Django: Save to a many-to-many field in a ModelForm's save() method?

Question

I'm writing a webblog app in Django. I currently have 2 models Post and PostMeta. Post is a standard post style model with fields such as author, title, content etc. It also contains a single many-to-many field called post_meta which is associated with my second model PostMeta. PostMeta is a simple name/value model with two fields, meta_key and meta_value.

What I'm trying to do is customize the Post model's form in the admin interface to be more intuitive. Specifically I want to abstract the creation of PostMeta associations rather than see the unintuitive select box which is redered by default for the admin. I want to instead show a text field in place of this select box where the user can enter a comma separated list of tags associated with the post. When the form is submitted I want to split the tag field's input into individual tags and save each as a PostMeta where meta_key will be set to "TAG" and meta_value will be one of the comma separated strings.

The problem I am having is I can't seem to get it to save correctly. I'm not sure if there is a problem with my syntax (I'm relativly new to python) or if there is somthing else I need to do which I may have missed. Here is a snippet of my admin.py:

class PostAdminForm(forms.ModelForm):
    tags = forms.CharField(max_length=200)
    class Meta:
        model = Post
    def save(self, commit=True):
        model = super(PostAdminForm, self).save(commit=False)
        if commit:
            model.save()
            splitTags = self.cleaned_data['tags'].split(',')
            for tag in splitTags:
                pm = PostMeta(meta_key="TAG", meta_value=tag)
                pm.save()
                model.post_meta.add(pm)
        return model

class PostAdmin(admin.ModelAdmin):
    model = Post
    form = PostAdminForm

admin.site.register(Post, PostAdmin)

Any advice or suggestions on how to make this work would be great. Still learning :\

Answer 1

The most immediate issue in your code is that when the save() method is called by the Django admin, the commit argument is almost always False. However, if you simply ignore the value of commit, you will not be able to do model.post_meta.add(pm) for newly created Posts, since the model will not have been created in the database yet (and hence cannot be referred to in the Post to PostMeta m2m table).

See my answer to a different post, which I think is also applicable in your case, and has quite a bit of code that you may find useful.