Django Models Problem

from django.db import models
from django.contrib.auth.models import User, Group


class Project(models.Model):
    project_title = models.CharField('Project name', max_length=200)
    project_number = models.IntegerField('Project number')
    project_path = models.CharField('Folder', max_length=200)
    project_manager = models.ForeignKey(User, default=None, null=True, limit_choices_to={User.objects.filter(groups__name='PM')}