How to extend an existing Ruby model to support persistence

Question

I have a gem which implements my entire business logic, so that I can use it in different applications. Now, one of these applications requires persistence. How do I easily extend my existing Ruby models to support persistence? Should I monkey patch them?

To give you a bit of a background, my model objects are usually just built from XML or JSON files, but now I need to store them in an relational database.

Are there common patterns for this problem? Should I write new model objects that support persistence and map between my legacy objects and the new model objects or should I extend the existing ones to be representable in a database?

Any tips, hints, and links are highly welcome.

Answer 1

I am not sure that I fully understand your question. However, the DataMapper library can be very easily used to add persistence to an already existing object model after the fact, for two reasons:

1. It doesn't rely on class inheritance (like e.g. ActiveRecord does) but on mixin inheritance, and you can inherit from as many mixins you like, which means you won't have to change the inheritance tree of your object model just to add DataMapper to it.
2. The object-relational-mapping is declared explicitly in the model, not inferred from the data-store. This means that you can have very complex mappings between the data-store and your models, unlike the rather simple 1:1 table == class, row == object, column == attribute mapping of ActiveRecord.

Now, whether or not you will manage to keep the persistence aspect fully orthogonal, and e.g. in a separate gem, that's another question. You could indeed keep it in a separate library that just opens up all the model classes and include DataMapper::Resource and declare all the properties. This will allow you to still deploy your object model gem without persistence, but the persistence gem will obviously be rather tightly coupled to the object model gem.