connect to two different data sources.  
I would suggest that to do this effectively you should create two  
Subclasses of ActiveRecord::Base.

class LegacyBase \<ActiveRecord::Base;end  
class MyAdditionsBase \<ActiveRecord::Base;end

You will need to set the connection in the Legacy database up by hand.  
If you are lucky this will just be a matter of calling  
LegacyBase.establish\_connection. Please see the documentation for that  
function.  
([ActiveRecord::Base](http://api.rubyonrails.org/classes/ActiveRecord/Base.html#M001075))

If you are not lucky (i.e. you legacy data source does not have a built  
in connection adapter) you will need to make your own. If you have to  
go through a WebService, this will defiantly be the case. I’m afraid I  
cannot help you here, but my instinct is that someone else has had to do  
this and there may be a plugin out there that will at least give you a  
place to start.

As I think about this, there is a point at which you access to the  
legacy data becomes so restrictive, that it may be easier write a class  
using Ruby’s native SOAP or XMLRPC client classes rather then fight  
ActiveRecord. If you cannot run general SQL queries through the  
WebServices, but only call pre-baked query function with parameters, I  
think this would be the way to go.

Next, create some models for you legacy data useing the LagacyBase class  
instead of AR::B. These should be fairly basic, but you will need to  
explicitly state what the primary\_key and foreign\_keys are, because I  
will bet that your legacy data does not conform to rails expectations.

Finally, I would suggest that anywhere you have created a new tables in  
the database you control, that you make a model on those tables and use  
a belongs\_to relationship with your legacy models. A little work with  
method\_missing might go a long way towards hiding the underlying  
relationship.

Something you ought to think through, is how to maintain integrity  
between the two databases. What happens when someone creates or deletes  
a record in the Legacy data source and doesn’t bother to tell you about  
it.

Hope this helps.

John M.