How to make a binding to the parent model (object)?

Question

Django

Hello! There are 2 pages 'project_list' (displays a list of projects) and 'project_detail' (displays detailed information about a particular project). The 'project_detail' page has a link that should take the user to a page where they can add a new member to this particular project. How to intercept the id of the current project in the view in order to associate new members with the current project? At the moment, in the url, I define the project by its slug. I created my own form based on the model to add new members to the project and then applied this form to the view, but I'm not sure if I did it right?
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})

ERROR :

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

Answer 1

not sure if i did it right?

correct
Only use slug - slightly overkill
IMO better id
If the project does not have a pronounced name that is included in the title of the tickets, for example