IntegrityError, NOT NULL constraint failed - django

Question

I want to add tags to posts in my Django app.
I can add tags through the admin interface, but when I try to do it through the form I created, I get an IntegrityError.
I couldn't find the solution in the existing topics with the same error. I ran makemigrations and migrate.
From models.py:
class Post(models.Model):
title = models.CharField(null=False, blank=False)
text = models.TextField()
class Tag(models.Model):
post = models.ForeignKey('my_app.Post', on_delete=models.CASCADE, related_name='tags')
tag_text = models.CharField()
The view:
def post_add_tags(request, pk):
post= get_object_or_404(Post, pk=pk)
if request.method == "POST":
form = TagForm(request.POST)
if form.is_valid():
tag = form.save()
tag.post= post
tag.save()
return redirect("single_post_view", pk=post.pk)
else:
form = TagForm()
return render(request, "add_tags.html", {"form": form})
The form from forms.py:
class TagForm(forms.ModelForm):
class Meta:
model = Tag
fields = ["tag_text"]
The template:
<form method="POST"> {% csrf_token %}
{{ form.as_p }}
<button type="submit" class="save btn btn-default">Add tags</button>
</form>
The error:
IntegrityError at /post/4/add_tags/
NOT NULL constraint failed: my_app_tag.post_id
I'm using Django version 2.2, Python 3.6.8.

Answer 1

do not save form until you set all required fields, specially foreign keys :
def post_add_tags(request, pk):
post= get_object_or_404(Post, pk=pk)
if request.method == "POST":
form = TagForm(request.POST)
if form.is_valid():
tag = form.save(commit=False)
tag.post= post
tag.save()
return redirect("single_post_view", pk=post.pk)
else:
form = TagForm()
return render(request, "add_tags.html", {"form": form})