rdbms-playground/docs/adr/0044-relationship-visualization.md

# ADR-0044: Relationship visualization (two-table connector diagrams)

## Status

Accepted (2026-06-09); **implemented 2026-06-10** (commits: `cad90ec`
`show relationship` full diagram, `a0ee323` `show table` compact
diagrams, + compound-FK bus routing and self-referential diagrams). A
second `/runda` DA pass over the implementation confirmed
ADR-compliance, UTF-8/byte-range safety, and edge-case routing; the §3
last-resort helper line was **considered and rejected** (see §3).
Closes `requirements.md` **V1**.

Resolves **ADR-0016 OOS-1** ("Relationship visualization — two
structures side-by-side with an arrow; its own ADR; will compose
`render_structure`"). Partially **supersedes ADR-0016 §5** (the
plain-text `References:` / `Referenced by:` relationship block) and
**extends ADR-0016 §4 and §6** (adds layout width-awareness and
per-span styling). Builds on ADR-0028 (styled output runs),
ADR-0043 (compound, list-based relationship endpoints), ADR-0013
(named 1:n relationships), and the V5/V5a `show` family. Honours
ADR-0009 (DSL conventions) and ADR-0002 (no engine name in
user-facing strings).

Closes the substantive open half of requirement **V1**
(`docs/requirements.md`): "a selected relationship as two tables
joined by a line."

## Context

The table-structure half of V1 is done: `show table <T>` renders a
box-drawn structure table (columns / types / constraints / indexes),
and relationships appear **as prose** beneath it —
`References:` / `Referenced by:` blocks formatted `A.col → B.col`
(`output_render.rs render_structure`, per ADR-0016 §5). The same
prose appears in V5a's `show relationship <name>` detail view.

The open piece is the **visual** form: drawing a relationship as two
tables side by side joined by a connecting line, with cardinality and
referential actions. ADR-0016 deliberately deferred this (OOS-1) and
**pre-sized the structure renderer to compose two of itself**. This
ADR cashes that in.

Three facts from the current architecture shape the design:

1. **Rendering is App-side; the worker returns structured data.**
   *(Corrected from the initial draft after direct verification — an
   earlier survey had this backwards.)* The database worker (`db.rs`)
   returns `TableDescription` / `DataResult`; the **App** (`app.rs`)
   calls the `output_render.rs` helpers to format them into
   `OutputLine`s. Verified: every `render_structure` /
   `render_data_table` call site is in `app.rs` (557, 1669, 1732,
   1752, 1814, 1678, …); **none in `db.rs`**. Width and theme
   therefore live App-side. The **one exception** is the V5/V5a
   `show <kind> [<name>]` family: `do_show_one` / `show_list` build
   prose `Vec<String>` **in the worker** from `RelationshipSchema` /
   index metadata, carrying *no* `TableDescription` — so
   `show relationship <name>` is the path that must be restructured
   (§6). The output buffer is a flat `VecDeque<OutputLine>` — a
   historical log of lines, not re-renderable widgets (live
   re-rendering is V4 territory).
2. **Relationships are list-based (ADR-0043).** `RelationshipSchema`
   /`RelationshipEnd` endpoints are `Vec<String>` column lists,
   positionally paired. Single-column FKs are the one-element case;
   the diagram must render compound FKs too.
3. **Only 1:n relationships exist.** Relationships are declared 1:n
   (ADR-0013); m:n (requirement C4) is unbuilt, and there is no
   UNIQUE-target 1:1. So cardinality is always **1 on the parent
   (referenced) side, n on the child (referencing/FK) side**.

The user has chosen the design direction across three decisions
(recorded below as made): **visual style**, **where it appears**, and
**multi-relationship layout**.

## Decision

### 1. Scope and trigger — diagrams where relationships are the subject

Relationship diagrams replace the prose relationship form on the
surfaces where **relationships are the subject of the command**; no
new command or sigil is added (ADR-0009: one sigil, keyword grammar).
This is the user-chosen **"relationship-relevant"** reach (over a
global replacement of every structure echo, or a show-commands-only
scope — see the DA pass).

Diagram surfaces:

- **`show relationship <name>`** (V5a) renders **one** diagram: the
  parent and child tables as **full structure boxes** joined by a
  connector. The canonical base unit.
- **`show table <T>`** keeps T's full structure box at the top
  (unchanged from ADR-0016 §5), then a **Relationships** section:
  **one compact connector diagram per relationship**, stacked
  vertically (§4).
- **Relationship DDL auto-shows** — the structure echo after
  `add 1:n relationship`, `drop relationship`, and a future
  `modify relationship` (C3a) — render their relationships as
  compact diagrams (§4), since the user just acted on a relationship.

Prose-retained surfaces (**unchanged** from ADR-0016 §5):

- **Incidental DDL auto-shows** — the structure echo after
  `create table`, `add`/`drop`/`rename`/`change column`,
  `add`/`drop index` — keep the terse `References:` /
  `Referenced by:` prose. A simple `add column` on a heavily-related
  table should not print a wall of diagrams.

So this **partially supersedes ADR-0016 §5**: the prose block is
replaced by diagrams on the relationship-subject surfaces and
retained on incidental ones. No information is lost on either —
relationship name, endpoints, cardinality, and `on delete` /
`on update` actions all appear in both forms.

Mechanically (§6) this is a **render mode** on `render_structure`
(`Diagram` vs `Prose`), selected by the calling command; the items/
side panel (`ui.rs render_items_panel`) is a navigation tree and is
untouched.

A future **user-configurable setting** (e.g. always-prose /
always-diagram / auto-by-width, for small-screen users) is a clean
follow-up and is **out of scope here** (OOS-7) — the per-command
default above is the v1 behaviour.

### 2. The base diagram — Style A (structure + connector)

A relationship diagram is two boxes plus a connector. Reading
convention, applied **uniformly** (this is what makes
multi-relationship lists scannable):

- **Child (FK holder) on the left, parent (referenced) on the
  right.** The connector arrow always points **left → right**, in the
  direction of the reference (child *references* parent).
- **Cardinality** sits at the connector ends: **`n`** at the child
  (left) end, **`1`** at the parent (right) end.
- **Referential actions** (`on delete …`, `on update …`) label the
  connector beneath the line. `no action` is shown verbatim
  (ADR-0009 wording), abbreviated to the action keyword when space is
  tight.

#### 2.1 Box anatomy — the table name must stand out

Every box (full or compact) carries the table name as a **bold title
row separated from the columns by a rule** (`├─┤`), styled in the
table-name theme class (§5). This directly addresses the requirement
that a name never read as just another column row.

Full structure box (used in `show relationship`): title row, then the
ADR-0016 column/type listing, with key markers (§2.2):

```
┌──────────────────────┐
│ orders               │   ← bold title row (table name)
├──────────────┬───────┤
│ id      (PK) │ int   │
│ customer_id ●│ int   │   ● marks the FK column in this relationship
│ total        │ real  │
└──────────────┴───────┘
```

Compact box (used in `show table`'s stacked list): title row, rule,
then **only the column(s) participating in this relationship** — the
focal table's full structure is already shown above, so the compact
box stays small and the focal table is not redrawn in full N times:

```
┌──────────────┐
│ orders       │   ← bold title row
├──────────────┤
│ customer_id ●│   only the FK column(s) for this relationship
└──────────────┘
```

#### 2.2 Key markers

- The PK is annotated `(PK)` (consistent with the existing structure
  view's constraint column).
- The **endpoint columns of *this* relationship** are marked with a
  filled dot `●` adjacent to the column name — on the child FK
  column(s) and the parent referenced column(s). The connector
  attaches at these `●` rows.

#### 2.3 The connector (single-column)

`show relationship Customers_Orders` (child `orders.customer_id`
→ parent `customers.id`):

```
        orders                                customers
┌──────────────────────┐                  ┌─────────────────────┐
│ orders               │                  │ customers           │
├──────────────┬───────┤            1 ┌──●│ id      (PK)        │
│ id      (PK) │ int   │              │   ├─────────────┬───────┤
│ customer_id ●│ int   │ n ───────────┘   │ name        │ text  │
│ total        │ real  │                  │ email       │ text  │
└──────────────┴───────┘                  └─────────────┴───────┘
 on delete cascade · on update no action
```

**Connector routing.** The two `●` endpoint rows are generally at
different heights. The connector leaves the child box's right edge at
the child-`●` row, travels horizontally into a **gutter channel**
between the boxes, jogs vertically to the parent-`●` row, then enters
the parent box's left edge with an arrowhead. Box tops are aligned.
(Exact glyphs/spacing are pinned by insta snapshots in
implementation; the mockups here are representative, not normative.)

#### 2.4 Compound foreign keys (ADR-0043)

For an n-column FK, the n child `●` columns and n parent `●` columns
are each listed, and **one connector is routed per positional pair**.
A summary line states the pairing explicitly so it is unambiguous
even if the routed lines are visually close:

```
              orders                              customers
┌──────────────────────────┐              ┌──────────────────────┐
│ orders                   │          1 ┌●│ region   (PK)        │
├──────────────┬───────────┤            │ │ id       (PK)        │
│ cust_region ●│ text      │ n ─────────┤ ├────────────┬─────────┤
│ cust_id     ●│ int       │ n ─────────┘ │ name       │ text    │
│ …            │ …         │              └────────────┴─────────┘
└──────────────┴───────────┘
 (cust_region, cust_id) ──► customers.(region, id)
 on delete cascade · on update no action
```

### 3. Width handling and the narrow-terminal fallback

The diagram is rendered **once, App-side, at the output-panel width
current when the command runs**. The `App` does **not** track the
panel width today — `note_output_viewport(visible_rows,
total_wrapped_rows)` records only row counts for scroll math
(verified; `App` has no width field). This ADR adds a
`last_output_width: u16` to `App`, set from `ui.rs` where the output
panel's `inner.width` is already computed (next to the existing
`note_output_viewport` call), with an `80` default before the first
render. The App-side render path then chooses layout per diagram:

- **Side-by-side** (§2) when both boxes plus the gutter fit the
  available width.
- **Vertical stack** when they do not: parent box above, child box
  below, connector running **downward** through a short vertical
  channel, cardinality and actions labelling it. This keeps a real
  diagram at any width instead of degrading to prose:

```
┌──────────────┐
│ orders       │
├──────────────┤
│ customer_id ●│ n
└──────────────┘
        │  on delete cascade
        ▼  on update no action
┌──────────────┐
│ customers    │
├──────────────┤
│ id      (PK)●│ 1
└──────────────┘
```

- **Last-resort helper — considered and rejected (2026-06-10,
  implementation).** The draft proposed that, when even a single
  *vertical* diagram cannot fit (a box wider than the pane), `show
  table` emit a one-line `run `show relationship <name>`` pointer per
  relationship. In implementation we **decided against it**: the
  vertical fallback already covers every realistic narrow terminal,
  and the only remaining case — a box wider than the *whole* pane —
  requires an extreme combination (very long identifiers on a tiny
  terminal) and is handled the same way as all other over-wide output
  (ratatui's right-edge truncation, ADR-0016 §4). A dedicated pointer
  would add a code path and a worse result (less information than even
  a truncated diagram) for a near-unreachable case. `show relationship`
  itself always renders the diagram; if its box is wider than the pane,
  the same truncation applies — no new truncation logic.

**No live reflow.** Because the output buffer is a historical line
log, resizing the terminal *after* a diagram is rendered does not
reflow it — identical to how every other rendered output behaves
today. Reflow-on-resize belongs to V4's re-renderable journal and is
explicitly out of scope (OOS-2).

This **extends ADR-0016 §4**: that ADR added no width-awareness
(relying on ratatui truncation); this ADR adds width-awareness for
the **layout decision** only. It still performs **no per-cell
truncation** (ADR-0016 OOS-4 stands).

### 4. `show table` multi-relationship layout — stacked compact diagrams

After T's full structure box, a **Relationships** heading, then one
**compact** diagram (§2.1) per relationship, stacked vertically.
Ordering matches the current prose order: **outbound** (T is the
child — "References") first, then **inbound** (T is the parent —
"Referenced by"), each group ordered by relationship name. The
child-left/parent-right rule (§2) is applied per diagram, so the
focal table appears on the left for its outbound relationships and on
the right for its inbound ones — consistent with the reference
direction.

```
orders
┌──────────────┬──────┐
│ orders       │      │
├──────────────┼──────┤
│ id      (PK) │ int  │
│ customer_id  │ int  │
│ total        │ real │
└──────────────┴──────┘

Relationships
┌──────────────┐  n          1  ┌─────────────┐
│ orders       │────────────────│ customers   │
├──────────────┤  on delete     ├─────────────┤
│ customer_id ●│  cascade       │ id   (PK)  ●│
└──────────────┘                └─────────────┘
┌──────────────┐  n          1  ┌─────────────┐
│ order_items  │────────────────│ orders      │
├──────────────┤  on delete     ├─────────────┤
│ order_id    ●│  cascade       │ id   (PK)  ●│
└──────────────┘                └─────────────┘
```

Stacking (rather than a single focal-centred subgraph with fan-out
connectors) is chosen because: it never produces crossing lines; it
scales to any number of relationships via the pane's existing
vertical scroll; each diagram is only two boxes wide, so it fits
nearly any terminal; and it keeps the per-diagram width logic of §3
trivial. The cost — the focal table name repeats per diagram — is
mitigated by the compact box showing only the participating
column(s).

### 5. Styling

Styling uses the **ADR-0028 styled-run mechanism** (`OutputSpan` /
`OutputStyleClass` on `OutputLine`), not raw text. New style classes
(theme-defined, legible light/dark per ADR-0016 §6 / NFR-7):

- **table name** — bold, accent colour (the "stand out" requirement);
- **key marker** (`●`, `(PK)`) — a distinct accent;
- **connector** (box-drawing line + arrowhead) — muted;
- **cardinality** (`1` / `n`) — emphasised;
- **referential action** label — muted/secondary.

This **extends ADR-0016 §6** (which set up `OutputKind::System`
styling but no per-element theming) by reusing the per-span
`OutputSpan` path ADR-0028 later introduced — which is **already
produced App-side** (`output_render.rs:299-332` builds styled runs
for the explain-plan tree; `app.rs` constructs `OutputLine`s with
`styled_runs: Some(..)`; `ui.rs:863` renders them). So **no
worker→UI contract change is needed**: the new `output_render`
diagram functions return styled lines directly on the App side
(§6).

### 6. Implementation

All rendering is **App-side** (per the corrected Context fact 1), in
`output_render.rs` (hand-rolled, ADR-0016 §7), returning styled
lines that `app.rs` pushes as `OutputLine`s with `styled_runs` set.
No worker `available_width` and no worker→UI contract change.

- **New renderer functions in `output_render.rs`**, composing the
  existing box primitives and emitting styled spans (§5):
  - `render_relationship_diagram(child, parent, rel, width, full) ->
    Vec<OutputLine>` — `full` selects full vs compact boxes;
    internally decides side-by-side vs vertical (§3) from `width`;
    routes the connector(s) including compound pairs (§2.4).
  - A shared helper for the box title row + key markers.
  - The two call paths supply `width` from the new
    `App::last_output_width` (§3).
- **`show table <T>` (already App-side).** The focal
  `TableDescription` reaches `app.rs` and is rendered by
  `render_structure` today. `render_structure` is refactored to emit
  the focal structure box, then a **Relationships** section built
  from the focal description's `RelationshipEnd`s (which already
  carry neighbour table name + participating columns + cardinality +
  actions — **enough for the compact box**; no neighbour
  full-structure fetch needed). `render_structure` gains a
  **relationship render mode** (`Diagram` | `Prose`); the caller
  selects it (§1): `show table` and the relationship DDL echoes pass
  `Diagram`, incidental DDL echoes pass `Prose`. The generic
  `handle_dsl_success(command, description)` (`app.rs:1669`) already
  has the `Command`, so the mode is a function of the command
  variant; the incidental-only call sites (`app.rs:557/1732/1752/
  1814`) pass `Prose`.
- **`show relationship <name>` (must be restructured).** Its
  worker path `do_show_one` currently returns prose `Vec<String>`
  from a `RelationshipSchema` only. To draw the **full** diagram the
  App needs **both** endpoint `TableDescription`s. The relationship
  detail reply is upgraded to carry the `RelationshipSchema` plus
  both endpoints' `TableDescription`s (the worker already has
  `do_describe_table`); the App renders the diagram. The
  "No relationship named `X`." not-found line is preserved.
- **Self-referential FKs** (`parent_table == child_table`; supported
  in the grammar): render as two boxes bearing the same table name
  (child-left copy with the FK column, parent-right copy with the
  referenced column), connector as usual — clearer than a self-loop
  glyph in a TUI. Covered by a snapshot test.
- **Out of these surfaces:** the items/side panel (`ui.rs:582`
  `render_items_panel`) is a navigation tree, not a relationship
  view, and is unchanged.
- **Database engine name** never appears in any rendered string
  (ADR-0002).

### 7. Testing

- **Insta snapshots** (Tier 2) pin exact rendered output for:
  single-column 1:n (`show relationship`); compound FK; the
  narrow-terminal vertical fallback; the last-resort helper line;
  a self-referential FK; and a `show table` with both an outbound and
  an inbound relationship (stacked compact list).
- **Unit tests** (`output_render.rs`): the side-by-side-vs-vertical
  **width-threshold** decision at boundary widths; connector routing
  for endpoints at differing row heights; compound-pair routing;
  key-marker placement.
- **Tier-3 integration**: `show relationship <name>` and
  `show table <T>` produce diagram output (not prose) end-to-end
  through the worker.
- **Existing tests/code to update** (enumerated from a DA grep — the
  supersession of ADR-0016 §5 is not abstract):
  - `output_render.rs:121,135` — the `References:` / `Referenced by:`
    prose-emitting code in `render_structure`;
  - `output_render.rs:78` — the docstring deferral note (OOS-1);
  - `output_render.rs:793` — unit test asserting `"Referenced by:"`;
  - `src/snapshots/…render_structure_with_relationships.snap` — the
    prose snapshot;
  - `tests/it/walking_skeleton.rs:477,530` — integration asserts on
    `"Referenced by:"` (and the comment at 433);
  - `src/dsl/command.rs:984` — a comment referencing the prose (no
    behaviour).
  If the scope fork resolves to "all `render_structure` call sites,"
  the DDL-echo snapshots for create-table / add-column / drop-index /
  change-column auto-shows also churn — to be re-recorded with
  `cargo insta`. No test is deleted to hide a regression; each change
  is a deliberate format update.

### 8. Out of scope

- **OOS-1.** Live reflow-on-resize of already-rendered diagrams —
  V4's re-renderable journal (requirement V4).
- **OOS-2.** Whole-database ER diagram / export (requirement V3).
- **OOS-3.** m:n relationships (requirement C4, unbuilt). Only the
  existing 1:n form is rendered.
- **OOS-4.** ASCII fallback for terminals without box-drawing
  (inherits ADR-0016 OOS-5).
- **OOS-5.** Per-cell colouring of column *data* values (ADR-0016
  OOS-3 / NFR-5) — unrelated to relationship structure.
- **OOS-6.** Cell-level truncation with ellipsis (ADR-0016 OOS-4
  stands).
- **OOS-7.** A user-configurable relationship-display setting
  (always-prose / always-diagram / auto-by-width — useful for small
  screens). The §1 per-command default is v1; the setting is a clean
  later follow-up (user-flagged 2026-06-09).

## Consequences

- Requirement **V1** is fully satisfied: relationships render as
  two-table connector diagrams in both `show relationship <name>`
  (full) and `show table <T>` (stacked compact), superseding the
  prose form.
- `output_render.rs` grows a relationship renderer that **composes**
  the box primitives exactly as ADR-0016 anticipated; the worker→UI
  show contract becomes styled-line-based, a small generalisation
  reusable by other styled `show` output later.
- The worker gains an `available_width` input for `show` — the first
  width-aware formatting in the codebase. The decision is a snapshot
  at command time; no resize machinery is introduced.
- Compound FKs (ADR-0043) get their first dedicated visualization,
  with explicit positional-pair labelling.
- The historical-log output model is preserved (no widget/reflow
  model); V4 remains the home for a re-renderable journal.
- Box-drawing is required (no ASCII fallback), consistent with
  ADR-0016.

## Devil's Advocate / runda pass (2026-06-09)

A planning-time DA pass (empirical, against the code) corrected three
foundational errors carried in from an initial code survey, and
surfaced one open fork:

1. **Rendering boundary was inverted** (fixed in Context 1 / §6):
   `render_structure` / `render_data_table` run **App-side**
   (`app.rs`), not in the worker. Rendering, width, and theme are all
   App-side — simpler than the draft assumed.
2. **Width was claimed "already tracked"; it is not** (fixed in §3):
   `note_output_viewport` records only rows. A new
   `App::last_output_width` (set from `ui.rs`) is required.
3. **`show relationship <name>` renders prose in the worker** (fixed
   in §6): `do_show_one` carries only a `RelationshipSchema`. The
   detail reply must be upgraded to include both endpoint
   `TableDescription`s for the full diagram.
4. **Styled-line contract was over-stated** (fixed in §5): styled
   runs are already produced App-side (explain plan); no worker→UI
   contract change.

**Resolved fork (user, 2026-06-09): "relationship-relevant" reach.**
Diagrams render on the surfaces where the relationship is the subject
(`show table`, `show relationship`, relationship DDL echoes);
incidental DDL echoes keep prose (§1) — avoiding a wall of diagrams
after an `add column`. A future user-configurable display setting is
noted as OOS-7. This also records V1's **deliberate scope expansion**
beyond its literal "a selected relationship" wording into
multi-relationship surfaces, to be reflected in `requirements.md`.

All design forks are now resolved and the architecture is corrected.
The user accepted this revised ADR on 2026-06-09; status is
**Accepted** and implementation proceeds test-first.