Challenge 2: Add Custom Validation — Possible Solution
====================================================================

# catalog/forms.py

from django import forms
from .models import Author

class AuthorForm(forms.ModelForm):
    class Meta:
        model = Author
        fields = ["name", "bio"]

    def clean_bio(self):
        bio = self.cleaned_data.get("bio", "")
        if bio and len(bio) > 500:
            raise forms.ValidationError(
                "Bio must be 500 characters or fewer (currently %(count)d)."
                % {"count": len(bio)}
            )
        return bio

# --- Usage in the shell, to demonstrate both outcomes ---

>>> from catalog.forms import AuthorForm
>>> short_bio_form = AuthorForm(data={"name": "Ann Leckie", "bio": "A sci-fi author."})
>>> short_bio_form.is_valid()
True

>>> long_bio_form = AuthorForm(data={"name": "Ann Leckie", "bio": "x" * 600})
>>> long_bio_form.is_valid()
False
>>> long_bio_form.errors
{'bio': ['Bio must be 500 characters or fewer (currently 600).']}

WHY THIS WORKS
--------------
- Django automatically calls any method named clean_<fieldname>() as part
  of is_valid()'s validation pass — clean_bio() specifically targets the
  bio field, running AFTER that field's own built-in validation (e.g.
  TextField's basic type check) has already passed.
- self.cleaned_data.get("bio", "") reads the already-validated value for
  this field — using .get() with a default avoids a KeyError in the edge
  case where bio failed its own built-in validation before clean_bio()
  even runs.
- raise forms.ValidationError(...) is how a clean_<field>() method
  reports a failure — Django catches it, attaches the message to that
  specific field's errors, and is_valid() correctly returns False.
- Returning bio at the end (the ONLY path that doesn't raise) is required
  — a clean_<field>() method must return the cleaned value, since
  whatever it returns becomes the new cleaned_data value for that field.