custom data coercion with django modelform

Question

Caveat: I don't have a deep knowledge, so if I'm barking up the wrong tree please let me know.

Anyways, I'm working on writing a tagging app. I want the user to be able to enter a list of space separated words rather than have to browse through a gigantic list of tags.

There are two models. One holds the name of the tags, and another holds tag assignments.

class Tags(models.Model):
    name = models.CharField(max_length=50)

    def __unicode__(self):
        return self.name

class Tagged(models.Model):
    tag = models.ForeignKey(Tags)
    app = models.CharField(max_length=256)
    app_item = models.IntegerField()

The modelform just displays the tag field as that is the only input needed from the user.

class TaggedForm(forms.ModelForm):
    class Meta:
        model = Tagged
        fields = ('tag',)
        widgets = {
            'tag': TextInput(),
        }

The problem I'm having is that while I am able to enter a list of space separated choices, the input is rejected as invalid.

Select a valid choice. That choice is not one of the available choices.

What I would like to do is get the data, coerce it into valid choices myself and return that cleaned data as a choice field (ie: take what's unexpected and user-friendly and make it expected and django friendly).

My question is, how can I do this and simply as possible?

Thanks.

Edit:

The solution follows the recommendation of Tisho.

The data is cleaned in the form and a custom save function handles the saving so the app only needs to pass in a few variables. It's still a bit rough around the edges (no permissions for instance), but it works.

class TaggedForm(forms.Form):
    tags = forms.CharField(max_length=50)

    def save(self, app, app_item, tags):

        # Convert tag string into tag list (whitespace and duplicates are removed here)
        split_tags = list(set(tags.strip().split(' ')))
        tag_list = []
        for tag in split_tags:
            if tag != ''  and tag != u'':
                tag_list.append(tag)

        # Get list of current tags
        current_tags = Tagged.objects.filter(app=app, app_item=app_item)

        # Separate new, stable, and old tags
        # (stable tags are deleted from tag_list leaving it populated by only new tags)
        old_tags = []
        if current_tags:
            for tag in current_tags:

                # Stable tags
                if str(tag.tag) in tag_list:

                    # Delete tag from tag_list (this removes pre-existing tags leaving just new tags)
                    del tag_list[tag_list.index(str(tag.tag))]

                # Old tags
                elif not str(tag.tag) in tag_list:
                    old_tags.append(tag.tag)

        # Remove old tags
        try:
            Tagged.objects.filter(tag__in=old_tags).delete()
        except Tagged.DoesNotExist:
            pass

        # Add new tags
        for tag in tag_list:

            # Get tag object
            try:
                tag=Tags.objects.get(name=tag)
                tag.save()

            # Create new tag
            except Tags.DoesNotExist:
                tag = Tags(name=tag)
                tag.save()

            # Add new tagging relationship
            try:
                new_tag = Tagged(tag=tag, app=app, app_item=app_item)
                new_tag.save()
            except Tags.DoesNotExist:
                pass

    def clean(self):

        # Get tags
        try:
            tags = self.cleaned_data['tags'].strip().split(' ')
        except KeyError:
            tags = False

        # Process tags
        if tags:

            # Variables
            tag_list = ''

            # Build tag list
            for tag in tags:
                if tag != '' or tag != u'':
                    tag_list += tag + ' '

            # Assign and return data
            self.cleaned_data['tags'] = tag_list
            return self.cleaned_data

        # Just return cleaned data if no tags were submitted
        else:
            return self.cleaned_data    

Usage: The tagging logic is kept out of the application logic

tagged_form = TaggedForm(request.POST)
if tagged_form.is_valid():
    tagged_form.save('articles', 1, request.POST.get('tags',''))

Answer 1

As long as you just need a list of tags here - you don't need a forms.ModelForm. A ModelForm will try to create a Model(Tags) instance, so it will require all the fields - app, app_item. A simple form will do work just fine instead:

class TagsForm(forms.Form):
    tags = forms.CharField(max_length=200)

and in the view just process the form:

if request.POST:
    form = TagsForm(request.POST)
    if form.is_valid():
        tags = form.cleaned_data['tags'].split(' ')
        # here you'll have a list of tags, that you can check/map to existing tags in the database(or to add if doesn't exist.. some kind of pre-processing).

I don't know what your real goal is, but from here you can display a second form:

class TaggedForm2(forms.ModelForm):
    class Meta:
        model = Tagged

and after you have the tags from the user input, to create a new form:

form = TaggedForm()
form.fields['tag'].widget = forms.widgets.SelectMultiple(choices=((t.id, t.name) 
                            for t in Tags.objects.filter(name__in=tags))) 

I'm not sure if this is even close to what you need, just adding some examples here..