Question

sorry if this question has been asked before, I searched but wasn’t sure on the proper name for what I’m looking for.

I’m pretty much a newcomer to rails and development in general, I’ve done some hacky programming but I wouldn’t say I know what I’m doing. I’ve always been at a loss on how to define “types” in my Rails models and in databases in general.

For example, say I have a model “Car” and it has the property “Colour”, where Colour is chosen from a known set rather than an RGB value or whatever. Something tells me that there should be another table (and associated model) for Colour and then have some kind of relationship between the two, but which relationship is appropriate? A car doesn’t BELONG to its colour, or vice versa.

Or of course I could just store an integer and look it up in code, but this feels wrong to me.

What did I miss? 🙂

Answer 1

Color may be a complex object or may be a very simple one, depending on what you need.

• Simple: Integer subsequently converted to any other format, or hex value like “a3ab34” stored in a string.
• Complex: You want to map colors to certain human titles.

For simple you’ll just have a column in cars named color, and override the reader method to return whichever color format you desire, like so.

class Car < ActiveRecord::Base
  def color(format = :integer)
     case format
     when :integer: super
     when :hex: # ... hex conversion code
     # ... more conditions ...
     end
  end
end

If we’re talking about complex case, then it’s different. You should then have a separate model called Color. Then Car would belongs_to :color like thenduks said above. It would be more semantically correct to say Car has_one :color but then you’re forcing colors table to store car_id which would be kind of backwards. In our case, Car will be storing color_id. So make sure you make a migration that adds color_id to table cars.

Then you should create the migration for colors table, which would include id, title and code for example, and which you’d have to pre-populate yourself using rails seeding stuff. (see http://railscasts.com/episodes/179-seed-data)