diff --git a/CHANGELOG.md b/CHANGELOG.md
index 56ef6e4..72ea93c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,17 @@
 # Changelog
 
+## Unreleased
+
+**Fixed**
+
+- `affects` on a keyed or structured source that takes a guess threw `INVALID_AFFECTS_TARGET` in a dev build, so the
+  action never reached its `await`, and did nothing in a production build, so `pending?` stayed false (#50,
+  S-affects-keyed).
+- A keyed source that takes a guess (`source rows key id = q.rows`) merges each answer by key instead of replacing every
+  row, so a row's readers re-run only when its own leaves change (#51).
+- The guide's action example (`guide/async.md` §3) lost an error thrown before the first `await`: the `onFlush` clear
+  ran after the catch. The catch now writes the error through `onFlush` as well (#52).
+
 ## 0.3.1
 
 **New**
diff --git a/guide/async.md b/guide/async.md
index 4105212..5465243 100644
--- a/guide/async.md
+++ b/guide/async.md
@@ -72,10 +72,16 @@ export action save(next: Profile) {
     profile := answer               // the truth
   } catch (e) {
     saveError := e.message          // lands in the same frame as the revert
+    onFlush(() => { saveError := e.message })   // and stays, if the body failed before its first await
   }
 }
 ```
 
+Write the error both ways. A failure after the `await` lands in the action's last instant, with the revert, and the
+`onFlush` run then writes the same value, which changes nothing. A failure before the first `await` (a `throw`, or a
+call that throws) lands in the action's first instant, so the clear above runs after it and would erase it; the second
+`onFlush` runs after the clear, in the same instant (EF-8: one step's callbacks run in call order), and puts it back.
+
 A handler can be the action itself, with no keyword: an arrow that writes `guess`, declares `affects`, or both
 writes and awaits is an action (AC-1b). Declare a named `action` when you call it from more than one place.
 
diff --git a/ref/compile/rt-server.ts b/ref/compile/rt-server.ts
index 93f0647..2ecf15b 100644
--- a/ref/compile/rt-server.ts
+++ b/ref/compile/rt-server.ts
@@ -136,7 +136,7 @@ export function createApp(opts: { api?: Record<string, (...a: any[]) => unknown>
     },
     question(name: string, compute: (prev: unknown) => unknown, o: { loadingValue?: unknown } | undefined, mode: { optimistic: boolean; structured: boolean; derived?: boolean }, key?: string): [Acc, Writer] {
       const fn = named(name, compute);
-      if (mode.optimistic && mode.structured) { const [store] = (createOptimisticStore as any)((draft: any) => { const r = fn(draft); if (r instanceof Flight || r instanceof Promise) return (async () => assignDraft(draft, r instanceof Flight ? await r.first : await r))(); assignDraft(draft, r); }, key ? [] : {}, key ? { key } : undefined); return ro(() => store); }
+      if (mode.optimistic && mode.structured) { const [store] = (createOptimisticStore as any)((draft: any) => { const r = fn(draft); if (r instanceof Flight || r instanceof Promise) return (async () => landDraft(draft, r instanceof Flight ? await r.first : await r, key))(); landDraft(draft, r, key); }, key ? [] : {}, key ? { key } : undefined); return ro(() => store); }
       if (mode.optimistic && mode.derived) { const [get] = createOptimistic(fn as any, o as any); return ro(get as Acc); }   // AC-3 over an equation: the client's createOptimistic(fn), so the owner trees (and hydration keys) match
       if (mode.optimistic) { const [truth] = createSignal(fn as any, o as any); const [get] = createOptimistic(() => (truth as Acc)()); return ro(get as Acc); }
       if (mode.structured) {
@@ -241,6 +241,21 @@ export function createApp(opts: { api?: Record<string, (...a: any[]) => unknown>
 }
 export type ServerApp = ReturnType<typeof createApp>;
 
+/** A keyed answer is merged into the draft by key: a row whose key is still there keeps its object and takes the new
+ *  leaves, so an equal answer changes nothing and a changed one changes only its leaves (#51). Any other answer is
+ *  assigned.
+ *  TODO(Solid rc.22): return the keyed answer from the derive and let Solid reconcile it, dropping this merge. On
+ *  2.0.0-rc.21 that path throws in holdStaging (memo over latest() of the rows); fixed on Solid next (dba2aad). */
+function landDraft(draft: any, v: unknown, key: string | undefined) {
+  if (!key || !Array.isArray(v) || !Array.isArray(draft)) return assignDraft(draft, v);
+  const had = new Map<unknown, any>(); for (const row of draft) if (row && typeof row === "object") had.set(row[key], row);
+  v.forEach((x: any, i) => { const row = x && typeof x === "object" ? had.get(x[key]) : undefined; if (row) { mergeLeaves(row, x); if (draft[i] !== row) draft[i] = row; } else draft[i] = x; });
+  draft.length = v.length;
+}
+function mergeLeaves(o: any, v: any) {
+  for (const k of Object.keys(o)) if (!(k in v)) delete o[k];
+  for (const k of Object.keys(v)) { const a = o[k], b = v[k]; if (a && b && typeof a === "object" && typeof b === "object" && Array.isArray(a) === Array.isArray(b)) { if (Array.isArray(a)) { a.length = b.length; } mergeLeaves(a, b); } else if (a !== b) o[k] = b; }
+}
 /** A store draft becomes `v`: keys `v` lacks are removed, the rest assigned (an array's length follows). */
 function assignDraft(draft: any, v: unknown) {
   if (!v || typeof v !== "object") return;
diff --git a/ref/compile/rt-web.ts b/ref/compile/rt-web.ts
index 576da10..044fc5e 100644
--- a/ref/compile/rt-web.ts
+++ b/ref/compile/rt-web.ts
@@ -177,7 +177,7 @@ export function createApp(opts: { root: any; api?: Record<string, (...a: any[])
     question(name: string, compute: (prev: unknown) => unknown, opts: { loadingValue?: unknown } | undefined, mode: { optimistic: boolean; structured: boolean; derived?: boolean }, key?: string): [Acc, Writer] {
       const fn = named(name, compute);
       if (mode.optimistic && mode.structured) {                        // AC-3 path targets: createOptimisticStore(fn, seed) — the derive writes its draft; the setter guesses per key
-        const [store, setStore] = (createOptimisticStore as any)((draft: any) => { const r = fn(draft); if (r instanceof Flight || r instanceof Promise) return (async () => assignDraft(draft, r instanceof Flight ? await r.first : await r))(); assignDraft(draft, r); }, key ? [] : {}, key ? { key } : undefined);
+        const [store, setStore] = (createOptimisticStore as any)((draft: any) => { const r = fn(draft); if (r instanceof Flight || r instanceof Promise) return (async () => landDraft(draft, r instanceof Flight ? await r.first : await r, key))(); landDraft(draft, r, key); }, key ? [] : {}, key ? { key } : undefined);
         const get = () => store; storeAccs.add(get);
         const plain = () => { throw new Error("a plain write to an optimistic store is not lowered yet"); };
         return [get, { set: plain, path: plain, guess: (v, keys = []) => setStore((d: any) => { if (keys.length === 0) return typeof v === "function" ? (v as (x: unknown) => unknown)(d) : v; /* a whole guess is the setter\'s return, which Solid diffs shallowly against the view as guesses */ let o = d; for (const k of keys.slice(0, -1)) o = o[k]; const last = keys[keys.length - 1]; o[last] = typeof v === "function" ? (v as (x: unknown) => unknown)(o[last]) : v; }) }];
@@ -265,7 +265,7 @@ export function createApp(opts: { root: any; api?: Record<string, (...a: any[])
     onSettled: (f: () => void) => onSettled(f),                             // EF-7
     onFlush: (f: () => void) => onFlush(f),                                 // EF-8
     refresh(name: string, get: Acc) { const key = lastKey.get(name); if (key) flights.delete(key); for (const h of hoistedOf.get(get) ?? []) refresh(h as any); const t = truthOf.get(get) ?? get; refresh((storeAccs.has(t) ? t() : t) as any); },
-    affects: (get: Acc) => affects(get as any),
+    affects: (get: Acc) => affects((storeAccs.has(get) ? get() : get) as any),   // a structured source's accessor wraps its store: Solid takes the store (#50), as `refresh` above
     preOr: (v: unknown, init: () => unknown) => v === undefined ? init() : v,
 
     // ---- modules (09 MO-1): a dependency's factory runs once per app; its exports are shared by every importer ----
@@ -316,6 +316,21 @@ export function createApp(opts: { root: any; api?: Record<string, (...a: any[])
 }
 export type App = ReturnType<typeof createApp>;
 
+/** A keyed answer is merged into the draft by key: a row whose key is still there keeps its object and takes the new
+ *  leaves, so an equal answer changes nothing and a changed one changes only its leaves (#51). Any other answer is
+ *  assigned.
+ *  TODO(Solid rc.22): return the keyed answer from the derive and let Solid reconcile it, dropping this merge. On
+ *  2.0.0-rc.21 that path throws in holdStaging (memo over latest() of the rows); fixed on Solid next (dba2aad). */
+function landDraft(draft: any, v: unknown, key: string | undefined) {
+  if (!key || !Array.isArray(v) || !Array.isArray(draft)) return assignDraft(draft, v);
+  const had = new Map<unknown, any>(); for (const row of draft) if (row && typeof row === "object") had.set(row[key], row);
+  v.forEach((x: any, i) => { const row = x && typeof x === "object" ? had.get(x[key]) : undefined; if (row) { mergeLeaves(row, x); if (draft[i] !== row) draft[i] = row; } else draft[i] = x; });
+  draft.length = v.length;
+}
+function mergeLeaves(o: any, v: any) {
+  for (const k of Object.keys(o)) if (!(k in v)) delete o[k];
+  for (const k of Object.keys(v)) { const a = o[k], b = v[k]; if (a && b && typeof a === "object" && typeof b === "object" && Array.isArray(a) === Array.isArray(b)) { if (Array.isArray(a)) { a.length = b.length; } mergeLeaves(a, b); } else if (a !== b) o[k] = b; }
+}
 /** A store draft becomes `v`: keys `v` lacks are removed, the rest assigned (an array's length follows). */
 function assignDraft(draft: any, v: unknown) {
   if (!v || typeof v !== "object") return;
diff --git a/ref/compile/rt.ts b/ref/compile/rt.ts
index 82c980a..5eecb91 100644
--- a/ref/compile/rt.ts
+++ b/ref/compile/rt.ts
@@ -187,7 +187,7 @@ export function createApp(opts: { api?: Record<string, (...a: any[]) => unknown>
     question(name: string, compute: (prev: unknown) => unknown, opts: { loadingValue?: unknown } | undefined, mode: { optimistic: boolean; structured: boolean; derived?: boolean }, key?: string): [Acc, Writer] {
       const fn = named(name, compute);
       if (mode.optimistic && mode.structured) {                        // AC-3 path targets: createOptimisticStore(fn, seed) — the derive writes its draft; the setter guesses per key
-        const [store, setStore] = (createOptimisticStore as any)((draft: any) => { const r = fn(draft); if (r instanceof Flight || r instanceof Promise) return (async () => assignDraft(draft, r instanceof Flight ? await r.first : await r))(); assignDraft(draft, r); }, key ? [] : {}, key ? { key } : undefined);
+        const [store, setStore] = (createOptimisticStore as any)((draft: any) => { const r = fn(draft); if (r instanceof Flight || r instanceof Promise) return (async () => landDraft(draft, r instanceof Flight ? await r.first : await r, key))(); landDraft(draft, r, key); }, key ? [] : {}, key ? { key } : undefined);
         const get = () => store; storeAccs.add(get);
         const plain = () => { throw new Error("a plain write to an optimistic store is not lowered yet"); };
         return [get, { set: plain, path: plain, guess: (v, keys = []) => setStore((d: any) => { if (keys.length === 0) return typeof v === "function" ? (v as (x: unknown) => unknown)(d) : v; /* a whole guess is the setter\'s return, which Solid diffs shallowly against the view as guesses */ let o = d; for (const k of keys.slice(0, -1)) o = o[k]; const last = keys[keys.length - 1]; o[last] = typeof v === "function" ? (v as (x: unknown) => unknown)(o[last]) : v; }) }];
@@ -273,7 +273,7 @@ export function createApp(opts: { api?: Record<string, (...a: any[]) => unknown>
     onSettled: (f: () => void) => onSettled(f),                             // EF-7
     onFlush: (f: () => void) => onFlush(f),                                 // EF-8
     refresh(name: string, get: Acc) { const key = lastKey.get(name); if (key) flights.delete(key); for (const h of hoistedOf.get(get) ?? []) refresh(h as any); const t = truthOf.get(get) ?? get; refresh((storeAccs.has(t) ? t() : t) as any); },
-    affects: (get: Acc) => affects(get as any),
+    affects: (get: Acc) => affects((storeAccs.has(get) ? get() : get) as any),   // a structured source's accessor wraps its store: Solid takes the store (#50), as `refresh` above
     preOr: (v: unknown, init: () => unknown) => v === undefined ? init() : v,
 
     // ---- modules (09 MO-1): a dependency's factory runs once per app; its exports are shared by every importer ----
@@ -324,6 +324,21 @@ export function createApp(opts: { api?: Record<string, (...a: any[]) => unknown>
 }
 export type App = ReturnType<typeof createApp>;
 
+/** A keyed answer is merged into the draft by key: a row whose key is still there keeps its object and takes the new
+ *  leaves, so an equal answer changes nothing and a changed one changes only its leaves (#51). Any other answer is
+ *  assigned.
+ *  TODO(Solid rc.22): return the keyed answer from the derive and let Solid reconcile it, dropping this merge. On
+ *  2.0.0-rc.21 that path throws in holdStaging (memo over latest() of the rows); fixed on Solid next (dba2aad). */
+function landDraft(draft: any, v: unknown, key: string | undefined) {
+  if (!key || !Array.isArray(v) || !Array.isArray(draft)) return assignDraft(draft, v);
+  const had = new Map<unknown, any>(); for (const row of draft) if (row && typeof row === "object") had.set(row[key], row);
+  v.forEach((x: any, i) => { const row = x && typeof x === "object" ? had.get(x[key]) : undefined; if (row) { mergeLeaves(row, x); if (draft[i] !== row) draft[i] = row; } else draft[i] = x; });
+  draft.length = v.length;
+}
+function mergeLeaves(o: any, v: any) {
+  for (const k of Object.keys(o)) if (!(k in v)) delete o[k];
+  for (const k of Object.keys(v)) { const a = o[k], b = v[k]; if (a && b && typeof a === "object" && typeof b === "object" && Array.isArray(a) === Array.isArray(b)) { if (Array.isArray(a)) { a.length = b.length; } mergeLeaves(a, b); } else if (a !== b) o[k] = b; }
+}
 /** A store draft becomes `v`: keys `v` lacks are removed, the rest assigned (an array's length follows). */
 function assignDraft(draft: any, v: unknown) {
   if (!v || typeof v !== "object") return;
diff --git a/ref/oracle/lower.ts b/ref/oracle/lower.ts
index 85063b2..e867ee0 100644
--- a/ref/oracle/lower.ts
+++ b/ref/oracle/lower.ts
@@ -37,6 +37,21 @@ function boundaryRead(f: () => unknown, match: (err: unknown) => boolean, alt: (
 /** A boundary's fallback that is decided when read, so a failure that changes class while the fallback shows is re-decided. */
 class Fallback { get: () => unknown; constructor(g: () => unknown) { this.get = g; } }
 function unwrapFallback(v0: unknown): unknown { let v = v0; if (v instanceof Fallback) v = v.get(); if (v instanceof Forward) throw v.err; return v; }
+/** A keyed answer is merged into the draft by key: a row whose key is still there keeps its object and takes the new
+ *  leaves, so an equal answer changes nothing and a changed one changes only its leaves (#51). Any other answer is
+ *  assigned.
+ *  TODO(Solid rc.22): return the keyed answer from the derive and let Solid reconcile it, dropping this merge. On
+ *  2.0.0-rc.21 that path throws in holdStaging (memo over latest() of the rows); fixed on Solid next (dba2aad). */
+function landDraft(draft: any, v: unknown, key: string | undefined) {
+  if (!key || !Array.isArray(v) || !Array.isArray(draft)) return assignDraft(draft, v);
+  const had = new Map<unknown, any>(); for (const row of draft) if (row && typeof row === "object") had.set(row[key], row);
+  v.forEach((x: any, i) => { const row = x && typeof x === "object" ? had.get(x[key]) : undefined; if (row) { mergeLeaves(row, x); if (draft[i] !== row) draft[i] = row; } else draft[i] = x; });
+  draft.length = v.length;
+}
+function mergeLeaves(o: any, v: any) {
+  for (const k of Object.keys(o)) if (!(k in v)) delete o[k];
+  for (const k of Object.keys(v)) { const a = o[k], b = v[k]; if (a && b && typeof a === "object" && typeof b === "object" && Array.isArray(a) === Array.isArray(b)) { if (Array.isArray(a)) { a.length = b.length; } mergeLeaves(a, b); } else if (a !== b) o[k] = b; }
+}
 /** A store draft becomes `v`: keys `v` lacks are removed, the rest assigned (an array's length follows). */
 function assignDraft(draft: any, v: unknown) {
   if (!v || typeof v !== "object") return;
@@ -257,7 +272,7 @@ export class Oracle {
             this.hoistAwaits(d.name, d.init, scope);
             const compute = this.computeOf(d.name, d.init, scope);
             if (this.optimisticTargets.has(d.name) && this.pathTargets.has(d.name)) {   // AC-3 path targets over ST-5 structured: createOptimisticStore(fn, seed) — the derive writes its draft; the setter guesses per key
-              const derive = async (draft: any) => { const r = compute(draft); const v = r instanceof Flight ? await r.first : await r; assignDraft(draft, v); };
+              const derive = async (draft: any) => { const r = compute(draft); const v = r instanceof Flight ? await r.first : await r; landDraft(draft, v, d.key); };
               const [store, setStore] = (createOptimisticStore as any)(derive, d.key ? [] : {}, d.key ? { key: d.key } : undefined);
               const plain = () => { throw new Unsupported("a plain write to an optimistic store"); };
               scope.vars.set(d.name, { k: "store", get: () => store, set: plain, setPath: plain, question: true, setGuess: setStore });
@@ -294,7 +309,7 @@ export class Oracle {
             const compute = this.computeOf(d.name, d.e, scope);
             const plain = () => { throw new Unsupported("a plain write to an equation"); };
             if (this.pathTargets.has(d.name) || d.key) {
-              const derive = async (draft: any) => { const r = compute(draft); const v = r instanceof Flight ? await r.first : await r; assignDraft(draft, v); };
+              const derive = async (draft: any) => { const r = compute(draft); const v = r instanceof Flight ? await r.first : await r; landDraft(draft, v, d.key); };
               const [store, setStore] = (createOptimisticStore as any)(derive, d.key ? [] : {}, d.key ? { key: d.key } : undefined);
               scope.vars.set(d.name, { k: "store", get: () => store, set: plain, setPath: plain, question: true, setGuess: setStore });
             } else {
@@ -508,7 +523,7 @@ export class Oracle {
       case "throw": throw untrack(() => this.ev(s.e, scope, env));
       case "action": this.lowerAction([], s.body, scope, env)(); return;
       case "refresh": { const b = scope.lookup(s.name); if (!b || !("get" in b)) throw new Error("INVALID_REFRESH_TARGET"); const key = this.lastKey.get(s.name); if (key) this.forget(key); const d = Scope.all.find(x => x.vars.get(s.name) === b); for (const h of (d && this.hoistedByName.get(d)?.get(s.name)) ?? []) refresh(h as any); refresh((b.k === "store" ? b.get() : ((b.k === "sig" || b.k === "memo") && (b as any).truth) || b.get) as any); return; }
-      case "affects": { const b = scope.lookup(s.name); if (!b || !("get" in b)) throw new Error("INVALID_AFFECTS_TARGET"); affects(b.get as any); return; }
+      case "affects": { const b = scope.lookup(s.name); if (!b || !("get" in b)) throw new Error("INVALID_AFFECTS_TARGET"); affects((b.k === "store" ? b.get() : b.get) as any); return; }   // a store binding's get wraps the store: Solid takes the store (#50)
       case "optimistic": case "write": {
         const b = s.target.ctx ? this.provision(s.target.name, scope) : scope.lookup(s.target.name);
         const v = untrack(() => this.ev(s.e, scope, env));          // W-6: a function value is Solid's updater form — the setters take it as is
diff --git a/scenarios/S-affects-keyed.md b/scenarios/S-affects-keyed.md
new file mode 100644
index 0000000..996c2e1
--- /dev/null
+++ b/scenarios/S-affects-keyed.md
@@ -0,0 +1,46 @@
+# S-affects-keyed — `affects` on a keyed source that takes a guess
+
+Rules exercised: AC-5, AC-3, AC-4, H-9.
+
+## Program
+
+```rx
+export default component App() {
+  source tick := 0
+  day = await fetch("d" + str(tick))
+  source rows key id = day.rows
+  source err := "none"
+  action move(id: number, at: number) {
+    try {
+      let i = rows.findIndex(r => r.id == id)
+      guess rows[i].at := at
+      affects rows
+      await fetch("save")
+      refresh day
+    } catch (e) { err := e.message }
+  }
+  errored fallback(e) { <p>error</p> } {
+    loading fallback { <p>loading</p> } {
+      <p>{for r in rows { <i>{r.at}</i> }}{pending? rows ? "saving" : ""} {err}</p>
+    }
+  }
+}
+```
+
+A keyed source over an answer, written by a guess, lowers to an optimistic store. The action declares its work on the
+whole list with `affects rows`, so `pending? rows` is true until the action ends (AC-5), and the save is asked.
+
+## Frames
+
+| instant | action | frame |
+| ------- | ------ | ----- |
+| 0 | mount | `loading` |
+| 1 | resolve "d0" {"rows":[{"id":1,"at":0},{"id":2,"at":3}]} | `03 none` |
+| 2 | handler `move(1, 5)` | `53saving none` |
+| 3 | resolve "save" 0 | `53saving none` |
+| 4 | resolve "d0" {"rows":[{"id":1,"at":5},{"id":2,"at":3}]} | `53 none` |
+
+Reported as solidjs/reactivescript#50: the compiled runtime handed `affects` the source's accessor, a wrapper around
+the store, where `refresh` already unwrapped it. Solid's dev build threw `INVALID_AFFECTS_TARGET` into the action's
+`catch`, so the save was never asked and the guess reverted (instant 2 read `03 none`, then the error); its
+production build ignored the call, so `pending? rows` never turned true.