find.js

  Student.find(search)
    .populate('sections')
    .then(students => {
      return students.filter(student => {
        let flag = false;
        student.sections.forEach(section => {
          if (section.year === req.query.year) {
            flag = true;
            console.log(flag);
          }
        })
        return flag;
      })
    })