Use Model Property in a Django Model Form

Question

I am attempting to use model properties like fields within a model form, but so far haven't had any luck. The result is that the form renders only the model fields, and not the property I defined. Any idea how to get the form to recognize the property added to the model? I expect to see the latitude property added as just another field in the form.

Models.py:

class Plot(models.Model):
        plot_id = models.AutoField(primary_key=True)
        plot_number = models.IntegerField(validators=[MinValueValidator(1)], null=False, unique=True)
        geometry = models.PointField(srid=2163, null=True, blank=True) 
        objects = models.GeoManager()

        @property
        def latitude(self):
            self.geometry.transform(4326)
            return self.geometry.y

        @latitude.setter
        def latitude(self, latitude):
            self.geometry.y = latitude

Forms.py:

class InventoryPlotForm(ModelForm):
    class Meta:
        model = ForestInventoryPlot
        exclude = {"geometry"}

Answer 1

Some years later. A complete answer, working in Django up to 2.2. As others have pointed out, only real db fields are included in the model form. So, you'll need to:

• Define a custom model form, add your @property field
• Exclude your geometry field
• In the __init__ of the form, get the value, and set it as initial.
• Customize your save method (on the form or admin)

Note: This works also in more complex cases, where you want to abstract away some more complex database structure...

class InventoryPlotForm(ModelForm):

    class Meta:
        model = ForestInventoryPlot
        exclude = ("geometry", )

    latitude = forms.WhateverField()

    def __init__(self, *args, **kwargs):
        instance = kwargs.get('instance', None)
        if instance:
            kwargs['initial'] = {'latitude': instance.latitude, }
        super().__init__(*args, **kwargs)

    def save(self, *args, **kwargs):
        self.instance.latitude = self.cleaned_data['latitude']
        return super().save(*args, **kwargs)

Answer 2

Your property is not a Field instance, and as such a ModelForm cannot automatically generate a form field for it. You will need to explicitly define a latitude field on the InventoryPlotForm and manage its retrieval and update in the form's __init__ and save methods. Alternatively, you can implement your own Widget class and tell your InventoryPlotForm to use it for the geometry field:

class InventoryPlotForm(ModelForm):
    class Meta(object):
        widgets = {
            'geometry': MyGeometryWidget,
        }
        ...
    ...

Answer 3

Only Django Fields are automatically generated by ModelForm.

• You need to define your field in the ModelForm
• Probably make geometry field hidden
• Write custom ModelForm Save method and separately save model fields and model properties.
• Modify your view to set the initial for the ModelForm

For example something like this:

class InventoryPlotForm(ModelForm):
    latitude = forms.CharField(max_length=52)           # Something like this

    def save(self, commit=True):
        model_instance = super(InventoryPlotForm, self).save(commit=False)
        result = super(InventoryPlotForm, self).save(commit=True)
        model_instance.latitude = self.cleaned_data['latitude']
        model_instance.save()
        return result

    class Meta:
        model = ForestInventoryPlot
        widgets = {'geometry': HiddenInput()}

in your view

...
form = InventoryPlotForm(instance=forestinventoryplot,
                         initial={"latitude":forestinventoryplot.latitude})
...