## Summary
Adds support for filtering records by fields on a related MANY_TO_ONE
object via the GraphQL API. Backend only — no frontend, no REST, no
view-filter persistence yet.
```graphql
{
people(filter: { company: { name: { like: "%Airbnb%" } } }) {
edges { node { id } }
}
}
```
### Where the work lands
- **Schema** — `relation-field-metadata-gql-type.generator.ts` now emits
`{relationName}: TargetFilterInput` alongside the existing
`{joinColumnName}: UUIDFilter` for MANY_TO_ONE relations. Mirrors the
order-by generator that already does this for sort. Lazy thunks in
`object-metadata-filter-gql-input-type.generator.ts` handle the cycle
between filter inputs.
- **Arg processor** — `FilterArgProcessorService` no longer hard-rejects
accessing a relation by its name. When the value is a nested object on a
MANY_TO_ONE field, it recurses into the target object's metadata so each
leaf still gets validated and coerced. Depth-capped at 1.
- **Query parser** — new `parseRelationSubFilter` branch in
`graphql-query-filter-field.parser.ts`. When triggered: looks up the
target object metadata, calls `ensureRelationJoin` against the outer
query builder, and recurses via a child
`GraphqlQueryFilterConditionParser` scoped to the target.
`and`/`or`/`not` inside the relation filter keep working because the
child dispatches through the same `parseKeyFilter`.
- **Shared join utility** — `ensureRelationJoin.util.ts` is a single
function that inspects `queryBuilder.expressionMap.joinAttributes` for
the alias before adding a `LEFT JOIN`. Rewired the existing inline
`qb.leftJoin` calls in the order parser and group-by service to use it,
so filter-driven joins no longer collide with sort-driven joins on the
same relation.
### Out of scope (explicit)
- ONE_TO_MANY reverse traversal (needs EXISTS subqueries)
- Aggregates (`company.people.count > 5` — needs HAVING)
- View-filter storage (no `relationPath` column on `ViewFilterEntity`)
- REST DSL changes
- Frontend filter-picker UX
- Nesting deeper than one hop (parser and arg-processor both reject)
### Open question for review
Permissions. The order-by-on-relation code path already lets users sort
People by Company.name without a Company read-permission check, and this
PR matches that behavior for filters — felt wrong to add a stricter gate
only on the filter side. If we want object-permission gating on the
relation target, it should be a follow-up that covers both paths
consistently. The only attack surface today is existence inference via
timing, identical to what sort already exposes.
## Test plan
- [x] `tsc --noEmit` — clean for changed files (5 unrelated pre-existing
errors on main untouched)
- [x] `oxlint --type-aware` + `prettier --check` — 0 errors on all 17
changed/new files
- [x] `jest filter-arg-processor.service.spec` — 229 tests pass (the new
optional `flatObjectMetadataMaps` arg is backwards-compatible)
- [x] Integration test (`filter-by-relation-field.integration-spec.ts`,
6 cases) — needs to be verified against a seeded test DB. Could not
exercise the happy path in my isolated worktree; depth-2 rejection
passed there.
- [ ] EXPLAIN ANALYZE on the integration test query to confirm the FK on
`person.companyId` is indexed for both standard and custom MANY_TO_ONE
relations.
### Integration test cases
1. Filter People by `company.name = "Airbnb"` (exact match)
2. Filter People by `company.name like "%irbnb%"`
3. Non-matching filter returns empty
4. Combined with a scalar filter at root via `and`
5. **Combined with `orderBy` on the same relation** — proves the
join-dedupe works (without `ensureRelationJoin`, TypeORM throws
"duplicate alias")
6. Depth-2 nesting (`company.accountOwner.name`) returns
`INVALID_ARGS_FILTER`
🤖 Generated with [Claude Code](https://claude.com/claude-code)
---------
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
