controller.rb

def index
  filters = {}
  filters[:categories_filter] = params[:categories].split(',') unless params[:categories].blank?
  filters[:questions_filter] = params[:questions].split(',') unless params[:questions].blank?
  @questions = QuestionFilter.new(filters).questions
end

gistfile1.rb

class QuestionFilter
  attr_accessible :options

	def initialize opts = {}
		@options = default_options.merge opts
	end

  def default_options
    {
      questions_filter: [],
      categories_filter: [],
      random: false,
      string_filter: nil
    }
  end

	def questions
    return @_questions unless @_questions.nil?

    @_questions = Question.scoped
    apply_categories_filter if options.categories_filter.size > 0
    apply_questions_filter if options.questions_filter.size > 0
    apply_random_filter if options.random
    apply_string_filter if options.string_filter
    @_questions
  end

  private

  def apply_categories_filter
    condition_params = options.
      categories_filter.
      inject({ id:[], name:[]}){ |hash,value|
        value.to_i > 0 ? hash[:id].push(value.to_i) : hash[:name].push(value)
        hash
      }
    condition_string = condition_params.inject([]){|array, (key,values)|
      array << "categories.#{key} IN (:#{key})" if values.size > 0
      array
    }
    @_questions = @_questions.where(condition_string.join('OR'), condition_params)
  end

  def apply_questions_filter
    @_questions = @_questions.
      where("questions.id IN (?)", options.questions_filter)
  end

  def apply_random_filter
    @_questions = @_questions.order('random()')
  end

  def apply_string_filter
    @_questions = @_questions.search_for(options.string_filter)
  end

end