Add per-query ConversionService support

[ggomarighetti]

Resume

This PR adds optional per-query ConversionService support for RSQL predicate conversion.

Today, custom value converters are registered through RSQLJPASupport.addConverter(...), which mutates the static conversion service owned by RSQLCommonSupport. That works for simple applications, but it makes conversion rules JVM-global. Applications that run multiple Spring contexts, execute tests in parallel, use application-specific value objects, or validate RSQL before delegating execution cannot guarantee that the conversion rules used during validation are the same rules used later by Specification.toPredicate(...).

The change keeps the current global converter registry as the backward-compatible fallback, while allowing callers to pass a query-specific ConversionService:

Specification<User> spec = RSQLJPASupport.toSpecification(QuerySupport.builder()
    .rsqlQuery("customerId==ACME-42")
    .propertyPathMapper(paths)
    .conversionService(applicationConversionService)
    .build());

Conversion now follows this order:

1. Query or visitor instance ConversionService, when present and able to convert.
2. Existing global defaultConversionService, preserving addConverter(...) behavior.
3. Existing built-in conversion fallbacks.

The same conversion path is used by default JPA operators and JPA custom predicates. QueryDSL also gets a new overload that accepts a per-query ConversionService.

Evidence

Not required

Reference

Closes #207

[Copilot]

Pull request overview

Note

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Adds support for a per-query Spring ConversionService so conversions can be isolated per request/query and take precedence over the library-wide (static) converter registry.

Changes:

• Add ConversionService plumbing to RSQLVisitorBase and thread it through JPA + QueryDSL predicate/spec builders.
• Extend QuerySupport to carry a per-query ConversionService.
• Add tests and README documentation describing precedence and usage.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
rsql-querydsl/src/test/java/io/github/perplexhub/rsql/RSQLQueryDslSupportTest.java	Adds a test ensuring a per-query ConversionService is preferred for QueryDSL predicates.
rsql-querydsl/src/main/java/io/github/perplexhub/rsql/RSQLQueryDslSupport.java	Adds a new toPredicate overload that accepts a ConversionService.
rsql-querydsl/src/main/java/io/github/perplexhub/rsql/RSQLQueryDslPredicateConverter.java	Adds constructor overload to inject/set a per-query ConversionService.
rsql-jpa/src/test/java/io/github/perplexhub/rsql/RSQLJPASupportTest.java	Adds tests ensuring QuerySupport.conversionService takes precedence (including for custom predicates).
rsql-jpa/src/main/java/io/github/perplexhub/rsql/RSQLJPASupport.java	Passes QuerySupport’s ConversionService into the JPA predicate converter.
rsql-jpa/src/main/java/io/github/perplexhub/rsql/RSQLJPAPredicateConverter.java	Adds constructor overload to accept and set ConversionService.
rsql-jpa/src/main/java/io/github/perplexhub/rsql/QuerySupport.java	Adds conversionService field + includes it in toString().
rsql-common/src/test/java/io/github/perplexhub/rsql/RSQLVisitorBaseTest.java	Adds tests for precedence/fallback between instance and global conversion services.
rsql-common/src/main/java/io/github/perplexhub/rsql/RSQLVisitorBase.java	Implements instance ConversionService precedence in convert(...).
README.md	Documents global vs per-query conversion behavior and usage examples.

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[ggomarighetti]

Hi @perplexhub, @nstdio,

I opened this PR as a follow-up to #207 to add optional per-query ConversionService support while keeping the current global converter registry as the backward-compatible fallback.

The goal is to let applications isolate conversion rules per query/context without changing existing behavior for users who rely on RSQLJPASupport.addConverter(...).

I also included focused tests for the common conversion path, JPA default operators, JPA custom predicates, and QueryDSL. I would appreciate your review or guidance if you prefer a narrower scope or a different API shape.

Thanks!