python-tdd/src/apps/dashboard/forms.py

from django import forms
from django.core.exceptions import ValidationError
from .models import Item

DUPLICATE_ITEM_ERROR = "You've already logged this to your list"
EMPTY_ITEM_ERROR = "You can't have an empty list item"

class ItemForm(forms.models.ModelForm):
    class Meta:
        model = Item
        fields = ("text",)
        widgets = {
            "text": forms.widgets.TextInput(
                attrs={
                    "placeholder": "Enter a to-do item",
                    "class": "form-control form-control-lg",
                }
            ),
        }
        error_messages = {"text": {"required": EMPTY_ITEM_ERROR}}

    def save(self, for_list):
        self.instance.list = for_list
        return super().save()

class ExistingListItemForm(ItemForm):
    def __init__(self, for_list, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.instance.list = for_list

    def clean_text(self):
        text = self.cleaned_data["text"]
        if self.instance.list.item_set.filter(text=text).exists():
            raise forms.ValidationError(DUPLICATE_ITEM_ERROR)
        return text