Question

I would like to be able to extract different information in my django form:

That’s my form:

<form action="" method="post">{% csrf_token %}
{{ form.as_p }}
<input type="submit" value="Submit" />
</form>

class InstanceForm(ModelForm):
    class Meta:
        model = models.BaseAsset
        widgets = {
            'labels': LabelIconCheckboxSelectMultiple()
        }

The model:

class AssetClass(models.Model):
    default_labels = models.ManyToManyField(Label, null=True, blank=True)
    pass

the M2M reference field

class Label(models.Model):
    explanation = models.CharField(null=True, max_length=63)
    svgpreview  = models.CharField(null=True, max_length=31)
    def __unicode__(self):
        return unicode(self.explanation)
    pass

Now, the HTML code generated by the {{ form.as_p }} is as follows:

<li><label for="id_labels_0"><input type="checkbox" name="labels" value="1" id="id_labels_0" /> Consult owner before using</label></li>
<li><label for="id_labels_1"><input type="checkbox" name="labels" value="2" id="id_labels_1" /> This item is broken</label></li>

Which means it’s clearly using the __unicode__ rendering of the model ‘Label’. How can I change that behavior in the Select widget, so that it would use a different function to populate it’s choices? I’m trying to get it, in the reasonably portable way, to print '<img src="{{label.svgpreview}}" alt="{{label.explanation}}"...>' next to the checkbox?

Answer 1

Reading django.forms.models.ModelChoiceField gives a hint:

# this method will be used to create object labels by the QuerySetIterator.
# Override it to customize the label.
def label_from_instance(self, obj):
    """
    This method is used to convert objects into strings; it's used to
    generate the labels for the choices presented by this object. Subclasses
    can override this method to customize the display of the choices.
    """
    return smart_unicode(obj)

ok, but how do I override it per-instance of ModelForm – this gets overridden in few places throughout django.forms

Considering the following code:

class InstanceForm(ModelForm):
    class Meta:
        model = models.BaseAsset
        widgets = {
            'labels': forms.CheckboxSelectMultiple()
        }


    def __init__(self, *args, **kwargs):
        def new_label_from_instance(self, obj):
            return obj.svgpreview

        super(InstanceForm, self).__init__(*args, **kwargs)
        funcType = type(self.fields['labels'].label_from_instance)
        self.fields['labels'].label_from_instance = funcType(new_label_from_instance, self.fields['labels'], forms.models.ModelMultipleChoiceField)

This is somewhat creepy – basically, it’s a more bizzare implementation of this:
Override a method at instance level

Please read the comments in the referenced thread to understand why this might be a bad idea in general..