Fix BookListSerializer update example logic to support creation

[JPisOP007]

Description

This PR fixes a bug in the documentation example for BookListSerializer.update() under the ListSerializer section of the serializers API guide.

Problem

The current documentation example crashes with a KeyError if list items are submitted without an id field. This occurs when trying to perform creations alongside updates via a ListSerializer.

Solution

1. Added list comprehensions to separate the list items:
   • Filtered data_mapping to only include validated data items containing an 'id' key (updates/deletes).
   • Filtered out new_items that do not contain an 'id' key (creations).
2. Iterated over new_items and called .create(data) on the child serializer (self.child.create(data)).
3. Updated the BookSerializer definition in the example to use id = serializers.IntegerField(required=False). This ensures validation succeeds when creating new items that do not yet have an ID.

refs #9469

[ulugbekbackend]

Logic looks correct overall — nice catch on the KeyError that happens when creating list items without an id.
A couple of small suggestions:

Avoid iterating validated_data twice. Right now data_mapping and new_items are built with two separate comprehensions, each looping over the full list. This could be combined into a single loop:

data_mapping = {}
new_items = []
for item in validated_data:
    if 'id' in item:
        data_mapping[item['id']] = item
    else:
        new_items.append(item)

This keeps the same result but only goes through validated_data once.

Is there a docs test covering this example? DRF sometimes has tests that verify code examples in the docs actually run correctly. If one exists for this section, it might need updating too — worth checking.

Otherwise this is a solid fix for a real bug in the documentation. Thanks for working on it!

[ulugbekbackend]

Overall logic is solid — good fix for the KeyError on creation without an id. Left a couple of inline notes.

[ulugbekbackend]

Nit: this could be a single loop instead of two separate comprehensions, to avoid iterating validated_data twice — e.g.:

data_mapping = {}
new_items = []
for item in validated_data:
    if 'id' in item:
        data_mapping[item['id']] = item
    else:
        new_items.append(item)

[ulugbekbackend]

Makes sense given the new create path.

[Copilot]

🟢 Approval recommended

The documentation example change directly addresses the reported KeyError scenario and remains consistent with DRF ListSerializer usage patterns.

Pull request overview

This PR fixes a documentation bug in the serializers API guide by updating the BookListSerializer.update() example to correctly handle mixed update + create submissions when some list items do not include an id.

Changes:

• Prevent KeyError in the example by building data_mapping only from items that include an id.
• Add explicit handling for “new” items (without id) by calling self.child.create(...) for each.
• Update the example BookSerializer so id is optional (required=False) to allow validation of create items.

File summaries

File	Description
docs/api-guide/serializers.md	Updates the ListSerializer multiple-update example to support create-without-id inputs without crashing.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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[Copilot]

🟡 Changes recommended

Creation and update processing must preserve the submitted item order.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Balanced