You're getting down voted because SO is really for specific programming queries & questions (the more code the better), but here are some ideas for you:
According to Rails' ActiveRecord associations, you'd be best using the has_many :through association, effectively creating a join model; like this:
#app/models/question.rb
class Question < ActiveRecord::Base
has_many :answers
has_many :users, :through => :answers
end
#app/models/answer.rb
class Answer < ActiveRecord::Base
belongs_to :user
belongs_to :question
end
#app/models/user.rb
class User < ActiveRecord::Base
has_many :answers
has_many :questions, :through => :answers
end