Docs: formula output on reference-typed properties is silently coerced to string

[mitselek]

TL;DR

A property declared type: reference with a formula produces string output, not a reference value. The declared type: reference is silently ignored.

Reproduction (probed 2026-05-18 against api.entu.app/polyphony)

1. Create temp entity type with property: { name: ref_via_formula, type: reference, formula: _parent }
2. Create a parent entity (any type)
3. Create an instance of the temp type under that parent
4. GET the instance

Returns:

"ref_via_formula": [
  { "string": "<parent's name>" }
]

No reference key — value is stored as string regardless of the declared type: reference.

Source

entu/api/utils/formula.js — the formula() function only returns { string: X } or { number: X }. No { reference: X } output path exists.

Implication

Apps designing on Entu can't expect formula-populated reference properties to behave as refs. They should declare formula-bearing properties as type: string for honest schema, or accept the type-vs-output mismatch.

Suggested doc note

At https://www.entu.ee/api/formulas/ (or properties config docs): add a note that formula output is always string or number; declaring type: reference on a formula property is silently ignored.

Related: polyphony case study D3 — https://github.com/entu/research/blob/main/docs/case-studies/2026-05-polyphony-on-entu.md