# Coding to an interface in RoR?

**URL:** <https://www.ruby-forum.com/t/coding-to-an-interface-in-ror/84614>\
**Category:** Rails\
**Created:** [February 6, 2007, 1:26pm UTC](https://www.ruby-forum.com/t/coding-to-an-interface-in-ror/84614 "2007-02-06T13:26:10Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![Joram](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Joram](https://www.ruby-forum.com/u/Joram)\
**Post date:** [February 6, 2007, 1:26pm UTC](https://www.ruby-forum.com/t/coding-to-an-interface-in-ror/84614/1 "2007-02-06T13:26:10Z")

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Hi,

I’m currently evaluating the conversion of a j2ee app to a RoR app.  
Without going into much detail, the app lets users compose a meal and  
shows  
the amounts of nutrients associated to that meal (eg salt,  
sodium, …).

These nutrients are read from a database.  
In the original app we use a “service” that abstracts all the database  
level code etc.  
This service implements an interface that defines the functionality.

That way, we can swap the services when needed, but the interface  
stays the same, so no adjustments are needed.  
(eg. McDonaldsService instead of ‘normal’ food, when the app is placed  
in a McDonalds to say something)  
And that’s where my problem lies. How can I achieve the same level of  
abstraction in Rails?

In concreto, how can I define my model so that I can change the  
original model to another model without changing much?  
The only solution I see at the moment is defining a very general  
model that fits all the different services,  
and put everything in a big table.  
However, that way we are losing the power of abstraction we had with  
j2ee.

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<div class="post-metadata">

**Author:** ![Joram](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Joram](https://www.ruby-forum.com/u/Joram)\
**Post date:** [February 6, 2007, 2:54pm UTC](https://www.ruby-forum.com/t/coding-to-an-interface-in-ror/84614/2 "2007-02-06T14:54:21Z")

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On 2/6/07, Joram [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Hi,
> 
> In the original app we use a “service” that abstracts all the database  
> level code etc.

Could you describe this service in more detail? Some of us might  
know specific details about the Java stuff you are doing.

> This service implements an interface that defines the functionality.

Well Ruby makes the whole java concept of interface disappear.  
Look up “duck typing” - if two Ruby classes have identical method  
definitions they are interchangeable. If they share subsets of method  
definitions, they are interchangeable in contexts where only those  
subsets are used.

> That way, we can swap the services when needed, but the interface  
> stays the same, so no adjustments are needed.  
> (eg. McDonaldsService instead of ‘normal’ food, when the app is placed  
> in a McDonalds to say something)  
> And that’s where my problem lies. How can I achieve the same level of  
> abstraction in Rails?

Well a rails user might consider that an obfuscation, not an  
abstraction. And  
the question of whether that abstraction is desirable comes to mind. It  
just  
strikes me as an unusual way to achieve separation of customer data. You  
could just have separate databases for each customer, or maintain a  
customer table that has relates to the master food/nutrient lookup  
table.

> In concreto, how can I define my model so that I can change the  
> original model to another model without changing much?

Are you changing the model or the data? I see nothing in the problem  
that  
would indicate that the food\_product, ingredients,  
nutritional\_information  
models are essentially different between (e.g.) McDonalds and Ben &  
Gerrys,  
just that each customer would offer a customer specific range of food  
products.

> The only solution I see at the moment is defining a very general  
> model that fits all the different services,  
> and put everything in a big table.

Thats pretty much how a Rails user would do it, but they would define  
multiple  
tables for each model and relate them correctly.

> However, that way we are losing the power of abstraction we had with  
> j2ee.

Do you need that power? Or is that abstraction solving a problem  
introduced  
by a weak previous implementation? Like inadequate distinction between  
customer data (objects) and actual model types (classes).

I am sorry if I came across a bit aggressive/blunt but your problem  
appears  
to be the kind of overcomplicated java-centric solution to a simple  
problem  
that I have to deal with far too frequently than I should have to.

It actually sounds like you basically maintain different databases/data  
models  
for each customer with different ORM strategies, and try to treat them  
as  
similar at the ‘service’ level by wrapping each ORM implementation in a  
common interface.

A Rails solution would seek to normalise your DB schema first by doing  
appropriate E-R modelling. In the Java/J2ee/Java Web Framework world  
it is more common to treat the database as an implementation detail, and  
go with code-first approaches (which gets a lot of Java projects into  
trouble).

regards,  
Richard.

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<div class="post-metadata">

**Author:** ![Joram](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Joram](https://www.ruby-forum.com/u/Joram)\
**Post date:** [February 6, 2007, 10:22pm UTC](https://www.ruby-forum.com/t/coding-to-an-interface-in-ror/84614/3 "2007-02-06T22:22:18Z")

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Thank you Richard for your long reply!

I’ve gathered some more information about the problem.

The “service” approach is used to abstract the database totally from  
the rest of the application.  
The benefit of doing that is that the datamodel can change in a  
drastic way, but the rest of the app is unaffected since a service is  
used to access the information.

The problem is that in fact the datamodel can change (eg another data  
source provider, forced by the government, so no discussion is  
possible about the database schema). By changing the datamodel, the  
objectmodel however should not change. What does change is the  
translation between the data- and objectmodel.

However, in Rails, when changing the datamodel the objectmodel is  
changed automatically (which is in most cases a good thing).  
But if the model changes (eg other combination of attributes to  
achieve the same functionality) the controller will likely have to  
change…  
So that’s what my question is about. Is there any way in Rails to  
define an ‘interface’ to a model, so I can use this interface in the  
controllers. When the model changes because the data model changes,  
the controllers should not be affected.

Thank you for your time!

Joram

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<div class="post-metadata">

**Author:** ![Joram](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Joram](https://www.ruby-forum.com/u/Joram)\
**Post date:** [February 6, 2007, 11:40pm UTC](https://www.ruby-forum.com/t/coding-to-an-interface-in-ror/84614/4 "2007-02-06T23:40:05Z")

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Joram wrote:

> However, in Rails, when changing the datamodel the objectmodel is  
> changed automatically (which is in most cases a good thing).  
> But if the model changes (eg other combination of attributes to  
> achieve the same functionality) the controller will likely have to  
> change…  
> So that’s what my question is about. Is there any way in Rails to  
> define an ‘interface’ to a model, so I can use this interface in the  
> controllers. When the model changes because the data model changes,  
> the controllers should not be affected.

Joram,

I hear and I believe that I understand what you are saying. There is no  
notion of an “interface” per se in Ruby, as others have stated.

One way to go is to add methods directly to your model classes which  
would provide a model-agnostic set of behavior for your controller to  
use.

Or you could build another layer and create a set of mediator/adapter  
classes that would be used “between” your controller and model objects.  
Basically, this would be the same set of methods as you might create  
given the first approach, but actually factored out into their own  
objects (which, frankly, they probably should be in the first place).  
Obviously, you would have to manage the marshaling/unmarshaling of data  
between the models and this new layer that the controller manipulates.

Bottom line, is that you are free to create other objects to supplement  
the Rails framework if you feel that you need them.

I’m curious to hear whether your database schema has already changed  
enough to justify such a heavily decoupled approach.

Thanks,  
Wes

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**Author:** ![Joram](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Joram](https://www.ruby-forum.com/u/Joram)\
**Post date:** [February 6, 2007, 11:11pm UTC](https://www.ruby-forum.com/t/coding-to-an-interface-in-ror/84614/5 "2007-02-06T23:11:24Z")

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On 2/6/07, Joram [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> However, in Rails, when changing the datamodel the objectmodel is  
> changed automatically (which is in most cases a good thing).  
> But if the model changes (eg other combination of attributes to  
> achieve the same functionality) the controller will likely have to  
> change…  
> So that’s what my question is about. Is there any way in Rails to  
> define an ‘interface’ to a model, so I can use this interface in the  
> controllers. When the model changes because the data model changes,  
> the controllers should not be affected.

Ruby has this built in. Let’s say you have a table to store an item  
name (string) and it’s price in dollars.

class Item \< ActiveRecord::Base

# columns are name and dollars

end

You originally have the price in dollars, but you change the datamodel  
to have the price in pesos:

class Item \< ActiveRecord::Base

# columns are name and pesos

CONVERSION\_VALUE = 10.9378 # 1 Dollar = 10.9378 pesos

def dollars  
pesos/CONVERSION\_VALUE  
end

def dollars=(value)  
self.pesos = value \* CONVERSION\_VALUE  
end  
end

This gives you the same interface as the model with dollars.

Jeremy

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<div class="post-metadata">

**Author:** ![Joram](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Joram](https://www.ruby-forum.com/u/Joram)\
**Post date:** [February 7, 2007, 8:03pm UTC](https://www.ruby-forum.com/t/coding-to-an-interface-in-ror/84614/6 "2007-02-07T20:03:24Z")

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On 2/6/07, Joram [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Thank you Richard for your long reply!

No problem.

> The “service” approach is used to abstract the database totally from  
> the rest of the application.

Essentially below the waterline, everything is custom, you get an  
implementation-specific PersistenceService, which returns only  
implementation-neutral types (models).

The database ORM is effectively wrapped by whatever you are using.

> The benefit of doing that is that the datamodel can change in a  
> drastic way, but the rest of the app is unaffected since a service is  
> used to access the information.

Well the implementation of everything below service layer is not  
trivial.

> The problem is that in fact the datamodel can change (eg another data  
> source provider, forced by the government, so no discussion is  
> possible about the database schema). By changing the datamodel, the  
> objectmodel however should not change. What does change is the  
> translation between the data- and objectmodel.

I understand. This is a strategy to re-use all code above the  
service-layer,  
and make it pretty much 100% implementation neutral. But this becomes  
only implementation neutral from a _type_ or _interface_ perspective. If  
the  
schema is very different from the original reference (i.e. totally  
different  
table relationships) several problems creep in:

- first you will lose a lot of performance without leaning heavily on  
Hibernate  
style performance features (like in memory object caching).
- you are also probably limiting your ability to perform unusual queries  
against the DB, as the implementation of the service layer has to  
translate the relationships, which means that query implementations will  
be limited
- you are doing a lot of CPU-intensive model translation in the context  
of your application

> However, in Rails, when changing the datamodel the objectmodel is  
> changed automatically (which is in most cases a good thing).

Well that would be a _feature_ of rails. Though Rails does have features  
for working with legacy databases, like defining getters in models and  
mapping them to differently named table fields. Though if relationships  
change there is not a lot you can do there.

> But if the model changes (eg other combination of attributes to  
> achieve the same functionality) the controller will likely have to  
> change…  
> So that’s what my question is about. Is there any way in Rails to  
> define an ‘interface’ to a model, so I can use this interface in the  
> controllers. When the model changes because the data model changes,  
> the controllers should not be affected.

Well you kind of can. This is close to the Java model you describe, but  
there is no service, or distinction between service models, and  
controller  
-used models.

1. You can just define arbitrary models with no relationships
2. map the models to the implementation specific table schema, using  
a simple vendor specific configuration code and/or file
3. Do Model.find(:foo) at the controller level and tease the  
relationships  
and data conversion together using model helpers (that use vendor  
specific libraries or mixins)
4. The models will present a consistent interface to controllers and  
views
5. It will also perform like an absolute dog, Rails isn’t meant for this  
kind of in-code model mapping, and you are likely to be pulling back  
bigger result sets than necessary

Depending on the size of the source database, you could simply  
create a second database (possibly even in memory) and translate  
the implementation specific database into the reference database.  
Not a perfect solution, keeping the reference database (that the  
models look at) up to date is a problem.

There is also the “I hate the people I work with, and in 2 weeks time  
I am going travelling the world” approach:

- do implementation specific metaprogramming
- use metaprogramming techniques to dynamically alter your models  
& relationships to match the implementation database
- quake in fear at the insanely ambitious use of metaprogramming,  
but keep your fingers crossed, as _it just might work_

There is also the other Rails approach:  
1 - writing controllers & views is simple, just make a bespoke  
implementation  
for each schema model, and keep the tests intact  
2 - refactor controllers and views to use the new models  
3 - bill accordingly

regards,  
Richard.

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<div class="post-metadata">

**Author:** ![Joram](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Joram](https://www.ruby-forum.com/u/Joram)\
**Post date:** [February 7, 2007, 7:03pm UTC](https://www.ruby-forum.com/t/coding-to-an-interface-in-ror/84614/7 "2007-02-07T19:03:16Z")

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Well since the database is abstracted why do you need active record  
models? Why not just use plain ruby objects or the Struct class if  
it’s just data?

However, if you want some sort of ‘interface’, I would do as such:

Define a module as your interface, but in this case it’s also getting  
implementation, not just the ‘contract’ the interface brings:

Module MyInterface  
def method1  
end

def method2  
end  
end

In your object that requires that interface include the module,

require ‘my\_interface’

class MyModel  
include MyInterface  
end

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<div class="post-metadata">

**Author:** ![Joram](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Joram](https://www.ruby-forum.com/u/Joram)\
**Post date:** [February 8, 2007, 11:12pm UTC](https://www.ruby-forum.com/t/coding-to-an-interface-in-ror/84614/8 "2007-02-08T23:12:20Z")

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Thank you Jeremy, Wes, Apsoto and Richard for your contributions!

You have given me insight into a difficult subject.  
I haven’t got much to add anymore at the moment, your information has  
given me a lot to think about!

Greetings,

Joram