WatermelonDB Model & Observation

Overview

This skill covers WatermelonDB models (database/models/), observation (reactive queries), and ensuring React re-renders when observed data changes. Use it when working with findAndObserve, query.observe(), withObservables, or any screen that subscribes to DB changes.

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Observation & React Re-rendering

findAndObserve and same-reference emission

• findAndObserve(id) (on a collection): Fetches a record by ID, returns an Observable that emits immediately on subscribe and whenever the record is updated or deleted.
• When a model is updated (e.g. via model.update() or @writer methods), the observable emits the same object reference with updated properties — it does not emit a new model instance.

React useState bailout

• useState uses Object.is to decide whether to re-render. Passing the same reference (e.g. setPhrase(model)) after an update means no re-render.
• Result: DB updates (text, note, language, etc.) don’t appear until the user navigates away and back, when a new subscription yields a fresh reference.

Fix: store a wrapper so each emit is a new reference

When subscribing to a single model (e.g. findAndObserve) and storing it in React state, don’t store the raw model. Store a wrapper so every emission updates state with a new object:

const [phraseState, setPhraseState] = useState<
  { phrase: Phrase; _key: number } | null
>(null);

useEffect(() => {
  if (!id) return;
  const sub = db.collections
    .get<Phrase>(PHRASE_TABLE)
    .findAndObserve(id)
    .subscribe((result) => {
      setPhraseState({ phrase: result, _key: result.updatedAt });
    });
  return () => sub.unsubscribe();
}, [id, db]);

const phrase = phraseState?.phrase ?? null;

• Use phrase (derived) everywhere in the component. Updates persisted to the DB will re-emit, update phraseState with a new wrapper, and trigger a re-render.

Query observe() and arrays

• query.observe() emits arrays of models. When the query result set changes, WatermelonDB typically emits a new array reference, so setState(results) usually triggers re-renders.
• If you build derived data (e.g. linked.filter(...), assignments) in the subscribe callback and setState that, you’re already passing new references — no extra wrapper needed.

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withObservables (HOC)

• withObservables(triggerProps, getObservables) injects observable values as props and always passes a new state object into setState (e.g. { values, isFetching }), so React re-renders on each emission even when model references are unchanged.
• Use it when you can observe a model (or query) passed as a prop: e.g. withObservables(['attempt'], ({ attempt }) => ({ attempt: attempt.observe(), ... })). See AttemptCard in features/lesson/components/AttemptCard.tsx.
• For route params (e.g. id) you typically subscribe manually in useEffect (e.g. findAndObserve(id)). In that case, use the wrapper pattern above instead of storing the raw model.

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Model patterns in this project

• Models: database/models/ (e.g. Phrase, Lesson, Attempt, Translation, Deck). Use @field, @writer, and static helpers (e.g. Phrase.findOrCreatePhrase, Lesson.addLesson).
• Observation: useDatabase() from @nozbe/watermelondb/react; then collection.findAndObserve(id) or query.observe().subscribe(...).
• Schema/tables: database/schema.ts; collection access via db.collections.get<Model>(TABLE).

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Quick reference

Scenario	Pattern	Re-render guarantee
Single model by id (e.g. detail screen)	findAndObserve + wrapper state { model, _key }	Yes
Query results (list)	query.observe() + setState(results) or derived structures	Yes (new array/refs)
Model passed as prop	withObservables(['model'], ({ model }) => ({ model: model.observe() }))	Yes (HOC uses new state shape)

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Resources

• DeepWiki: Nozbe/WatermelonDB — findAndObserve, observe(), withObservables, model updates.
• Project: PhraseDetailScreen, LessonDetailScreen, SetDetailScreen (findAndObserve + wrapper); AttemptCard (withObservables).