Как правильно сделать привязку к родительской модели (объекту)?

Здравствуйте! Есть 2 страницы 'project_list'(выводится список проектов) и 'project_detail'(выводится детальная информация о конкретном проекте). На странице 'project_detail' есть ссылка, которая должна перебрасывать пользователя на страницу, где он мог бы добавить нового члена в данный конкретный проект. Как правильно в view перехватывать id текущего проекта, чтобы ассоциировать новых членов с текущим проектом? На текущий момент в url я определяю проект по его slug. Я создал собственную форму на основе модели для добавления новых членов в проект и затем эту форму применил в view, но не уверен правильно ли я вообще cделал?

models.py:

class Project(models.Model):
    ***FIELDS***
    members = models.ManyToManyField(User, through='Membership',)

class Membership (models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE,)
    project = models.ForeignKey(Project, on_delete=models.CASCADE)
    role = models.CharField(max_length=20, choices=ROLE_CHOICES,)

forms.py:

class ProjectMembersForm(forms.ModelForm):
    class Meta:
        model = Membership
        exclude = ('project',)

urls.py:

url(r'^projects/(?P<project_id>\d+)/$', project_detail, name='project_detail'),
url(r'^projects/(?P<project_id>\d+)/members/add/$', add_project_member, name='add_project_member'),

views.py:

def add_project_member(request, project_id):
    project = get_object_or_404(Project, pk=project_id)
    if request.method == 'POST':
        form = ProjectMembersForm(request.POST)
        if form.is_valid():
            new_member = form.save(commit=False)
            new_member.save()
            return render(request, 'project/project_detail.html', {'new_member': new_member})
    else:
        form = ProjectMembersForm()
    return render(request, 'project/project_members.html', {'form': form})

ОШИБКА:

Traceback (most recent call last):
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\core\handlers\exception.py", line 39, in inner
    response = get_response(request)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\core\handlers\base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\core\handlers\base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "C:\Users\Nurzhan\PycharmProjects\RMS\project\views.py", line 71, in add_project_member
    new_member.save()
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\models\base.py", line 796, in save
    force_update=force_update, update_fields=update_fields)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\models\base.py", line 824, in save_base
    updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\models\base.py", line 908, in _save_table
    result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\models\base.py", line 947, in _do_insert
    using=using, raw=raw)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\models\manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\models\query.py", line 1045, in _insert
    return query.get_compiler(using=using).execute_sql(return_id)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\models\sql\compiler.py", line 1054, in execute_sql
    cursor.execute(sql, params)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\utils.py", line 79, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\utils.py", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\utils\six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35-32\lib\site-packages\sql_server\pyodbc\base.py", line 539, in execute
    return self.cursor.execute(sql, params)
django.db.utils.IntegrityError: ('23000', '[23000] [Microsoft][ODBC SQL Server Driver][SQL Server]�� ������� �������� �������� NULL � ������� "project_id", ������� "RMS.dbo.project_membership"; � ������� ��������� �������� NULL. ������ � INSERT. (515) (SQLExecDirectW)')
[11/Mar/2017 21:35:38] "POST /ru/account/dashboard/projects/3/members/add/ HTTP/1.1" 500 143162