par.rb

###############
##   WRONG   ##
###############
  
  
  class Users::CoursesController
    def edit
      @course = Course.find(params[:id])
    end
  end

  class Users::LessonsController
    def new
      @lesson = Lesson.new(lesson_params)
    end
  end

###############
##   RIGHT   ##
###############

  class Users::CoursesController
    def edit
      @course = current_user.courses.find(params[:id])
    end
  end

  class Users::LessonsController
    def new
      course = current_user.courses.find(params[:course_id])
      @lesson = course.lessons.build(lesson_params)
    end
  end