STI where inheritance column values are not the same as the name of the class

I recently tried to retrofit STI on a database table that had already existed for a while. Here’s a basic outline of the scenario.

  1. I had a class ‘Code’ and a database table ‘codes’.
  2. ‘Code’ had an attribute ‘units’, which could be either ‘$’ or ‘%’
  3. I wanted the STI classes to be Code::Dollar or Code::Percent
    I successfully implemented this with the following:
  class Code
self.inheritance_column = 'units'
class << self
def find_sti_class(units)
unit_class_for[units]
end
def sti_name
unit_class_for.invert[self]
end
def unit_class_for
{
'$' => Code::Dollar,
'%' => Code::Percent
}
end
end
end

This works perfectly if I use it in the following way:

Code::Dollar.new(initialization_hash)
Code::Percent.new(initialization_hash)

However, if I do just Code.new(units: '$') or something.build(units: '$'), I get an error like the following:

ActiveRecord::SubclassNotFound: Invalid single-table inheritance type: $ is not a subclass of Code

What I really want is for Code.new(units: '$') to return me a Code::Dollar object.

I was able to trace the lookup of the class name intoActiveRecord::Inheritance::ClassMethods#subclass_from_attrs. From there I could see that it was trying to build the class name from the units value in the database, which obviously doesn’t work as there isn’t a class named $ or %.

What I’m really trying to do is setup STI to work correctly when the value of the database column doesn’t correspond to a class name. As there is already another method called find_sti_class, it seems curious that we couldn’t use it inside of subclass_from_attrs in order to make it work in this way. I did try it and was successful, but as find_sti_class is a private method, I did not submit a patch using this.

So, after all of that, I guess what I’m after is finding out if doing such a thing is possible in Rails as is. If not, would a patch to make it possible be desired by people other than myself? And if that patch made find_sti_class part of the public interface, would that be likely to be accepted?

This would be super useful! I’ve been working on a legacy project, and the ability to map class name to what’s stored in the DB would be fantastic.

If it could be written so that there’s a single method to overwrite to provide a custom mapping from column value to class and it had no impact on performance then it should stand a pretty good chance.

Andrew White

Rails Core Team Member

OK, thanks for the feedback! I’ll work on a patch that does that.

I’ve got part of a solution here. It doesn’t explicitly setup mapping from columns, I’ve left that to however the end user implements find_sti_class. My changes just move find_sti_class from private to protected, and I also changed subclass_from_attributes to use that while still preserving the old behaviour.

I’m not super happy with it but it does seem to work. Feedback would be appreciated!

Well that was embarassing. I should probably include a link to my solution: https://github.com/ericroberts/rails/compare/sti-class-name