migration.backfill_outside_row: Computed backfill reads more than its row

Computed backfill reads more than its row.

Area 5, Entities, stores and migrations; not declinable; the next step is usually the agent's.

A backfill reads only the record it fills:

example

migration {    Draft.starred: {        backfill: Note.starred    }}

jadpo checkcheck failed: 1 error, 4 files

error The backfill for Draft.starred can't read Note.starred

--> database/migrations/20261020T1401_add_starred/changes.jadpo:3:19

  |3 |         backfill: Note.starred  |                   ^^^^^^^^^^^^ another entity's field

reason: A backfill works each Draft record's value out from that record's own fields, constants and pure functions, so it gives the same value whenever and wherever the migration runs.
next: Work the value out from Draft's own fields, or a constant (agent)
rule: migration.backfill_outside_row
help: https://jadpo.dev/docs/errors/migration.backfill_outside_row

Nor does it read a setting through a function it calls, a crypto operation's key: included, since each deployment sets its own and jadpo migrate apply is given none:

example

use notes/types { Label }library draft_labels {    function default_label() -> Label {        return config.draft_label    }}

example

migration {    Draft.label: {        backfill: draft_labels.default_label()    }}

jadpo checkcheck failed: 1 error, 6 files

error The backfill for Draft.label can't read config.draft_label

--> database/migrations/20261020T1401_add_label/changes.jadpo:3:19

  |3 |         backfill: draft_labels.default_label()  |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ reads a setting, through `draft_labels.default_label`

reason: A backfill gives each Draft record the value its own fields give, whenever and wherever the migration runs, and draft_labels.default_label reads config.draft_label, which each deployment sets for itself.
next: Work the value out without config.draft_label: from Draft's own fields, or a constant (agent)
rule: migration.backfill_outside_row
help: https://jadpo.dev/docs/errors/migration.backfill_outside_row

Nor a field the same migration adds, which no record already stored has a value for while the backfill runs:

example

migration {    Draft.title: {        backfill: "Untitled"    }    Draft.subheading: {        backfill: Draft.title    }}

jadpo checkcheck failed: 1 error, 4 files

error The backfill for Draft.subheading can't read Draft.title

--> database/migrations/20261020T1401_add_titles/changes.jadpo:6:19

  |6 |         backfill: Draft.title  |                   ^^^^^^^^^^^ a field this migration adds

reason: Draft.title is added by this same migration, so no Draft record already stored has a value for it while the backfill runs.
next: Work the value out from the fields Draft had before this migration, or a constant (agent)
rule: migration.backfill_outside_row
help: https://jadpo.dev/docs/errors/migration.backfill_outside_row