This migration will duplicate a "base document" and all its translations

i18n-duplicate.ts

import {uuid} from '@sanity/uuid'
import {SanityDocumentLike} from 'sanity'
import {defineMigration, create} from 'sanity/migrate'

// Update these constants to match your Document Internationalization Plugin configuration
const BASE_FIELD = `__i18n_base`
const REFS_FIELD = `__i18n_refs`
const LANGUAGE_FIELD = `_lang`
const ID_SUFFIX = '__i18n'

/**
 * Script for i18n duplications that exceed the max request size of a single transaction
 * The shape of the documents created in this script only work with Sanity Document Internationalization v1
 * 
 * This migration will duplicate a "base document" and all its translations
 * The base document's ID must be provided as an argument when running the script
 * 
 * Place this at the base of your Sanity Studio project in
 * ./migrations/i18n-duplicate/index.ts
 *
 * Dry run:
 * npx sanity@latest migration run i18n-duplicate --id=<documentId>
 * 
 * Execute:
 * npx sanity@latest migration exec i18n-duplicate --no-dry-run --id=<documentId> 
 */
export default defineMigration({
  title: 'i18n-duplicate',

  async *migrate(documents, context) {
    // Get ID from args
    const id = process.argv.find((arg) => arg.startsWith('--id='))?.split('=')[1]

    if (!id) {
      throw new Error('Missing ID argument')
    }

    // Query for base document
    let baseDocument = await context.client.fetch<SanityDocumentLike>(`*[_id == $id][0]`, {id})

    if (!baseDocument) {
      throw new Error('Base document not found')
    } else if (!Array.isArray(baseDocument?.[REFS_FIELD])) {
      throw new Error('Base document has no translation references')
    }

    let translationDocuments = await context.client.fetch<SanityDocumentLike[]>(`*[_id in $ids]`, {
      ids: baseDocument[REFS_FIELD].map((ref) => ref._ref),
    })

    if (!translationDocuments.length) {
      throw new Error('No translations found')
    }

    // Create new ID's in every document, preserving draft status
    baseDocument._id = baseDocument._id.startsWith('drafts.') ? `drafts.${uuid()}` : uuid()
    translationDocuments.forEach((doc) => {
      // translations have ID's like "4b344817-22e6-4e28-9c59-5dda1a313a75__i18n_fr"
      // where the ID begins with the base document's ID

      // retrieve the everything after the suffix on this translation's ID, eg "_fr"
      const idSuffix = doc._id.split(ID_SUFFIX)[1]

      // don't reference draft documents
      const publishedBaseId = baseDocument._id.replace('drafts.', '')
      const newId = publishedBaseId + ID_SUFFIX + idSuffix

      // set new ID on translation, keeping draft status
      doc._id = doc._id.startsWith('drafts.') ? `drafts.${newId}` : newId
    })

    // Update refs in base document
    baseDocument[REFS_FIELD] = translationDocuments.map((doc) => ({
      _key: doc[LANGUAGE_FIELD],
      // don't reference draft documents
      _ref: doc._id.replace('drafts.', ''),
      _type: 'reference',
    }))

    // Update base document ref in translations
    translationDocuments.forEach((doc) => {
      doc[BASE_FIELD] = {
        // don't reference draft documents
        _ref: baseDocument._id.replace('drafts.', ''),
        _type: 'reference',
      }
    })

    const payload: SanityDocumentLike[] = [baseDocument, ...translationDocuments]

    yield payload.map((doc) => create(doc))
  },
})