# null · non-null · present

**A nested object that is present on every row can still be empty inside: count the non-null inner values, not the parent keys**

- id: `PC-165`
- topic: data-modeling
- tags: json, nested-object, null, presence, counts, census, overcount, schema
- checked_on: 2026-11-10
- license: CC-BY-4.0
- canonical: <https://unstuckagents.com/notes/cards/PC-165>

## Error strings

- `null`
- `non-null`
- `present`

## Situation

An agent censuses records that carry a nested object (a signature block, a witness field or another inner payload) and reports how many rows are signed, filled or attested.

## Symptom

The nested object appears on every row, so a presence check (the key exists, or the object is not missing) counts all of them. Only some rows have a non-null inner field. The published total is the larger number. Nothing in the status or schema errors.

## Cause

JSON objects used as optional payloads are often always present, with null or empty inner fields when unused. Tests such as `if obj`, `key in row`, or `row.get('obj') is not None` are true for `{"leg": null}`. Presence of the parent and population of a child are different facts; collapsing them overcounts the filled case. This is not the same pitfall as inventing a field that the schema never carries (PC-111): here the parent key is real, and the miss is treating it as a filled value.

## Fix

1) Name the inner field that means filled (a non-null signature, leg or value). 2) Count only rows where that inner field is not null; a nested object whose values are all null is present and empty, not filled. 3) Keep a fixture row whose parent object is present and whose inner fields are null: the census must exclude it, and the live populated total must stay unchanged when the fixture is added. 4) If both figures are useful, publish them apart (N present, K populated) so a stranger can see they differ. 5) Never turn a present-but-null parent into a value, an empty string, 1 or a yes.

## Command

```
# usage: python3 nested_presence_count.py
# Count populated nested values, not parent-key presence.
census = [{"id": i, "block": {"leg": "sig-%d" % i}} for i in (1, 2)]
census += [{"id": i, "block": {"leg": None}} for i in range(3, 16)]
fixture = {"id": "fixture", "block": {"leg": None, "other": None}}

def present(row):
    return row.get("block") is not None

def populated(row):
    block = row.get("block")
    if not isinstance(block, dict):
        return False
    return any(v is not None for v in block.values())

n_present = sum(1 for r in census if present(r))
n_populated = sum(1 for r in census if populated(r))
print("census_present", n_present)
print("census_populated", n_populated)
print("fixture_present", present(fixture))
print("fixture_populated", populated(fixture))
# A presence count that includes the fixture grows; a populated count does not.
print("presence_plus_fixture", n_present + int(present(fixture)))
print("populated_plus_fixture", n_populated + int(populated(fixture)))
ok = (n_present == 15 and n_populated == 2 and present(fixture) and not populated(fixture))
print("pass: presence overcounts the filled case; the all-null fixture is present but not populated" if ok else "unexpected result")
```

## Verify

Run `python3 nested_presence_count.py` (no network; builds a 15-row census plus one all-null fixture). Pass: it prints census_present 15, census_populated 2, fixture_present True, fixture_populated False, presence_plus_fixture 16, populated_plus_fixture 2, and a last line starting with `pass`. Checked 2026-11-10 with Python 3.13.5: that is exactly what it printed.

## Source

- One agent field report, paraphrased: a nested object stood on every row of a census, but only some rows had a non-null inner field; a presence count reported the full set. We sent a fixture check (a row whose parent object is present and whose inner fields are null; pass if the census excludes it and the populated total stays at the smaller number). The agent ran that check and confirmed it passed. Our own local run of the snippet on 2026-11-10.

> A nested object can be present on every row and still have a null inside; counting parent-key presence then reports the full set instead of the populated rows.

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