Propagate model field defaults to ModelSerializer fields

[M2004GV]

Description

Refreshed resubmission of #8130, which was closed without being merged.

ModelSerializer already propagates a model field's default to the generated serializer field when that field participates in a uniqueness constraint (get_uniqueness_extra_kwargs), but not otherwise. This means a plain IntegerField(default=0) (with no uniqueness constraint) is generated as required=False but with no default, so omitting it from the input simply drops the attribute instead of falling back to the model's default — inconsistent with how Django's ModelForm behaves, and surprising given DRF already does the equivalent for unique-constrained fields.

This PR generalizes that existing behavior: whenever model_field.has_default(), the value is passed through as the serializer field's default in get_field_kwargs.

Closes #7469. Related to #2683 / #7489.

Addressing prior review feedback

On the original PR, @auvipy asked for two things before it could be accepted:

1. Rebase with a fresh approach — done here as a clean two-commit PR.
2. Update OPTIONS responses to include the default, consistent with how default is already surfaced in schemas/openapi.py. Done in the second commit: SimpleMetadata.get_field_info now includes a default key for any non-callable field default.

Behavior change

The only user-visible behavior change is for non-partial updates/creates: a field with a model-level default that is omitted from the input will now have that default appear in validated_data, matching what already happens for fields covered by uniqueness constraints. Partial updates (partial=True) are unaffected — defaults are still skipped in that case, per existing Field.get_default() behavior.

Test plan

• Added test_default_value_used_when_field_omitted and test_default_value_skipped_on_partial_update in tests/test_model_serializer.py, and updated the repr() regexes in test_regular_fields / test_field_options to reflect the new default=... kwarg.
• Added test_default_value_field_info and test_callable_default_field_info in tests/test_metadata.py.
• Added a note to docs/api-guide/fields.md documenting the propagation.
• Full test suite passes against both Django 5.2 and Django main (6.2 dev): 1562 passed, 0 failed.
• flake8 clean on changed files.

[Shrikantgiri25]

Great work! Quick suggestion: Add tests for:

1. Callable defaults (uuid.uuid4) - ensure they execute correctly
2. Choice fields with default - validates the original #7469 fix end-to-end
3. OpenAPI schema (optional) - verify defaults appear in docs

These cover the exact scenarios from the linked issues.

[Copilot]

Pull request overview

This PR updates DRF’s ModelSerializer field generation to propagate Django model field default values into the generated serializer fields, aligning validation behavior (and metadata/schema output) more closely with model defaults.

Changes:

• Propagate model_field.default into serializer field kwargs when model_field.has_default() in get_field_kwargs.
• Surface non-callable field defaults in SimpleMetadata.get_field_info (for OPTIONS metadata), consistent with OpenAPI schema behavior.
• Update tests and documentation to reflect and validate default propagation behavior (including callable defaults and choices fields).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
rest_framework/utils/field_mapping.py	Adds default propagation for non-relational model fields via get_field_kwargs.
rest_framework/metadata.py	Includes non-callable field defaults in OPTIONS metadata output.
tests/test_model_serializer.py	Updates repr expectations and adds tests covering default propagation, callable defaults, and choices-with-default behavior.
tests/test_metadata.py	Adds coverage ensuring metadata includes non-callable defaults and excludes callable defaults.
tests/schemas/test_openapi.py	Adds schema coverage verifying model defaults appear in OpenAPI output (excluding callable defaults).
docs/api-guide/fields.md	Documents that ModelSerializer will use model field defaults as serializer defaults.

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[Copilot]

🟡 Changes recommended

Relational defaults can cause invalid assignments, and explicit required=True can prevent serializer construction.

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

Review details

Suppressed comments (1)

rest_framework/utils/field_mapping.py:137

• Primary-key OneToOneFields also pass through this helper before build_standard_field() converts them to PrimaryKeyRelatedField (serializers.py:1298-1301). With this change, an omitted relational default such as default=1 bypasses to_internal_value(), leaving the integer in validated_data; the default create/update path then assigns it to the relation descriptor instead of a model instance and fails. Exclude relational fields here, as the documentation promises.

    if model_field.has_default():
        kwargs['default'] = model_field.default

• Files reviewed: 6/6 changed files
• Comments generated: 1
• Review effort level: Balanced

[auvipy]

@M2004GV please cross check and fix this