Fix snake case array discriminator

[koxudaxi]

Fixes: #3091

Summary by CodeRabbit

• Documentation

  • Adjusted CLI reference example formatting for the single-line docstring output to improve readability.

• Bug Fixes

  • Validated and normalized discriminator field names when collapsing root models to ensure generated discriminators use valid field names.

• Tests

  • Added a regression test and expected output covering discriminator-in-array scenarios with snake_case fields targeting Pydantic v2.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Run ID: 2cdb7e26-9377-461d-bbf1-11f8e9cc5bf2

📥 Commits

Reviewing files that changed from the base of the PR and between 5af17f7 and 00f4c97.

📒 Files selected for processing (2)

• docs/cli-reference/field-customization.md
• tests/main/openapi/test_main_openapi.py

✅ Files skipped from review due to trivial changes (1)

• docs/cli-reference/field-customization.md

🚧 Files skipped from review as they are similar to previous changes (1)

• tests/main/openapi/test_main_openapi.py

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📝 Walkthrough

Walkthrough

Normalize discriminator property names during root-model collapse by validating them with ModelResolver.get_valid_field_name_and_alias; add a golden Pydantic v2 artifact and a regression test for discriminator-in-array snake_case behavior; adjust a documentation example's snippet formatting.

Changes

Discriminator Snake-Case Normalization

Layer / File(s)	Summary
Core Implementation src/datamodel_code_generator/parser/base.py	Discriminator property names in Parser.__collapse_root_models are now passed through ModelResolver.get_valid_field_name_and_alias(...) and the validated field name is assigned to copied_data_type.discriminator.
Expected Output tests/data/expected/main/openapi/discriminator/in_array_snake_case_pydantic_v2.py	Adds golden Pydantic v2 models and enums for discriminated union-in-array with snake_case field aliasing and Field(discriminator='object_type').
Regression Test tests/main/openapi/test_main_openapi.py	New test test_main_openapi_discriminator_in_array_snake_case validates generation from discriminator_in_array_snake_case.yaml with --snake-case-field, --collapse-root-models, and Pydantic v2 flags, asserting output matches the golden file.
Documentation docs/cli-reference/field-customization.md	Adjusted example code block header/import whitespace for the --use-single-line-docstring section to match current rendered output.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• koxudaxi/datamodel-code-generator#3078: Related fixes to discriminator validation and parser sorting logic touching Parser.__collapse_root_models.
• koxudaxi/datamodel-code-generator#3022: Related changes to collapse logic and Pydantic v2 DataModelField recognition in Parser.__collapse_root_models.
• koxudaxi/datamodel-code-generator#2722: Related discriminator normalization and mapping changes during OpenAPI parsing.

Suggested reviewers

• ilovelinux

Poem

🐰 I nibbled names to make them right,
turned camel-hooks to snake_case light.
Discriminators now align,
tests and golden outputs sign.
Hooray — models import without fright!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Fix snake case array discriminator' is directly related to and accurately summarizes the main change, addressing the bug fix for discriminators in array items with snake_case field conversion.
Linked Issues check	✅ Passed	The PR successfully addresses issue #3091 by normalizing discriminator field names through ModelResolver.get_valid_field_name_and_alias in Parser.__collapse_root_models, ensuring discriminator property names are correctly converted to snake_case when collapsing root models with array items discriminators.
Out of Scope Changes check	✅ Passed	All changes are scoped to fixing the discriminator snake_case issue: core fix in base.py, documentation example update, test artifact addition, and regression test. No unrelated or out-of-scope changes detected.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/snake-case-array-discriminator

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[github-actions]

📚 Docs Preview: https://pr-3125.datamodel-code-generator.pages.dev

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 11 untouched benchmarks
⏩ 98 skipped benchmarks¹

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_{Comparing fix/snake-case-array-discriminator (00f4c97) with main (2a2ed06)}

Footnotes

1. 98 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2a2ed06) to head (00f4c97).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #3125   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           88        88           
  Lines        18536     18541    +5     
  Branches      2141      2141           
=========================================
+ Hits         18536     18541    +5     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[github-actions]

Breaking Change Analysis

Result: No breaking changes detected

Reasoning: This PR fixes a bug where discriminator field names in collapsed root models (array items) were not normalized through get_valid_field_name_and_alias(). Previously, with --snake-case-field and --collapse-root-models, the discriminator value in generated code used the raw schema property name (e.g., objectType) instead of the snake_case Python field name (e.g., object_type). This produced invalid Pydantic v2 code that would fail at runtime, since Pydantic requires the discriminator to match the Python field name, not the alias. The generated output changes only in the specific scenario where it was previously broken, making this a correctional bug fix rather than a breaking change.

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This analysis was performed by Claude Code Action

[github-actions]

🎉 Released in 0.57.0

This PR is now available in the latest release. See the release notes for details.