Cannot add index for locale path

[jefferyto]

Describe the Bug

When I try to add an index for a locale path using the collection-level indexes option (as described in https://payloadcms.com/docs/database/indexes#localized-fields-and-mongodb-indexes), I get an error saying the field was not found:

/home/node/app/node_modules/payload/src/collections/config/sanitizeCompoundIndexes.ts:22
        throw new InvalidConfiguration(`Field ${path} was not found`)
              ^


InvalidConfiguration: Field title.en was not found
    at sanitizeCompoundIndexes (file:///home/node/app/node_modules/payload/dist/collections/config/sanitizeCompoundIndexes.js:16:23)
    at sanitizeCollection (file:///home/node/app/node_modules/payload/dist/collections/config/sanitize.js:209:40)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async sanitizeConfig (file:///home/node/app/node_modules/payload/dist/config/sanitize.js:191:33)
    at async buildConfig (file:///home/node/app/node_modules/payload/dist/config/build.js:12:16) {
  data: null,
  isOperational: true,
  isPublic: false,
  status: 500,
  [cause]: null
}

I wanted to set this index because I want to sort by the English field value, but do not want multiple indexes for every locale (#13464).

Link to the code that reproduces this issue

https://github.com/jefferyto/payload-index-locale-path

Reproduction Steps

1. Set up localization in payload.config.ts
2. Add a collection with a localized field
3. Set the collection-level indexes option with a locale path for the localized field, e.g.:

import type { CollectionConfig } from 'payload'

export const Posts: CollectionConfig = {
  slug: 'posts',
  fields: [
    {
      name: 'title',
      type: 'text',
      localized: true,
    },
  ],
  indexes: [
    { fields: ['title.en'] },
  ],
}

Which area(s) are affected? (Select all that apply)

area: core

Environment Info

Binaries:
  Node: 22.19.0
  npm: 10.9.3
  Yarn: 1.22.22
  pnpm: N/A
Relevant Packages:
  payload: 3.58.0
  next: 15.4.4
  @payloadcms/db-mongodb: 3.58.0
  @payloadcms/email-nodemailer: 3.58.0
  @payloadcms/graphql: 3.58.0
  @payloadcms/next/utilities: 3.58.0
  @payloadcms/payload-cloud: 3.58.0
  @payloadcms/richtext-lexical: 3.58.0
  @payloadcms/translations: 3.58.0
  @payloadcms/ui/shared: 3.58.0
  react: 19.1.0
  react-dom: 19.1.0
Operating System:
  Platform: linux
  Arch: x64
  Version: #32-Ubuntu SMP PREEMPT_DYNAMIC Fri Aug 29 14:21:26 UTC 2025
  Available memory (MB): 7225
  Available CPU cores: 4

[jefferyto]

Assuming title.en is a valid path (locale as the last segment), I noticed there aren't any test cases for this scenario in getFieldByPath.spec.ts.

I also noticed perhaps getFieldByPath.ts#L36 should break out of the while loop instead of returning null.

[r1tsuu]

I think this could be useful, though it can be supported only in MongoDB and not Postgres. I'm converting this to a discussion since it's not a bug but a feature request. In the meantime you can either just use index: true on the title field, or insert a custom MongoDB index manually/with a migration script.

Assuming title.en is a valid path (locale as the last segment), I noticed there aren't any test cases for this scenario in getFieldByPath.spec.ts.

It is a valid path in the Mongoose schema but not in the Payload schema, this function operates on Payload fields.