# `Collection.set`

> Contract 2.0.0 · package sha256 `a857eca95b35921109faf997094bb4192f12527d9d0fcd4be7e97f06425bad6e` · generated from `configurator-runtime.d.ts` by `services/onsite-scripter/scripts/build-contract-docs.mjs`; edit the source, not this file.

Method of [`Collection`](Collection.md) · stable · since 2.0.0 · called as `session.collection(name, options).set`

```ts
set(spec: CollectionSpec, options?: C & SignalOption): Promise<CollectionReport>;
```

Keyed reconcile to the whole `spec` (not a merge patch): items are added, changed in place or
removed by `key`, including items that `load()` added. Outcome: one commit, look-only;
replacing and in the write queue. New content loads hidden and swaps in one frame. Asset
failures: [`problems`](ReportFields.problems.md). Timing: resolves at the commit point, after its loads.
Errors: rejects HoloInputError on an invalid spec ([`validate('collection', spec)`](validate.md) checks its
shape, keys and parents; `{ dryRun: true }` checks all this call checks and changes nothing).
Asset failures are [`problems`](ReportFields.problems.md) ('asset-failed', `path` the
item's address, or the stage layer's for its `stage`, [`Problem.path`](Problem.path.md)).

## Options

Through [`Session`](Session.md):

| Option | Type | Default | What it does |
| --- | --- | --- | --- |
| `actor` | `Actor` | the `expectGroup`'s actor when one is given, else 'app' | Who made the write. |
| `dryRun` | `boolean` | false | Runs and reports, then rolls back by design ('dry-run'). |
| `expectGroup` | `HistoryGroup` | undefined | A guard, checked at the call: this write belongs to the undo group you are running. |
| `expectRevision` | `number` | undefined (no check) | If-match on [`session.revision`](Session.revision.md). |
| `origin` | `string` | null | Your tag, echoed unchanged in [`Report.origin`](ReportFields.origin.md) and `cause.origin`. |
| `signal` | `AbortSignal` | undefined | Cancels this call; the member's JSDoc says what an abort does. |
| `supersede` | `boolean` | false (the write is queued behind the transaction and applies after it closes) | While a transaction is open: roll it back (it resolves 'superseded'); the writes queued behind it apply in call order, then this one, which supersedes an… |
| `undo` | `'rebase' \| (string & {})` | a recorded-state change gets one step labelled 'Change'; a look-only write gets none; the session's first model (cause 'load') gets none, whatever the label: it is the floor of history | An undo label makes this write one undo step. |

Through [`Tx`](Tx.md):

| Option | Type | Default | What it does |
| --- | --- | --- | --- |
| `signal` | `AbortSignal` | undefined | Cancels this call; the member's JSDoc says what an abort does. |

Refused here (typed `never`, so passing one fails to compile): `actor`, `checkpoint`, `dryRun`, `expectGroup`, `expectRevision`, `origin`, `supersede`, `undo`.

## Abort (`signal`)

This call is a write (`SignalOption.signal`, [the kinds of call](../read-this-first.md#conventions)).

> A write: before the commit point it rolls back and rejects AbortError; after it, no effect, except that a running camera move stops where the last frame drew it (`CameraMoveOptions`).
>
> Through a `tx` the commit point is the transaction's: aborting a write still loading rejects AbortError and stages nothing (catch it: unobserved, it rolls the transaction back, spec section 6.1, O6); aborting one already staged unstages it, its paths keeping the latest other write to them as after a failed savepoint (O7), and its Promise stays resolved 'staged'.

## Example

Imports: values (`has`, `validate`, the `Holo*Error` classes, the pure helpers) from `@holostep-configurators/runtime-client`, types from `@holostep-configurators/runtime-client/contract`, `/math` and `/testing` names from those entries; `boot` from the runtime module the root's `loadRuntimeAtFloor` resolves to. A type your page defines is your app's: declare it there, never import it from the package.

Compiled under the contract's strict settings (`examples/declaration/07-Collection.set.ts`):

```ts
import type { Session } from '@holostep-configurators/runtime-client/contract';
declare const session: Session;
declare const signal: AbortSignal;
declare const postsX: readonly number[];                   // metres along the fence
const fence = session.collection('fence', { role: 'product', signal });
const r = await fence.set({ items: postsX.map((x, i) => ({
  key: `post-${i}`, kind: 'mesh' as const, meta: { segment: i },
  shape: { kind: 'cylinder' as const, radius: 0.04, height: 1.2 },
  at: { position: { x, y: 0.6, z: 0 } },
  material: { baseColor: '#6b4f3a', roughness: 0.8 },
})) });
console.log(r.added.length, r.updated.length, r.removed.length);   // item keys, by reconcile
```

## Generic-API test (spec 3.1)

| Rule | Result |
| --- | --- |
| 1. Neutral name | pass: `set` names no product |
| 2. A precedent | pass: [glTF 2.0 node hierarchy](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#nodes-and-hierarchy) (row `K1`); [Unity Object.Destroy](https://docs.unity3d.com/ScriptReference/Object.Destroy.html) (row `K1`) |
| 3. Two apps, or labs | pass: 28 apps (ledger: app-04c2774145, app-0c773a4ee3, app-37356bf09f, app-378d190f1b, app-4762caacdc, app-4f16d2f6da, app-4fd23bc869, app-510d750508 and 20 more) |
| 4. The cheaper fix first | decided per need in Appendix B: rows `K1` at tier stable (a member only where no rule, data, asset or app code serves the need) |