def get_students_by_group(request, id):
    '''return students that belongs to group'''
    students = Student.objects.filter(user=request.user, groups__id=id)
    return Response(StudentSerializer(students, many=True).data)

def get_groups_by_student(request, id):
    '''return groups by given student'''
    groups = Group.objects.filter(user=request.user, student__id=id)
    return Response(GroupSerializer(groups, many=True).data)

# -----------------------------------

def get_items_by_item(ObjType, ObjSerializer, request, id):
    '''return groups that belongs to anoter type item'''
    objects = ObjType.objects.filter(user=request.user, ????)
    return Response(ObjSerializer(objects, many=True).data)