class KeepUnselectableModelFormMixin:
    """
    Keep unselectable items of queryset-based fields.

    Mixin for 'ModelForm'.

    Attribute
        keep_unselectable_fields (list of field names)

    This adding is performed in 'save' method. Specifically, if 'commit' arg
    is False, it's done in 'save_m2m' method.

    These fields must have a 'queryset' attribute (the selectable items), like
    'ModelMultipleChoiceField' (default field for ManyToMany model fields).
    """
    keep_unselectable_fields = []

    def get_unselectable(self, field_name):
        """
        Returns 'field_name' model field items of instance which can't be
        selected with the corresponding form field.

        Should be used before 'form.save' call, or 'form.save_m2m' call if
        'commit=False' was passed as argument to 'form.save'.
        """
        if self.instance.pk:
            previous = getattr(self.instance, field_name).all()
            selectable = self.fields[field_name].queryset
            return previous.exclude(pk__in=[o.pk for o in selectable])
        else:
            # Instance is being created, there is no previous item.
            return []

    def save(self, commit=True):
        # Use 'commit=False' to get the 'save_m2m' method.
        instance = super().save(commit=False)

        _save_m2m = self.save_m2m

        def save_m2m():
            # Get the unselectable items.
            # Force evaluate because those items are going to change.
            unselectable_f = {
                f_name: list(self.get_unselectable(f_name))
                for f_name in self.keep_unselectable_fields
            }
            # Default 'save_m2m' use 'set' method of m2m relationships with
            # fields' cleaned data.
            _save_m2m()
            # Add the unselectable elements.
            for f_name, unselectable in unselectable_f.items():
                getattr(instance, f_name).add(*unselectable)

        # Implement the default behavior of 'save' method, with our own
        # 'save_m2m'.
        if commit:
            instance.save()
            save_m2m()
        else:
            self.save_m2m = save_m2m
        return instance