[Protobuf] Fix Discriminator Issue and add capability Enum Extraction

[Ocsidot]

Overview

This PR resolves critical a critical issue in the Protobuf schema code generator related to discriminator handling.

Following this fix, during our investigation we also found another issue related on enum management: without extrating enum in separated messages, if enums have similar values, a conflict appears. Thus, we propose to add a new parameter to allow enum extraction, and proper support for enums within array types. The changes ensure correct protobuf generation when models use inheritance, discriminators, and inline/extracted enums.

Issues Fixed

This PR addresses multiple related issues in protobuf code generation:

1. Import Path Generation: Fixed protobuf import paths for discriminator-based models
2. Enum Value Collision: Improved logic when extracting enums to prevent naming collisions between enum values
3. Enum Extraction with Inheritance/Discriminator: Fixed incorrect handling of inline enums in models using inheritance or discriminators
4. Array of Enums Support: Added proper support for enums within array types when using extractEnumsToSeparateFiles option

Changes Made

1. Improved Protobuf Generation Core

• Enhanced inheritance management for protobuf models
• Improved discriminator handling to correctly process complex type hierarchies
• Implemented enum extraction to separate files with proper message wrapping
• Added comprehensive unit tests for the new functionality

2. Fixed Enum Value Collision

• Corrected enum extraction logic when models use inheritance or discriminators
• Ensures parent model properties are properly included when processing enum values
• Prevents duplicate or missing field assignments in generated protobuf schemas

3. Added Array of Enums Support

• Added detection for arrays of enums (property.isArray && property.items.isEnum)
• Properly applies enum wrapper naming conventions (ParentModel_FieldName.Enum) to array items
• Ensures consistent enum reference handling between direct properties and array elements
• Correctly updates protobuf data types for array elements

4. Fixed Enum Extraction in Complex Scenarios

• Recomputes model hierarchy after processing inherited properties
• Properly handles wrapper message naming for both inline and referenced enums
• Corrects data type references when enums are already wrapped with .Enum suffix
• Improves template compatibility for arrays of extracted enums

Enum Extraction Pattern

When extractEnumsToSeparateFiles is enabled, the generator now properly wraps inline enums to prevent naming collisions:

// Generated enum in separate file (collision-free)
message MyModel_Status {
  enum Enum {
    UNSPECIFIED = 0;
    ACTIVE = 1;
    INACTIVE = 2;
  }
}

message MyModel {
  MyModel_Status.Enum status = 1;
  repeated MyModel_Status.Enum previous_statuses = 2;
}

Testing

• Updated existing unit tests in ProtobufSchemaCodegenTest
• Added test coverage for models with inheritance, discriminators, and enum arrays
• Verified import path generation and wrapper message naming conventions
• All sample files regenerated successfully

Files Modified

• modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ProtobufSchemaCodegen.java
• modules/openapi-generator/src/test/java/org/openapitools/codegen/languages/protobuf/ProtobufSchemaCodegenTest.java
• Generated protobuf-schema sample files

Backwards Compatibility

✅ Maintains full backwards compatibility:

• Changes are internal to the protobuf generator logic
• Configuration options remain unchanged
• Existing non-discriminator models continue to generate identically
• Only fixes generation for previously broken scenarios

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
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  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
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---

Summary by cubic

Fixes discriminator handling and import paths in the Protobuf schema generator, and adds an option to extract enums to separate files with full support for arrays. This prevents enum name collisions and generates valid schemas for models using inheritance and discriminators.

• Bug Fixes

  • Correct import paths for allOf + discriminator models, including child references.
  • Fix inline enum handling under inheritance/discriminator hierarchies.
  • Properly reference enums inside arrays.
  • Avoid duplicate model imports.

• New Features

  • New option: extractEnumsToSeparateFiles, wrapping enums as MessageName.Enum to avoid collisions.
  • Support arrays of enums with the same wrapper pattern.
  • Improved multi-level inheritance and discriminator processing.
  • Updated templates and added targeted unit tests.

^{Written for commit 0902089. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found across 11 files

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="modules/openapi-generator/src/test/resources/3_0/protobuf-schema/pet.proto">

<violation number="1" location="modules/openapi-generator/src/test/resources/3_0/protobuf-schema/pet.proto:15">
P1: Imported proto file `models/characteristics.proto` is missing, leaving `Characteristics` unresolved and making the schema uncompilable.</violation>
</file>

---

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[cubic-dev-ai]

P1: Imported proto file models/characteristics.proto is missing, leaving Characteristics unresolved and making the schema uncompilable.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At modules/openapi-generator/src/test/resources/3_0/protobuf-schema/pet.proto, line 15:

<comment>Imported proto file `models/characteristics.proto` is missing, leaving `Characteristics` unresolved and making the schema uncompilable.</comment>

<file context>
@@ -12,14 +12,20 @@ syntax = "proto3";
 
 package openapitools;
 
+import public "models/characteristics.proto";
+
 message Pet {
</file context>