Add custom form fields that are not part of the model

Either in your admin.py or in a separate forms.py you can add a ModelForm class and then declare your extra fields inside that as you normally would. I've also given an example of how you might use these values in form.save():

from django import forms
from yourapp.models import YourModel


class YourModelForm(forms.ModelForm):

    extra_field = forms.CharField()

    def save(self, commit=True):
        extra_field = self.cleaned_data.get('extra_field', None)
        # ...do something with extra_field here...
        return super(YourModelForm, self).save(commit=commit)

    class Meta:
        model = YourModel

To have the extra fields appearing in the admin just:

Edit your admin.py and set the form property to refer to the form you created above.
Include your new fields in your fields or fieldsets declaration.

Like this:

class YourModelAdmin(admin.ModelAdmin):

    form = YourModelForm

    fieldsets = (
        (None, {
            'fields': ('name', 'description', 'extra_field',),
        }),
    )

UPDATE:

In Django 1.8 you need to add fields = '__all__' to the metaclass of YourModelForm.

It it possible to do in the admin, but there is not a very straightforward way to it. Also, I would like to advice to keep most business logic in your models, so you won't be dependent on the Django Admin.

Maybe it would be easier (and maybe even better) if you have the two seperate fields on your model. Then add a method on your model that combines them.

For example:

class MyModel(models.model):

    field1 = models.CharField(max_length=10)
    field2 = models.CharField(max_length=10)

    def combined_fields(self):
        return '{} {}'.format(self.field1, self.field2)

Then in the admin you can add the combined_fields() as a readonly field:

class MyModelAdmin(models.ModelAdmin):

    list_display = ('field1', 'field2', 'combined_fields')
    readonly_fields = ('combined_fields',)

    def combined_fields(self, obj):
        return obj.combined_fields()

If you want to store the combined_fields in the database you could also save it when you save the model:

def save(self, *args, **kwargs):
    self.field3 = self.combined_fields()
    super(MyModel, self).save(*args, **kwargs)

Django 2.1.1 The primary answer got me halfway to answering my question. It did not help me save the result to a field in my actual model. In my case I wanted a textfield that a user could enter data into, then when a save occurred the data would be processed and the result put into a field in the model and saved. While the original answer showed how to get the value from the extra field, it did not show how to save it back to the model at least in Django 2.1.1

This takes the value from an unbound custom field, processes, and saves it into my real description field:

class WidgetForm(forms.ModelForm):
    extra_field = forms.CharField(required=False)

    def processData(self, input):
        # example of error handling
        if False:
            raise forms.ValidationError('Processing failed!')

        return input + " has been processed"

    def save(self, commit=True):
        extra_field = self.cleaned_data.get('extra_field', None)

        # self.description = "my result" note that this does not work

        # Get the form instance so I can write to its fields
        instance = super(WidgetForm, self).save(commit=commit)

        # this writes the processed data to the description field
        instance.description = self.processData(extra_field)

        if commit:
            instance.save()

        return instance

    class Meta:
        model = Widget
        fields = "__all__"

You can always create new admin template, and do what you need in your admin_view (override the admin add URL to your admin_view):

url(r'^admin/mymodel/mymodel/add/$','admin_views.add_my_special_model')

Add custom form fields that are not part of the model

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