I recommend a has many through association, with a link to the answer in the intermediate table. Thus:
class User < ActiveRecord::Base
has_many :user_questions
has_many :questions, through: :user_questions
end
class UserQuestion < ActiveRecord::Base
belongs_to :user
belongs_to :question
belongs_to :answer
end
In this way, you can create questions that you can associate with a user or not, and also link to their answer, where it exists.
