Thedwick

Ok, I’ve been banging my head against this one for a little while, so I want to make sure to share the solution.

In my Ruby on Rails project, I have a RegisteredUser class and a Person class. They have a one-to-one relationship with each other that I was trying to represent like so:

class Person < ActiveRecord::Base
  has_one :registered_users
end
class RegisteredUser < ActiveRecord::Base
  belongs_to :person
end

see the problem? I didn’t either for a while, but maybe the title of this post will give you a clue. The problem is with the pleurality of my has_one. It should be registered_user instead of registered_users. You really have to watch the plurality of relationships in Rails. If it’s one-to-one, then both sides better be singular. If it’s one-to-many, then the many side better be plural and the one side better be singular. It all makes a difference.

The error message I was getting was simply the not-very-helpful:

test_add_reporter_by_name(PersonControllerTest):
NoMethodError: undefined method `registered_user' for #<person :0x48ff2c4>
    d:/ruby/lib/ruby/gems/1.8/gems/activerecord-1.15.1/lib/active_record/base.rb:1848:in `method_missing'
    D:/dev/rails/mystats/config/../app/controllers/person_controller.rb:54:in `add_reporter_by_name'
    d:/ruby/lib/ruby/gems/1.8/gems/actionpack-1.13.1/lib/action_controller/base.rb:1095:in `send'
    d:/ruby/lib/ruby/gems/1.8/gems/actionpack-1.13.1/lib/action_controller/base.rb:1095:in `perform_action_without_filters'
...snip...
    d:/ruby/lib/ruby/gems/1.8/gems/actionpack-1.13.1/lib/action_controller/test_process.rb:353:in `post'
    test/functional/person_controller_test.rb:79:in `test_add_reporter_by_name'

One other gotcha I ran into…if you have a has_and_belongs_to_many relationship (ie. a many-to-many) then you need to make sure your join table in between does not have an identity column. So, in my case I have another many-to-many relationship between RegisteredUser and Person that I call “authorized reporters”. This relationship says which RegisteredUsers are allowed to report a status for which People:

  has_and_belongs_to_many :authorized_reporters, :class_name =>'RegisteredUser', :join_table => 'authorized_reporters'

In between I have a join table called authorized_reporters:

class CreateAuthorizedReporters < ActiveRecord::Migration
  def self.up
    create_table :authorized_reporters, :id => false do |t|
      t.column :registered_user_id, :integer, :null => false
      t.column :person_id, :integer, :null => false
    end
  end
  def self.down
    drop_table :authorized_reporters
  end
end

There is one very important part of that migration: “, :id => false”. Without it, the many-to-many works in one direction, but not in the other (I don’t remember which direction works).