Django에서 새 레코드를 만들려고 할 때 외래 키 오류

일대 다 관계로 연결된 두 가지 모델 인 활동과 사용자가 있습니다. 사용자는 하나의 활동에만 연결할 수 있지만 활동은 여러 사용자 항목에서 참조 할 수 있습니다.

// Activity
id | username_id | activity | initial_date | ending_date
--  ------------   --------   ------------  ------------
1  |             |   test   |   2020-01-20 |    None
45 |             |   test45 |   2020-01-25 |    None

// User
username_id |       email       | username | last_login | date_joined |is_admin | is_active | is_staff | is_superuser
------------ -----------------  ----------  ------------  -----------  --------- --- --------------------------------
    2        |  [email protected]   |   ab     |  2020-01-20 |  2020-01-15|   No    |      No   |     No   |   No      
    3        |  [email protected]   |   ab     |  2020-01-21 |  2020-01-18|   No    |      No   |     No   |   No      

//models.py 

from django.utils import timezone
from django.contrib.auth import get_user_model
User = get_user_model()

class Activity(models.Model):
    id           = models.AutoField(primary_key=True)
    username_id  = models.ForeignKey(User, on_delete=models.CASCADE)
    activity     = models.CharField(null=False, max_length=140)
    initial_date = models.DateField(default=timezone.now(), null=False)
    ending_date  = models.DateField(default=timezone.now(), null=False)

class User(AbstractBaseUser):
   username_id  = models.AutoField(primary_key=True)
   email        = models.EmailField(verbose_name="email", max_length=60, unique=True)
   username     = models.CharField(max_length=30, unique=True)   

   last_login   = models.DateTimeField(verbose_name='last login', auto_now=True)
   date_joined  = models.DateTimeField(verbose_name='date joined', auto_now_add=True)
   is_admin     = models.BooleanField(default=False)
   is_active    = models.BooleanField(default=True)
   is_staff     = models.BooleanField(default=False)
   is_superuser = models.BooleanField(default=False)

   USERNAME_FIELD = 'email' 
   REQUIRED_FIELDS = ['username']


//forms.py
from .models import Activity
from django import forms

class ActivityModelForm(forms.ModelForm):
    model = Activity
    fields = ['username_id', 'activity', 'initial_date', 'ending_date']

def __init__(self, *args, **kwargs):
    super(ActivityModelForm, self).__init__(*args, **kwargs)
    self.fields['username_id'].required = False
    self.fields['activity'].required = True
    self.fields['initial_date'].required = False
    self.fields['ending_date'].required = False

표에서 볼 수 있듯이 username_id 열에는 빈 값이 있으므로 views.py새 활동을 저장하려고 할 때 코드를 수정 했지만 이제 오류가 발생합니다.ValueError: Cannot assign "3": Activity.username" must be a user instance.

def create_activity(request):
    form_create = ActivityModelForm(request.POST or None)
    username_id = None
    if request.user.get_username():
        username_id = request.user.id

    if form_create.is_valid():
        obj = form_create.save(commit=False)
        obj.username = username_id
        obj.save()
        form_create = ActivityModelForm()
    template_name = 'activity/formActivity.html'
    context = {'form': form_create}
    return render(request, template_name, context}