query-field-node-object-form-removed.md

@objectstack/spec	major
@objectstack/objectql	patch
@objectstack/metadata-protocol	patch
@objectstack/driver-mongodb	patch
@objectstack/driver-sql	patch
@objectstack/driver-memory	patch

refactor(data)!: a select-list entry is a field name — the nested-select object form is removed (#4196)

FieldNode declared two forms for one entry of QueryAST['fields']:

type FieldNode =
  | string                                                  // "name"
  | { field: string; fields?: FieldNode[]; alias?: string }; // nested select

The object form was declared-but-inert. Nothing produced it, and nothing read .fields or .alias — every consumer on the path treats the list as string[]: objectql's formula projection and its two known-field filters, driver-sql's select(), driver-memory's projectFields. driver-mongodb keyed its projection with the entry itself, so an object entry asked for a column literally named "[object Object]", and the REST ingress stringified each entry before comparing it to the field map, so the same entry came back as 400 INVALID_FIELD: Unknown field '[object Object]' — a rejection naming something the caller never wrote. An author who wrote fields: [{ field: 'owner', fields: ['name'] }] got it accepted by validation and then dropped or mangled, depending on the driver (ADR-0078 silently-inert declaration; ADR-0049 enforce-or-remove).

The capability the object form described is already served, by a different key. Removing the second spelling rather than lowering it into the first is Prime Directive #12: one capability, one contract.

FROM → TO

Was	Now
fields: [{ field: 'owner', fields: ['name'] }]	expand: { owner: { object: 'user', fields: ['name'] } }
fields: [{ field: 'owner' }]	fields: ['owner']
fields: [{ field: 'owner', fields: ['name'] }], one column only	fields: ['owner.name'] (dotted path)
fields: [{ field: 'total', alias: 't' }]	aggregations / windowFunctions — they carry the live alias

The one-line fix: a fields[] entry is a string. Move nested selection to expand, which the engine resolves through batch $in queries (default max depth 3).

There is no os migrate meta step, and deliberately so: QueryAST is a request shape, never stored in stack metadata, so the chain has no source to rewrite. It is registered as an ADR-0087 D3 semantic migration (query-field-node-object-form-retired) on the protocol-17 step instead — the EnhancedApiError.fieldErrors / BatchOptions.validateOnly precedent. Callers move their own select lists, and both channels tell them how:

• The parse. FieldNodeSchema narrows to z.string() with an error map that answers an object entry with the prescription above, not "expected string, received object". z.input becomes string, so tsc fails at the authoring site first.
• The ingress. assertProjectionFieldsExist judges the entry's shape before consulting the object's field map — it is wrong about the shape, not about this object, and a registry-less host would otherwise pass it to a driver that cannot read it. The 400 now names the retired form instead of the field "[object Object]".

No runtime behaviour changes for anything that ever worked; the defensive unwrapping the drivers had grown against a shape nothing sends goes with it.