Coding to an interface in RoR?

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.

On 2/6/07, Joram [email protected] 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.

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

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

On 2/6/07, Joram [email protected] 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

On 2/6/07, Joram [email protected] 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.

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

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

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