Question

Nothing is worse than not understanding why something works. I asked this question a couple of days ago and figured out a fix, but I don’t know why it works:

How to debug a Django MultiValueDictKeyError on Formset POST.

Why does the formset need a pointer field reference to the superclass of the formset’s model form model? I have other formsets just like this that don’t.

To make this a standalone question, here are the details:

When I post my formset, I get a MultiValueDictKeyError. Specifically:

MultiValueDictKeyError at /core/customers/1/update/documents/
"Key u'documents-0-attachment_ptr' not found in <QueryDict: {u'documents-1-last_modified_date': [u''], u'documents-1-name': [u''], u'documents-MAX_NUM_FORMS': [u''], u'documents-0-attachment_file': [u''], u'documents-INITIAL_FORMS': [u'1'], u'documents-1-document_type': [u''], u'documents-0-notes': [u''], u'documents-1-notes': [u''], u'submit': [u'Submit changes'], u'documents-0-DELETE': [u'on'], u'documents-1-attachment_file': [u''], u'documents-0-document_type': [u''], u'documents-TOTAL_FORMS': [u'2'], u'documents-0-name': [u'test'], u'documents-1-creation_date': [u''], u'documents-0-creation_date': [u'2012-12-01 23:41:48'], u'csrfmiddlewaretoken': [u'NCQ15jA7erX5dAbx20Scr3gWxgaTn3Iq', u'NCQ15jA7erX5dAbx20Scr3gWxgaTn3Iq', u'NCQ15jA7erX5dAbx20Scr3gWxgaTn3Iq'], u'documents-0-last_modified_date': [u'2012-12-01 23:41:48']}>"

The key part is that Django is looking for the key documents-0-attachment_ptr in the post data. This is confusing — a Document is a subclass of an Attachment. All of the other post data is as expected. Why is Django needing pointer data in my formset?

Here is the form used in the formset:

class DocumentInlineForm(forms.ModelForm):  # pylint: disable=R0924
    attachment_file = forms.FileField(widget=NoDirectoryClearableFileInput)
    notes = forms.CharField(
        required=False,
        widget=forms.Textarea(attrs={'rows': 2,}), 
    )
    helper = DocumentInlineFormHelper()

    class Meta: # pylint: disable=W0232,R0903
        fields = (
            'attachment_file', 
            'creation_date',
            'document_type',
            'last_modified_date',
            'name',
            'notes',
        )
        model = Document

And here is the Document model:

"""
Handles document model definitions.
"""
from django.db import models
from eee_core.models.attachments import Attachment
from django.db.models.signals import pre_save
from datetime import datetime
from django.utils.timezone import utc

class Document(Attachment):
    """
    A document is an attachment with additional meta data.
    """
    creation_date = models.DateTimeField(
        blank=True, 
        null=True,
    )
    document_type = models.CharField(
        blank=True, 
        choices=(
            ('CONTRACT', 'Contract'),
            ('INVOICE', 'Invoice'),
            ('FACILITY', 'Facility change form'),
            ('LOA', 'Letter of authorization'),
            ('USAGE', 'Usage history document'),
            ('OTHER', 'Other'),
        ),
        default=None, 
        null=True, 
        max_length=8, 
    )
    last_modified_date = models.DateTimeField(
        blank=True, 
        null=True,
    )
    notes = models.TextField(
        blank=True,
        null=True,
    )

    class Meta(Attachment.Meta): # pylint: disable=W0232,R0903
        """
        Sets meta fields for model.
        """
        app_label = 'core'

    def __str__(self):
        return unicode(self).encode('utf-8')

    def __unicode__(self):
        return unicode(self.name)

def pre_save_callback(sender, instance, *args, **kwargs): # pylint: disable=W0613
    if not isinstance(instance, Document):
        return

    if not instance.creation_date:
        instance.creation_date = datetime.utcnow().replace(tzinfo=utc)

    instance.last_modified_date = datetime.utcnow().replace(tzinfo=utc)

pre_save.connect(pre_save_callback, dispatch_uid='document_pre_save')

Additional info:

Curiously, the inital post of the formset works fine. It is only on update posts — when there are initial forms in the formset — when I get this error. It also happens when I try to delete forms from the formset.

Also, the formset is a generic inline formset using django crispy forms.

Update

There was a request for the template code used. Here is the simplified version:

{% load crispy_forms_tags %}
{% load url from future %}
<form action="" method="post" enctype="multipart/form-data">
    {{ formset.management_form }}
    {% for subform in formset.forms %}
      {{ subform.id }}
      {% crispy subform %}
    {% endfor %}
    <div class="btn-toolbar">
        <input class='btn btn-primary' type="submit" name="submit" value="Submit changes" />
    </div>
</form>

I stopped this error by adding attachment_ptr to the field list of my form. So DocumentInlineForm is now:

class DocumentInlineForm(forms.ModelForm):  # pylint: disable=R0924
    attachment_file = forms.FileField(widget=NoDirectoryClearableFileInput)
    notes = forms.CharField(
        required=False,
        widget=forms.Textarea(attrs={'rows': 2,}), 
    )
    helper = DocumentInlineFormHelper()

    class Meta: # pylint: disable=W0232,R0903
        fields = (
            'attachment_ptr',
            'attachment_file', 
            'creation_date',
            'document_type',
            'last_modified_date',
            'name',
            'notes',
        )
        model = Document

Maybe it is something I didn’t know before, but does Django require you to provide a pointer to the superclass in all forms that use a subclassed model? This surprises me.

Answer 1

Don’t forget that a subclass of a non-abstract model is really just a one-to-one relationship between the two models. So naturally Django needs some sort of pointer from one to the other, otherwise it can’t possibly know which row in the parent table to update when the child changes.