gistfile1.txt

question controller
31    def create
 32       params[:question][:subjects].delete_if { |n| n == "" }
 33       Question.create(question_params);
 34       #     Question.create(:title => params[:question][:title],
 35       #                     :summary => params[:question][:summary],
 36       #                     :subject_id => params[:question][:subject])
 37    end
 38
 39    def question_params
 40       params.require(:question).permit(:title,:summary,:subjects)
 41    end


question model 
 1 class Question < ActiveRecord::Base
  2   belongs_to :user
  3   has_many :QuestionSubjects
  4   has_many :subjects, :through => :QuestionSubjects
  5   has_many :answers
  6 end

Subject model
class Subject < ActiveRecord::Base
  2    has_many :QuestionSubjects
  3    has_many :questions, :through => :QuestionsSubject
  4 end

QuestionSubject Model

class QuestionSubject < ActiveRecord::Base
  2   belongs_to :question
  3   belongs_to :subject
  4 end