# Add a typed column. Sparse by default (written only on rows that have a value — use for

`POST /datanotes/operations/add_column/` (operation `add_column`)

Add a typed column. Sparse by default (written only on rows that have a value — use for
metrics, e.g. `{"name": "metric_mood", "type": "integer", "min": 1, "max": 5, "label":
"Mood", "aliases": ["mood"], "description": "Mood of the day, 1 = bad, 5 = great"}`).
`description` is required: what the column holds, for the agents and people who read the schema
(the engine writes it for `identity` "uuid" / "created" columns).
`dense: true` in the spec adds it to every row with its `default`. Call `describe_table` first: never add
a column whose meaning an existing column (name/label/aliases/description) already covers.
Column names are permanent identifiers; only `rename_column` changes them.
Enum columns: `values` and, for each, its meaning in `value_descriptions` (`{"stated": "the user said it", …}`): describe_table shows them and a refused value lists them.
Generated link columns: `generated: "link({occurred_at:YYYY-MM-DD})"` with `ref` = the days table links each row to the row of `ref` named by the template (its folder read from the schema; the target may not exist yet).
Filtered link targets (link columns): `ref_where` (a `where` every target must match) and `ref_by` (a column of the row picks the target level: `{"column": "kind", "levels": ["area", "project", "workstream"]}` or `{"column": "kind", "map": {"area": null, "project": ["area"]}}`; null = top level, no link; `target_column` when the target names it differently: an enum target column must have every value the map allows, a string one takes any value). A link to no row of the table, or to the row itself, is refused too. Existing rows that break them refuse the change; `list_link_targets` lists the valid targets of a row.

The schema changes only through operations like this one: never edit metastore.yaml by hand (no validation, no undo).

## Parameters

| Name | In | Type | Required | Description |
| --- | --- | --- | --- | --- |
| `X-Datanotes-Actor` | header | `string` | no | JSON: on whose behalf the write is made, recorded in the journal (`harness`, `model`, `session`). |

## Request body

`application/json`, required

| Property | Type | Required | Default | Description |
| --- | --- | --- | --- | --- |
| `table` | `string` | yes |  |  |
| `column` | `object` | yes |  | The column's spec: name, type, description and its other attributes; `dense: true` writes it on every row. |
| `dry_run` | `boolean` | no | `false` |  |

## Responses

| Status | Description | Body |
| --- | --- | --- |
| 200 | Done. | `OpResult` (application/json) |
| 400 | Bad input: the body is not a JSON object, or an argument is unknown, missing or of the wrong type. | `Error` (application/json) |
| 401 | Missing or wrong bearer token. | `Error` (application/json) |
| 403 | A named token beyond its permissions (the engine's administration, tables outside its `write` patterns, schema changes without `ddl`). | `Error` (application/json) |
| 404 | Unknown table, view, migration, note or folder. | `Error` (application/json) |
| 409 | Ambiguous table name: it matches several tables (`paths` or `tables` lists them). | `AmbiguousName` (application/json) |
| 422 | Refused (validation failures, conflicts, an unmet `expect`): nothing written. Same body as 200, with `ok: false` and the reasons. | `OpResult` (application/json) |
| 500 | Unexpected error (`{error}`), or a write that failed partway and could not be rolled back: `rolled_back: false`, `failed` lists the files left (undo_migration on `migration_id` retries the rollback). | `OpResult \| Error` (application/json) |
| 503 | A write that failed partway (a file another program holds) and was rolled back: `rolled_back: true`, nothing written. Send it again later. Same body as 200, with `ok: false` and `failed`. | `OpResult` (application/json) |

The schemas named here are in [openapi.json](https://datanotes.dev/openapi.json) (`components.schemas`).