Help with abstracting ActiveRecord models

rubyonrails-talk
1

Hi all. I'm looking for suggestions of how other people have implemented a situation like the following. I'd like to create a base activerecord model, let's call it Widget, from which other classes are based. I'd like to store the name of the final class in the database and either use that information to load the proper class or to include a module that overrides certain methods of the base class. For example:

Widget < AR: Base RSSWidget < Widget HTMLWidget < Widget

w = Widget.first

Currently w is a generic widget; it doesn't know if it's really a RSSWidget or an HTMLWidget. Really, what I'm more interest in is if something, say Page, has many widgets, then I can iterate through all widgets for an instance of page and each will cast itself to the appropriate subclass (or include the proper module).

Is there an AR hook that I can use immediately after the model is loaded from the database to include the necessary module and either deserialize any additional information or load additional properties from other tables? I've read that AR has an abstract_class property, but it almost looks like it sees this problem from the other direction, with the tables belonging to the inherited classes, but maybe I've misunderstood it.

Any suggestions of how this may be done would be greatly appreciated.

Thanks, -Jeff

2

It seems to me you're thinking in Java or C++. Ruby isn't about what type or class an object is based on so much as what the object can do. I don't see very much object casting when I read other people's Ruby. Mostly it's just the primitives, string to ints, etc.

Google "ruby duck typing", and check out respond_to? and module mixins.

3

Look up single table inheritance. It, by definition and implementation, stores the name of the class in a database field. The syntax for declaration is simply inheritance, so nothing special there. The only requirement is that all database fields for the subclasses be present in the table for the base class. Best of all, when you do:

w = RSSWidget.first

w.class == RSSWidget

=> true

Does that meet the criteria you’ve spelled out? Because if it does, it’s been in Rails since before 1.0.

4

And if it doesn't meet his criteria, when if ever did STI get into Rails? <G>

5

if rails.include? STI    puts "sigh of relief -- must be before today" else    puts "pain of embarrassment -- must be wrong" end

# or...

if requirements =~ /STI/    puts "STI must have been included in Rails before 1.0" else    puts "STI implementation date not computable from given information" end

<G>