Mutations & Optimistic State

update (Immer-style draft proxy)

// Single item -- mutate the draft, do NOT reassign it
todoCollection.update(todo.id, (draft) => {
  draft.completed = true
  draft.completedAt = new Date()
})

// Multiple items
todoCollection.update([id1, id2], (drafts) => {
  drafts.forEach((d) => {
    d.completed = true
  })
})

// With metadata
todoCollection.update(
  todo.id,
  { metadata: { reason: 'user-edit' } },
  (draft) => {
    draft.text = 'Updated'
  },
)

delete

todoCollection.delete(todo.id)
todoCollection.delete([id1, id2])
todoCollection.delete(todo.id, { metadata: { reason: 'completed' } })

All three return a Transaction object. Use tx.isPersisted.promise to await persistence or catch rollback errors.

Core Patterns

1. createOptimisticAction -- intent-based mutations

Use when the optimistic change is a guess at how the server will transform the data, or when you need to mutate multiple collections atomically.

import { createOptimisticAction } from '@tanstack/db'

const likePost = createOptimisticAction<string>({
  // MUST be synchronous -- applied in the current tick
  onMutate: (postId) => {
    postCollection.update(postId, (draft) => {
      draft.likeCount += 1
      draft.likedByMe = true
    })
  },
  mutationFn: async (postId, { transaction }) => {
    await api.posts.like(postId)
    // IMPORTANT: wait for server state to sync back before returning
    await postCollection.utils.refetch()
  },
})

// Returns a Transaction
const tx = likePost(postId)
await tx.isPersisted.promise

Multi-collection example:

const createProject = createOptimisticAction<{ name: string; ownerId: string }>(
  {
    onMutate: ({ name, ownerId }) => {
      projectCollection.insert({ id: crypto.randomUUID(), name, ownerId })
      userCollection.update(ownerId, (d) => {
        d.projectCount += 1
      })
    },
    mutationFn: async ({ name, ownerId }) => {
      await api.projects.create({ name, ownerId })
      await Promise.all([
        projectCollection.utils.refetch(),
        userCollection.utils.refetch(),
      ])
    },
  },
)

2. createPacedMutations -- auto-save with debounce / throttle / queue

import { createPacedMutations, debounceStrategy } from '@tanstack/db'

const autoSaveNote = createPacedMutations<string>({
  onMutate: (text) => {
    noteCollection.update(noteId, (draft) => {
      draft.body = text
    })
  },
  mutationFn: async ({ transaction }) => {
    const mutation = transaction.mutations[0]
    await api.notes.update(mutation.key, mutation.changes)
    await noteCollection.utils.refetch()
  },
  strategy: debounceStrategy({ wait: 500 }),
})

// Each call resets the debounce timer; mutations merge into one transaction
autoSaveNote('Hello')
autoSaveNote('Hello, world') // only this version persists

Other strategies:

import { throttleStrategy, queueStrategy } from '@tanstack/db'

// Evenly spaced (sliders, scroll)
throttleStrategy({ wait: 200, leading: true, trailing: true })

// Sequential FIFO -- every mutation persisted in order
queueStrategy({ wait: 0, maxSize: 100 })

3. createTransaction -- manual batching

import { createTransaction } from '@tanstack/db'

const tx = createTransaction({
  autoCommit: false, // wait for explicit commit()
  mutationFn: async ({ transaction }) => {
    await api.batchUpdate(transaction.mutations)
  },
})

tx.mutate(() => {
  todoCollection.update(id1, (d) => {
    d.status = 'reviewed'
  })
  todoCollection.update(id2, (d) => {
    d.status = 'reviewed'
  })
})

// User reviews... then commits or rolls back
await tx.commit()
// OR: tx.rollback()

Inside tx.mutate(() => { ... }), the transaction is pushed onto an ambient stack. Any collection.insert/update/delete call joins the ambient transaction automatically via getActiveTransaction().

4. Mutation handler with refetch (QueryCollection pattern)

const todoCollection = createCollection(
  queryCollectionOptions({
    queryKey: ['todos'],
    queryFn: () => api.todos.getAll(),
    getKey: (t) => t.id,
    onInsert: async ({ transaction }) => {
      await Promise.all(
        transaction.mutations.map((m) => api.todos.create(m.modified)),
      )
      // IMPORTANT: handler must not resolve until server state is synced back
      // QueryCollection auto-refetches after handler completes
    },
    onUpdate: async ({ transaction }) => {
      await Promise.all(
        transaction.mutations.map((m) =>
          api.todos.update(m.original.id, m.changes),
        ),
      )
    },
    onDelete: async ({ transaction }) => {
      await Promise.all(
        transaction.mutations.map((m) => api.todos.delete(m.original.id)),
      )
    },
  }),
)

For ElectricCollection, return { txid } instead of refetching:

onUpdate: async ({ transaction }) => {
  const txids = await Promise.all(
    transaction.mutations.map(async (m) => {
      const res = await api.todos.update(m.original.id, m.changes)
      return res.txid
    }),
  )
  return { txid: txids }
}

Common Mistakes

CRITICAL: Passing an object to update() instead of a draft callback

// WRONG -- silently fails or throws
collection.update(id, { ...item, title: 'new' })

// CORRECT -- mutate the draft proxy
collection.update(id, (draft) => {
  draft.title = 'new'
})

CRITICAL: Hallucinating mutation API signatures

The most common AI-generated errors:

• Inventing handler signatures (e.g. onMutate on a collection config)
• Confusing createOptimisticAction with createTransaction
• Wrong PendingMutation property names (mutation.data does not exist -- use mutation.modified, mutation.changes, mutation.original)
• Missing the ambient transaction pattern

Always reference the exact types in references/transaction-api.md.

CRITICAL: onMutate returning a Promise

onMutate in createOptimisticAction must be synchronous. Optimistic state is applied in the current tick. Returning a Promise throws OnMutateMustBeSynchronousError.

// WRONG
createOptimisticAction({
  onMutate: async (text) => {
    collection.insert({ id: await generateId(), text })
  },
  ...
})

// CORRECT
createOptimisticAction({
  onMutate: (text) => {
    collection.insert({ id: crypto.randomUUID(), text })
  },
  ...
})

CRITICAL: Mutations without handler or ambient transaction

Collection mutations require either:

1. An onInsert/onUpdate/onDelete handler on the collection, OR
2. An ambient transaction from createTransaction/createOptimisticAction

Without either, throws MissingInsertHandlerError (or the Update/Delete variant).

HIGH: Calling .mutate() after transaction is no longer pending

Transactions only accept new mutations while in pending state. Calling mutate() after commit() or rollback() throws TransactionNotPendingMutateError. Create a new transaction instead.

HIGH: Changing primary key via update

The update proxy detects key changes and throws KeyUpdateNotAllowedError. Primary keys are immutable once set. If you need a different key, delete and re-insert.

HIGH: Inserting item with duplicate key

If an item with the same key already exists (synced or optimistic), throws DuplicateKeyError. Always generate a unique key (e.g. crypto.randomUUID()) or check before inserting.

HIGH: Not awaiting refetch after mutation in query collection handler

The optimistic state is held only until the handler resolves. If the handler returns before server state has synced back, optimistic state is dropped and users see a flash of missing data.

// WRONG -- optimistic state dropped before new server state arrives
onInsert: async ({ transaction }) => {
  await api.createTodo(transaction.mutations[0].modified)
  // missing: await collection.utils.refetch()
}

// CORRECT
onInsert: async ({ transaction }) => {
  await api.createTodo(transaction.mutations[0].modified)
  await collection.utils.refetch()
}

Tension: Optimistic Speed vs. Data Consistency

Instant optimistic updates create a window where client state diverges from server state. If the handler fails, the rollback removes the optimistic state -- which can discard user work the user thought was saved. Consider:

• Showing pending/saving indicators so users know state is unconfirmed
• Using { optimistic: false } for destructive operations
• Designing idempotent server endpoints so retries are safe
• Handling tx.isPersisted.promise rejection to surface errors to the user

References

• Transaction API Reference -- createTransaction config, Transaction object, PendingMutation type, mutation merging rules, strategy types
• TanStack DB Mutations Guide