Auto-generate reference field from _id's last 6 characters in Sanity

You're on the right track! The best approach here is to generate the _id yourself before creating the document. This allows you to derive the reference field from the ID and include it in a single create() call—no need for a second API call or transaction.

Here's how to do it:

import {v4 as uuidv4} from 'uuid'

const customId = uuidv4() // Generate a UUID like Sanity does
const reference = customId.slice(-6).toUpperCase() // Last 6 chars, uppercased

const order = {
  _id: customId,
  _type: 'order',
  reference: reference,
  // ... other fields
}

await client.create(order)

This is actually a recommended pattern for content migrations and works perfectly for your use case. Sanity allows you to specify custom _id values when creating documents—they don't have to be auto-generated.

Why this is the best approach:

1. Single API call: Everything happens in one create() operation
2. Atomic: The document is created with the reference field already set
3. Clean: No need for transactions or patches
4. Supported: Custom IDs are a documented feature—Sanity accepts any valid ID format (alphanumeric plus ., _, - up to 128 characters)

Important notes:

• Don't use drafts. prefix for the custom ID—that's reserved for draft documents
• The _id you set is permanent and never changes
• Make sure your IDs are unique within your dataset
• If you're creating documents in Sanity Studio, it will auto-generate UUIDs, but for programmatic creation via the client, you have full control

Your initial instinct about using transactions was creative, but the issue you identified is real: you can't reference the _id of a document being created within the same transaction before it's actually created. By generating the ID upfront, you sidestep this entirely.