Django "Enter a list of values" form error when rendering a ManyToManyField as a Textarea

Question

I'm trying to learn Django and I've ran into some confusing points. I'm currently having trouble creating a movie using a form. The idea of the form is to give the user any field he'd like to fill out. Any field that the user fills out will be updated in its respective sql table (empty fields will be ignored). But, the form keeps giving me the error "Enter a list of values" when I submit the form. To address this, I thought stuffing the data from the form into a list and then returning that list would solve this.

The first idea was to override the clean() in my ModelForm. However, because the form fails the is_valid() check in my views, the cleaned_data variable in clean() doesn't contain anything. Next, I tried to override the to_python(). However, to_python() doesn't seem to be called.

If I put __metaclass__ = models.SubfieldBase in the respective model, I receive the runtime error

"TypeError: Error when calling the metaclass bases metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases"

My approach doesn't seem to work. I'm not sure how to get around the 'Enter a list of values" error! Any advice?

Here is the relevant code (updated):

models.py

""" Idea:
A movie consists of many equipments, actors, and lighting techniques. It also has a rank for the particular movie, as well as a title. 

A Theater consists of many movies.

A nation consists of many theaters. 
"""

from django.db import models
from django.contrib.auth.models import User

class EquipmentModel(models.Model):
        equip = models.CharField(max_length=20)
#       user = models.ForeignKey(User)

class ActorModel(models.Model):
        actor = models.CharField(max_length=20)
#       user = models.ForeignKey(User)

class LightModel(models.Model):
        light = models.CharField(max_length=20)
#       user = models.ForeignKey(User)

class MovieModel(models.Model):
#       __metaclass__ = models.SubfieldBase   
        rank = models.DecimalField(max_digits=5000, decimal_places=3)
        title = models.CharField(max_length=20)

        equipments = models.ManyToManyField(EquipmentModel, blank=True, null=True)
        actors = models.ManyToManyField(ActorModel, blank=True, null=True)
        lights = models.ManyToManyField(LightModel, blank=True, null=True)

class TheaterModel(models.Model):
        movies = models.ForeignKey(MovieModel)

class NationModel(models.Model):
        theaters = models.ForeignKey(TheaterModel)

=====================================
forms.py

""" 
These Modelforms tie in the models from models.py

Users will be able to write to any of the fields in MovieModel when creating a movie.
Users may leave any field blank (empty fields should be ignored, ie: no updates to database).
"""

from django import forms
from models import MovieModel
from django.forms.widgets import Textarea

class MovieModelForm(forms.ModelForm):
      def __init__(self, *args, **kwargs):
             super(MovieModelForm, self).__init__(*args, **kwargs)
             self.fields["actors"].widget = Textarea()
             self.fields["equipments"].widget = Textarea()
             self.fields["lights"].widget = Textarea()

       def clean_actors(self):
             data = self.cleaned_data.get('actors')
             print 'cleaning actors'
             return [data]


      class Meta:
            model = MovieModel

=============================================
views.py

""" This will display the form used to create a MovieModel """

from django.shortcuts import render_to_response
from django.template import RequestContext
from forms import MovieModelForm

def add_movie(request):
       if request.method == "POST":
             form = MovieModelForm(request.POST)

             if form.is_valid():
                    new_moviemodel = form.save()
                    return HttpResponseRedirect('/data/')

       else:
             form = MovieModelForm()

       return render_to_response('add_movie_form.html', {form:form,}, context_instance=RequestContext(request))

Answer 1

The probable problem is that the list of values provided in the text area can not be normalized into a list of Models.

See the ModelMultipleChoiceField documentation.

The field is expecting a list of valid IDs, but is probably receiving a list of text values, which django has no way of converting to the actual model instances. The to_python will be failing within the form field, not within the form itself. Therefore, the values never even reach the form.

Is there something wrong with using the built in ModelMultipleChoiceField? It will provide the easiest approach, but will require your users to scan a list of available actors (I'm using the actors field as the example here).

Before I show an example of how I'd attempt to do what you want, I must ask; how do you want to handle actors that have been entered that don't yet exist in your database? You can either create them if they exist, or you can fail. You need to make a decision on this.

# only showing the actor example, you can use something like this for other fields too

class MovieModelForm(forms.ModelForm):
    actors_list = fields.CharField(required=False, widget=forms.Textarea())

    class Meta:
        model = MovieModel
        exclude = ('actors',)

    def clean_actors_list(self):
        data = self.cleaned_data
        actors_list = data.get('actors_list', None)
        if actors_list is not None:
            for actor_name in actors_list.split(','):
                try:
                    actor = Actor.objects.get(actor=actor_name)
                except Actor.DoesNotExist:
                    if FAIL_ON_NOT_EXIST: # decide if you want this behaviour or to create it
                        raise forms.ValidationError('Actor %s does not exist' % actor_name)
                    else: # create it if it doesnt exist
                        Actor(actor=actor_name).save()
        return actors_list

    def save(self, commit=True):
        mminstance = super(MovieModelForm, self).save(commit=commit)
        actors_list = self.cleaned_data.get('actors_list', None)
        if actors_list is not None:
            for actor_name in actors_list.split(","):
                actor = Actor.objects.get(actor=actor_name)
                mminstance.actors.add(actor)

        mminstance.save()
        return mminstance

The above is all untested code, but something approaching this should work if you really want to use a Textarea for a ModelMultipleChoiceField. If you do go down this route, and you discover errors in my code above, please either edit my answer, or provide a comment so I can. Good luck.

Edit:

The other option is to create a field that understands a comma separated list of values, but behaves in a similar way to ModelMultipleChoiceField. Looking at the source code for ModelMultipleChoiceField, it inhertis from ModelChoiceField, which DOES allow you to define which value on the model is used to normalize.

## removed code because it's no longer relevant. See Last Edit ##

Edit:

Wow, I really should have checked the django trac to see if this was already fixed. It is. See the following ticket for information. Essentially, they've done the same thing I have. They've made ModelMutipleChoiceField respect the to_field_name argument. This is only applicable for django 1.3!

The problem is, the regular ModelMultipleChoiceField will see the comma separated string, and fail because it isn't a List or Tuple. So, our job becomes a little more difficult, because we have to change the string to a list or tuple, before the regular clean method can run.

class ModelCommaSeparatedChoiceField(ModelMultipleChoiceField):
    widget = Textarea
    def clean(self, value):
        if value is not None:
            value = [item.strip() for item in value.split(",")] # remove padding
        return super(ModelCommaSeparatedChoiceField, self).clean(value)

So, now your form should look like this:

class MovieModelForm(forms.ModelForm):
    actors = ModelCommaSeparatedChoiceField(
               required=False, 
               queryset=Actor.objects.filter(), 
               to_field_name='actor')
    equipments = ModelCommaSeparatedChoiceField(
               required=False,
               queryset=Equipment.objects.filter(),
               to_field_name='equip')
    lights = ModelCommaSeparatedChoiceField(
               required=False, 
               queryset=Light.objects.filter(),
               to_field_name='light')

    class Meta:
        model = MovieModel