Save/update a form to an EmbeddedDocument

Question

Here is a simple scenario: I have a blog post with comments attach to it. Now I want to use mongoforms to let user submit their comments. Therefore I have this models.py:

class Post(Document):
    title = StringField(max_length=60)
    body = StringField()
    created = DateTimeField(default=datetime.datetime.now)
    comments = ListField(EmbeddedDocumentField('Comment'))

class Comment(EmbeddedDocument):
    author = StringField(max_length=60)
    body = StringField()
    created = DateTimeField(default=datetime.datetime.now)

And I'm using django-mongoforms to support the similar ModelForm functionality, within forms.py:

class CommentForm(MongoForm):
    class Meta:
        document = Comment
        fields = ('author', 'body')

In the views.py, I just want to save what user has entered, more specifically the Author and Body fields of a comment, here is the method to add a comment:

def add_comment(request, id):
    post = Post.objects.with_id(id)
    if request.method == 'POST':
        form = CommentForm(request.POST, instance=post.comments)
        if form.is_valid():
            form.save()
    return HttpResponseRedirect(post.get_absolute_url())

Then once I click Submit button here comes the error message:

instance must be a mongoengine document, not BaseList

So questions are:

1. How to covert BaseList to Document in MongoEngine? I have looked through the document without a hint.
2. What's the best practices here generally to handle the EmbeddedDocument? Will ReferenceDocument help?

Also there is a related questions here but without an answer: What is the proper way to update a listfield of embedded documents in mongoengine?

Answer 1

As comments are embedded you'll need to find and update the existing comment in the post object. You can then overwrite the old comment with the new comment (where i is the index) eg:

post.comments[i] = new_comment

then just do a post.save() and mongoengine will convert that to a $set operation.

Alternatively, you could just write the $set directly eg:

Post.objects(pk=post.pk).update(set__comments__i=comment)

Forms.py

I still use the forms.py but I would pass commit=False to save that will return the comment - then append to the post and save the post like so:

def add_comment(request, id):
    post = Post.objects.with_id(id)
    if request.method == 'POST':
        form = CommentForm(request.POST)
        if form.is_valid():
            comment = form.save(commit=False)
            post.comments.append(comment)
            post.save()
    return HttpResponseRedirect(post.get_absolute_url())