Dynamically Displaying Fields in a ModelForm

Published on 2009-04-21 13:31:00+00:00
Programming   django   web  

The Problem: I want to dynamically include some fields in a ModelForm based on some external criteria. Sometimes I want the fields displayed, sometimes I don't.

I'm going to try to explain this scenario through a (albeit contrived) example. I have a Model that looks like the following:

class Suff(models.Model):  
    foo = models.CharField(max_length=255)  
    bar = models.BooleanField(default=False, blank=True)  
    def is\_foo\_bar(self):  
        ''' is this model's foo attribute set to 'bar' '''  
        return self.foo == 'bar'  

Normally, if I needed a Form for this Model, I would subclass a ModelForm like the following:

class StuffForm(models.ModelForm):  
    class Meta:  
        model = Stuff  
        fields = ('foo', 'bar')  

However, if I do NOT want the 'bar' field to be displayed by default I would need to remove it from the ModelForms list of fields (or use something like exclude = ('bar', ) ). But, if this form is created with an instance of Stuff whose foo attribute contains the string bar, I would like for the Form's 'bar' field to be displayed.

I originally tried to accomplish this task by overridding StuffForm's __init__ method, and adding a new BooleanField when the desired circumstances arose... However, I stumpled across Ross Poulton's Dynamic ModelForms in Django, and then I realized it would be much easier to prevent a ModelForm's Field from being displayed than it would be to dynamically create one.

In order to accomplish this, the StuffForms's __init__ method would look something like the following:

def \_\_init\_\_(self, \*args, \*\*kwargs):  
    super(StuffForm, self).__init__(\*args, \*\*kwargs)  
    # If this form is created without an instance, OR  
    # If the instance's foo field is != 'bar'  
    if not kwargs.has_key('instance') or (kwargs.has_key('instance') and \  
        not kwargs['instance'].is_foo_bar()):  
        # Remove this field from the form.  
        del self.fields['bar']  

Done. I get all the benefits of a ModelForm, and the bar field is not displayed unless it should be.