Question

I’m setting up a Rails 3.1 application in which users can change the “theme” of their page to one of several premade themes. I’m having trouble figuring out which ActiveRecord association properly models the relationship between a User and a Theme.

I want to write something like:

class User < ActiveRecord::Base
  has_one :theme

class Theme < ActiveRecord::Base
  belongs_to_many :users

But “belonds_to_many” isn’t a real association, and has_one puts the foreign key in the Theme table. (according to Association Basics)

What is the canonical way to for an ActiveRecord “User” to have a single “Theme”, where a “Theme” can be used by many “Users”?

Answer 1

The user should belong to the theme in this case.

class User < ActiveRecord::Base
  belongs_to :theme
end

class Theme < ActiveRecord::Base
  has_many :users
end

In this case, the user table would have a field called theme_id.

The belongs_to feels weird in this case, when thinking of it from an English language perspective, since a user shouldn’t really belong to a theme. However, the general rule of thumb is that the model containing the foreign key belongs to the model which doesn’t.