Why

Creating a node with a single-element relationship-only uniqueness_constraints entry (e.g. [["parent"]]) crashes schema processing with AttributeError: 'NodeSchema' object has no attribute 'get' and returns a 500. The docs list relationship-only constraints as supported syntax, so the implementation and docs disagree. The bug has been present since at
least 0.16.1.

Goal: Make relationship-only single-element uniqueness constraints process without crashing.

Non-goals:

• Auto-promoting a relationship-only constraint to an HFID (HFID logic only triggers when schema_path.is_type_attribute is true, which a pure relationship path is not — so the constraint correctly stays as a constraint and isn't lifted to HFID).
• Reconciling whether relationship-only single-element constraints should be a first-class HFID source.

Closes #4483

What changed

Behavioral changes

• Schema processing no longer crashes when a node defines uniqueness_constraints: [["<relationship>"]] with no other entries.
• Mixed constraints like ["parent", "name__value"] were already working — that path skipped the broken branch via the if len(constraint_paths) > 1: continue filter, which is why the bug only surfaces for the single-element relationship case.

Implementation notes

• One-line fix in _derive_human_friendly_id: pass schema=self (the SchemaBranch) to node.parse_schema_path instead of schema=node. parse_schema_path treats its schema argument as a SchemaBranch and calls .get(name=..., duplicate=True) on it when resolving a relationship peer; passing the NodeSchema instead made that call raise.
• The other two parse_schema_path call sites in the same file already pass schema=self, so this was a one-spot typo, not a systemic issue.

What stayed the same

• parse_schema_path is unchanged.
• No API, GraphQL, or DB schema changes.
• HFID derivation rules are unchanged — relationship-only constraints still do not auto-promote to HFID.

How to review

• backend/infrahub/core/schema/schema_branch.py — the one-line fix at the _derive_human_friendly_id call site.
• backend/tests/unit/core/schema/test_schema_branch.py — new test class that builds a hierarchical schema with uniqueness_constraints=[["parent"]] and asserts process_human_friendly_id does not raise. The test fails on stable with the original AttributeError.

No risky areas — the change strictly narrows the failure surface; mixed-constraint and attribute-only paths were already exercising the correct argument.

How to test

# Run the new unit test
uv run pytest backend/tests/unit/core/schema/test_schema_branch.py::TestDeriveHumanFriendlyIdWithRelationshipOnlyConstraint -v

# Full schema unit suite to confirm no regressions
uv run pytest backend/tests/unit/core/schema/ backend/tests/unit/core/test_schema_defaults.py

To reproduce on stable before the fix, load a schema like:

nodes:
  - name: Site
    namespace: Testing
    inherit_from: [TestingLocation]
    parent: TestingCountry
    uniqueness_constraints:
      - [parent]

Schema processing raises AttributeError: 'NodeSchema' object has no attribute 'get' → HTTP 500. With this PR the same schema loads cleanly.

Impact & rollout

• Backward compatibility: None broken. Schemas that previously crashed now load; schemas that previously worked are unaffected.
• Performance: None. Single attribute access change inside a path already taken on every schema reload.
• Config/env changes: None.
• Deployment notes: Safe to deploy. No migrations, no coordinated release needed.

Checklist

• Tests added/updated
• Changelog entry added (changelog/4483.fixed.md)
• External docs updated — N/A, docs already describe the intended (now-working) behavior
• Internal .md docs updated — N/A
• I have reviewed AI generated content

Summary by cubic

Fixes a crash when a node uses a single-element relationship-only uniqueness_constraints (e.g., [["parent"]]) by passing the SchemaBranch to parse_schema_path during HFID derivation. Adds a fixture-based unit test that processes a simple Car/Person schema to verify the constraint resolves without error and does not promote to an HFID.

^{Written for commit 8e29c5a. Summary will update on new commits. Review in cubic}