Render partial from Helper in Rails 4

I have read a lot of guides but I can not understand why this my method does not work:

module StudentsHelper
    def get_training_projects
        @training_projects_student = TrainingProject.where(student_id: @student.id)
        @training_projects_student.count
    end

    def get_training_courses
        @training_course_student = TrainingCourse.joins(:courses_students).where(courses_students: { student_id: @student.id })
        @training_course_student.count
    end

    def storico_studente
        n = get_training_courses + get_training_projects
        i_projects = 0
        i_courses = 0
        until i_courses + i_projects < n
            if @training_course_student[i_courses].data_inizio < @training_projects_student[i_projects].data_inizio
                render partial: "training_course", collection:@training_course_student[i_cousers]
                i_courses += 1
            else
            render partial: "training_project", collection: @training_course_student[i_projects]
            i_projects += 1
            end
        end
    end
end

The "_training_project.html.erb" and "_training_course.html.erb" files are in the folder students.