gistfile1.rb

class User
  require DataMapper::Resource

  property :id, Serial
  property :...

  has n, :interests
  has n, :questions, :through => :interests
end

class Qustion
  require DataMapper::Resource

  property :id, Serial
  property :...

  has n, :interests
  has n, :users, :through => :interests
end

class Interest
  require DataMapper::Resource

  property :user_id, Integer, :key => true
  property :question_id, Integer, :key => true

  belongs_to :users
  belongs_to :questions
end