Challenge 3: Validate Incoming Data — Possible Solution
====================================================================

# catalog/serializers.py

from rest_framework import serializers
from .models import Book

class BookSerializer(serializers.ModelSerializer):
    price = serializers.DecimalField(max_digits=6, decimal_places=2, min_value=0)

    class Meta:
        model = Book
        fields = ["id", "title", "price", "in_stock"]

# --- Demonstrating validation with a negative price ---

>>> from catalog.serializers import BookSerializer

>>> serializer = BookSerializer(data={
...     "title": "Discounted Book",
...     "price": "-5.00",
...     "in_stock": True,
... })

>>> serializer.is_valid()
False

>>> serializer.errors
{'price': [ErrorDetail(string='Ensure this value is greater than or equal to 0.', code='min_value')]}

# Compare to a valid submission:

>>> valid_serializer = BookSerializer(data={
...     "title": "Regular Book",
...     "price": "9.99",
...     "in_stock": True,
... })
>>> valid_serializer.is_valid()
True
>>> valid_serializer.errors
{}

WHY THIS WORKS
--------------
- min_value=0 on the DecimalField adds a validator that runs as part of
  is_valid() — a submitted price below zero fails validation before the
  serializer ever attempts to save anything, exactly like Course 1's
  clean_price() custom ModelForm validation achieved a very similar rule
  by hand.
- serializer.is_valid() returns a plain boolean, and serializer.errors is
  only meaningfully populated AFTER is_valid() has been called (calling
  .errors before .is_valid() would either be empty or raise, depending on
  DRF version) — this mirrors the is_valid()/cleaned_data pattern from
  Django's own Forms in Course 1 almost exactly, since DRF's serializers
  were deliberately designed to feel familiar to anyone who already knows
  Django Forms.
- The error message itself ("Ensure this value is greater than or equal
  to 0.") is DRF's built-in validator message for min_value — no custom
  clean_price()-style method was needed here, since DecimalField's own
  min_value argument covers this specific rule directly.