Automatically backing up the last entry to a field in django admin

I have a model that serves as a price database for a part:

class Part(models.Model):
    name = models.CharField("name", max_length=128)

class Price(models.Model):
    value = models.DecimalField(max_digits=10, decimal_places=2)
    part = models.ForeignKey(Part, on_delete=models.CASCADE)
    is_active = models.BooleanField(default=True)

in the admin I want to have a field that can change the price, but also a list where the old prices are visible. I kind of solved it with Django built in methods, but it seems rather complicated for such a simple request:

class FakePlainTextWidget(forms.Widget):
    """
    this widget masks an input field for prices so they are only being listed and not really editable or look that way 
    """
    def __init__(self, attrs=None):
        super().__init__(attrs)

    def render(self, name, value, attrs, renderer=None):
        s = f"""border: none; background-color: transparent; pointer-events: none; -webkit-appearance: none; -moz-appearance: textfield;"""
        return format_html(f"""<input type="number" name="{attrs["id"][3:]}" value="{value}" id="{attrs["id"]}" style="{s}"> €""")

class PriceInlineForm(forms.ModelForm):
    class Meta:
        model = Price
        fields = "__all__"  
    
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        if self.instance and not self.instance.is_active:
            # make fields read-only if price is_active=False
            for field in self.fields:
                self.fields[field].widget = FakePlainTextWidget()

    def save(self, commit=False):
        instance = super().save(commit=False)
        instance.is_active = False
        instance.save(update_fields=["is_active"])
        Price.objects.create(value=self.cleaned_data["value"],  part=instance.part)
        return instance


class PriceInline(admin.TabularInline):
    model = Price
    can_delete = False
    exclude = ["is_active",]
    extra = 0  # No extra empty forms
    min_num = 1  # Ensure at least one form is displayed
    max_num = 1  # Ensure only one form can be added
    form = PriceInlineForm

    def get_queryset(self, request):
        qs = super().get_queryset(request)
        return qs.order_by("-id") # so active input is first


@admin.register(Part)
class PartAdmin(admin.ModelAdmin):
    inlines = [PriceInline]

I feel like there should be a much simpler way to achieve the same?

Back to Top