fix(mongodb): serve :of-type, reference :identifier, :[type] and :above/:below

[smunini]

Summary

MongoDB refused search modifiers that SQLite, PostgreSQL and Elasticsearch all serve, so the same request succeeded or failed depending on HFS_STORAGE_BACKEND.

• The three modifiers named in MongoDB rejects :of-type, reference :identifier and reference :[type] that the other backends support #1408 are token :of-type, reference :identifier and the reference :[type] qualifier.
• A fourth case, reference :above/:below, was found while fixing them.

These were reachable over plain HTTP

These results come from a real hfs on MongoDB, with main's MongoDB code before the fix and this branch after.

Request	Before	After
Observation?subject:Patient=p1	400 unsupported modifier Patient	200, the Observation whose subject is Patient/p1
Observation?subject:Group=p1	400	200, the Observation whose subject is Group/p1
Observation?subject:identifier=http://example.org/mrn|12345	400	200, correct match
Patient?identifier:of-type=…v2-0203|MR|12345	400 unsupported modifier ofType	200, correct match
Observation?subject:below=Patient	400	200, correct match
Chained (subject:Patient.identifier=…) and _has	worked	unchanged

• REST passes :[type] to the backend as a modifier, not as a type-qualified value, so the refusal was reachable over HTTP.
• The chained form already worked, because REST resolves chains before the backend sees them.

Why the Inferno MongoDB legs stayed green: the US Core test kit never sends these forms.

• Its "reference with type" search puts the type in the value (patient=Patient/<id>).
• It sends no :identifier or :of-type.
• A real client that sends subject:Patient=… was refused on MongoDB.

Root cause

The builders were never written, so the modifiers fell into the UnsupportedModifier catch-all. Reference :above/:below were refused one step earlier, in validate_query_support.

Nothing was missing from the index. The writer has always stored the identifier type system and code on identifier rows, and a partial index for them already exists. No reindex is needed.

Fix

• :of-type
  • It compares the type system, the type code and the value.
  • An empty part is not compared, as on SQLite and PostgreSQL.
  • Anything other than three parts matches nothing. The condition is never dropped.
• :[type]
  • subject:Patient=1 builds the same filter as subject=Patient/1, so it cannot match Group/1.
  • Qualified reference values are now version-agnostic (strip_reference_version), as on every other backend.
• :identifier
  • It has the SQLite and PostgreSQL meaning: the reference's target carries the identifier.
  • A MongoDB filter document cannot join, so the targets are resolved first, then turned into one index-bounded $in. The lookup is tenant-scoped and bounded to the parameter's declared target types.
  • More than 10,000 targets is refused with TooManyResults. The result is never truncated.
• Reference :above/:below reuse the uri shapes.
• modifiers_for_type now advertises what is actually served.

New test suite

New cross-backend suite: crates/persistence/tests/search/modifier_parity_suite.rs.

• It walks every modifier that SearchModifier::is_valid_for allows, on every parameter type, on all four backends.
• Each backend's known differences are stated explicitly in the suite, so a backend cannot silently fall behind again.
• It also covers AND-combination, search_count and tenant isolation.

Verification

Run after merging current main, on real containers:

• cargo test -p helios-persistence --lib --features sqlite,postgres,mongodb,elasticsearch: 1782 passed
• mongodb_tests (full): 206 passed
• postgres_tests (full): 268 passed
• elasticsearch_tests (full): 141 passed
• sqlite_tests + search_suite: 219 passed
• cargo +1.98.1 clippy -p helios-persistence --features R4,R4B,R5,R6,sqlite,postgres,mongodb,elasticsearch,s3 --all-targets with the CI flags: clean. This compiles code gated on several FHIR versions.
• cargo fmt --check: clean

Not run on this branch alone:

• helios-rest tests. No REST code is touched.
• The full --workspace --all-features compile. That runs on the combined tree of the open PRs.

Fixes #1408

[codecov]

Codecov Report

❌ Patch coverage is 98.44789% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...es/persistence/src/backends/mongodb/search_impl.rs	98.60%	6 Missing ⚠️
crates/persistence/src/backends/mongodb/backend.rs	95.45%	1 Missing ⚠️

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