A field on the same document

An account has createdAt. List pages want the cohort (YYYY-MM) without parsing dates on every read. Keep _cohort on the account, and refresh it when createdAt changes.

from is enough: there is no join and no previous value to subtract.

import type { Collection, Model, N, O } from '@omegup/msync'
import { $simpleMerge, from, monthPart, root } from '@omegup/msync'

type Account = O<{
  _id: string
  name: string
  createdAt: Date
  _cohort: string | N
}>

export const makeAccountCohortStream = (accounts: Collection<Account & Model>) =>
  from(
    {
      collection: accounts,
      projection: { createdAt: ['createdAt', 1] },
    },
    'account-cohort',
  )
    .get()
    .out(
      $simpleMerge<O<{ _cohort: string }>>()(accounts, {
        _cohort: ['_cohort', monthPart(root<Account>().of('createdAt').expr())],
      }),
    )

$simpleMerge writes _cohort onto the same _id. The document is already there, so a missing target fails the merge. The next change of createdAt rewrites that account only.

{ _id: "a1", createdAt: 2026-03-02 }   →   { _cohort: "2026-03" }

root<T>() is the current document, typed as T. .of('key') steps into a field. .expr() turns the path into a value you can pass to monthPart, $sum, $merge, and the rest.

Next: copy the account name onto each order.