How to force-save an "empty"/unchanged django admin inline?

I have some inlines in one of my admin models which have default values which likely won't need to be changed when adding a new instance with "Add another ...". Unfortunately django won't recognize these inline as new objects unless some value has changed. This forces me to add an inline, change an arbitrary value, save, change the value back and save again to reac开发者_运维百科h the desired effect.

The only solution I've come up with so far is to add a hidden 'changed'-field which would be populated via JavaScript when adding a new inline. As this feels very hackish I hope there is a more elegant solution.

It took me quite some time to figure out but it is actually really simple.

from django.contrib import admin
from django.forms.models import BaseInlineFormSet, ModelForm

class AlwaysChangedModelForm(ModelForm):
    def has_changed(self):
        """ Should returns True if data differs from initial. 
        By always returning true even unchanged inlines will get validated and saved."""
        return True

class CheckerInline(admin.StackedInline):
    """ Base class for checker inlines """
    extra = 0
    form = AlwaysChangedModelForm

@daniel answer is great, however it will try to save the instance that is already created ever if no changes is made, which is not necessary, better to use it like:

class AlwaysChangedModelForm(ModelForm):
    def has_changed(self, *args, **kwargs):
        if self.instance.pk is None:
            return True
        return super(AlwaysChangedModelForm, self).has_changed(*args, **kwargs)

Combined @Daniels, @HardQuestions' and @jonespm answers:

class MarkNewInstancesAsChangedModelForm(forms.ModelForm):
    def has_changed(self):
        """Returns True for new instances, calls super() for ones that exist in db.
        Prevents forms with defaults being recognized as empty/unchanged."""
        return not self.instance.pk or super().has_changed()