docs: ADR-0045 m:n convenience command (C4); accepted

create m:n relationship from <T1> to <T2> [as <name>] generates a
junction table (compound PK over the two FK column sets, CASCADE FKs)
plus two 1:n relationships, in one do_create_table call = one undo
step. Forks user-confirmed; /runda DA pass verified the reuse against
code and the no-PK-tables-exist-in-advanced-mode fact (parent-PK guard
retained). Self-referential m:n refused; FK cols named {table}_{pkcol}.
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# ADR-0045: `create m:n relationship` convenience command (C4)
## Status
Accepted (2026-06-10). Closes `requirements.md` **C4**. All four
design forks were escalated and user-confirmed at the recommended
option (compound-over-FKs junction PK; `CASCADE` actions; auto-name +
optional `as`; both modes). Two follow-up points were also confirmed
in a `/runda` DA pass: **self-referential m:n is refused outright**
(user: "refuse — full stop"; it is a beginner-facing convenience, not
the place for directional-naming complexity), and the FK column naming
is **`{parent_table}_{pk_column}`**. The DA pass additionally
established — against the user's initial assumption — that **PK-less
tables *are* reachable** (advanced-mode SQL `create table t (a int)`
declares no PK; `sql_create_table.rs` asserts `pk.is_empty()`), so the
parent-PK guard (D7) is retained as a correctness check.
Builds on ADR-0013 (named 1:n relationships, the relationship
metadata table, the rebuild-table primitive), ADR-0043 (compound,
list-based FK references — the junction may reference compound parent
PKs), ADR-0011 (`Type::fk_target_type()` for FK column typing), and
the existing `do_create_table` executor (which already accepts
`foreign_keys: Vec<SqlForeignKey>` and writes relationship metadata
per FK). Honours ADR-0003 (mode model), ADR-0009 (DSL conventions),
ADR-0002 (no engine name in user-facing strings), and ADR-0024
(unified grammar / `CommandNode` registration, completion, hints,
help-id, usage-id wiring).
## Context
A many-to-many relationship is modelled in a relational database by a
**junction table** (a.k.a. associative / bridge table) that holds one
foreign key to each of the two parents. Today a learner can build this
by hand: `create table` the junction, then `add 1:n relationship`
twice. That is three commands and requires the learner to already know
the junction-table pattern — exactly the concept C4 is meant to
*teach*.
C4 (`requirements.md`): *"`create m:n relationship from <T1> to <T2>`
produces an auto-named junction table the user can rename; pulls
primary keys and FK definitions automatically."*
The relationship machinery this builds on is freshly solid: ADR-0043
made the relationship model list-based (compound-aware) across six
layers, and ADR-0044 gave relationships a visual representation. C4 is
the natural convenience layer on top.
## Decision
Add a DSL command:
```
create m:n relationship from <T1> to <T2> [as <name>]
```
It generates a **junction table** with one FK column per primary-key
column of each parent, a **compound primary key** over all of those FK
columns, and **two 1:n relationships** (junction → T1, junction → T2),
all in a single transaction (= one undo step). The junction is a
normal table: `rename table`, `drop table`, `show table`, `insert`,
etc. all work on it afterward.
### D1 — Junction primary key: compound over the FK columns (fork, user-chosen)
The junction's PK is the **combination of all its FK columns**. For
`create m:n relationship from Students to Courses` (both PK `id`):
```
Students_Courses
Students_id int ┐ PRIMARY KEY (Students_id, Courses_id)
Courses_id int ┘
FOREIGN KEY (Students_id) REFERENCES Students(id)
FOREIGN KEY (Courses_id) REFERENCES Courses(id)
```
This is the textbook junction: the `(Students_id, Courses_id)` pair is
unique, so a student cannot be linked to the same course twice. It is
the most pedagogically correct model and needs no surrogate key.
*Rejected:* a surrogate `serial` PK + `UNIQUE` over the FK pair (adds a
key the learner did not ask for); two FK columns with no PK (allows
duplicate links — wrong lesson).
### D2 — Referential actions: `CASCADE` on delete and update (fork, user-chosen)
Both generated FKs default to `ON DELETE CASCADE ON UPDATE CASCADE`. A
junction row is meaningless without both ends, so deleting a parent
(a Student or a Course) removes its link rows automatically — the
natural junction semantics, and a clean teaching demonstration of
cascade. There is no syntax in this command to override the actions;
the learner who wants different actions builds the junction by hand
(or drops + re-adds a relationship). This keeps the convenience
command convenient.
### D3 — Naming: auto-name `<T1>_<T2>`, optional `as <name>` (fork, user-chosen)
The junction table is auto-named `{T1}_{T2}` (e.g. `Students_Courses`).
An optional `as <name>` clause overrides it — consistent with
`add 1:n relationship [as <name>]` and saving a follow-up
`rename table`. The two generated relationships are auto-named by the
existing relationship-name resolver (e.g.
`Students_id_to_Students_Courses_Students_id`), exactly as `create
table` with inline FKs already names them.
### D4 — FK column naming: `{parent_table}_{pk_column}`
Each FK column is named `{parent_table}_{pk_column}` — one per PK
column of each parent. This disambiguates the common case where both
parents share a PK column name (both `id``Students_id`,
`Courses_id`), and generalises to compound parent PKs: a parent
`Sections(course_id, term)` contributes `Sections_course_id` and
`Sections_term`. Column types come from each parent PK column's
`Type::fk_target_type()` (ADR-0011): `serial → int`, `shortid → text`,
others identity — so the junction columns are plain storable types,
never auto-generating.
### D5 — Mode availability: both simple and advanced
`create m:n relationship` is a `CommandCategory::Simple` DSL command,
reachable in **both** input modes — the same posture as the sibling
relationship commands (`drop relationship … Mode::Advanced` is a tested
path today). There is no SQL spelling for it; an advanced-mode user who
prefers raw SQL still has `CREATE TABLE` + `FOREIGN KEY`. The command
is purely additive teaching sugar, so making it available everywhere is
harmless and consistent.
### D6 — Implementation: one `do_create_table` call, not a batch
The junction table and both relationships are created by **building a
single `Command`/worker request that `do_create_table` already
handles**: `do_create_table` accepts `columns`, `primary_key`, and
`foreign_keys: Vec<SqlForeignKey>`, and already inserts relationship
metadata for each FK. So the executor:
1. Canonicalises `T1`, `T2` (`require_canonical_table`) and reads each
parent's PK (`read_schema().primary_key`). **D7 — parent-PK guard:**
if either parent's `primary_key` is empty, error with a friendly
"`<table>` has no primary key, so it cannot anchor an m:n
relationship" *before* building any FK. This is a real case — a
table created via advanced-mode SQL `create table t (a int)` has no
PK (`sql_create_table.rs` asserts `pk.is_empty()` for that form) —
not a theoretical one, so the guard is a correctness requirement,
not defensive padding.
2. Builds the junction `ColumnSpec`s (one per parent PK column, typed
via `fk_target_type`), the compound `primary_key` list, and two
`SqlForeignKey` values (`on_delete = on_update = Cascade`).
3. Calls the create-table path, which creates the table + both FKs +
all metadata in **one transaction** — naturally one undo step, no
`BeginBatch`/`EndBatch` bracketing needed.
This reuses the most-tested machinery and inherits its persistence,
metadata, and FK-validation behaviour for free.
A new typed command variant `Command::CreateM2nRelationship { t1, t2,
name }` carries the parsed form; the runtime/executor expands it to the
junction definition. (We do **not** lower it to `Command::CreateTable`
at parse time — keeping a distinct command preserves command identity
per the X5 "unique commands for every unique case" principle, and lets
the teaching echo speak in m:n terms.)
## Grammar, AST, and cross-cutting wiring
A new command is not done when it parses — it must light up every
surface a learner touches. Enumerated here so none is missed
(verification in **Testing**).
- **Grammar node** `CREATE_M2N` (`ddl.rs`): a separate `CommandNode`
with `entry: create`, shape `m:n relationship [as <name>] from <T1>
to <T2>`, registered in `REGISTRY` as `CommandCategory::Simple`. A
separate node (not a branch inside `CREATE`) keeps the tested
create-table builder untouched; the walker already dispatches
multiple nodes per entry word. The `m:n` opener mirrors `1:n`
(`Word("m")`, `Punct(':')`, `Word("n")`).
- **AST builder** `build_create_m2n``Command::CreateM2nRelationship`.
- **Command + worker plumbing:** `Command::CreateM2nRelationship`
variant; `Request::CreateM2nRelationship`; runtime
`execute_command_typed` arm; `Database::create_m2n_relationship`
public API; executor `do_create_m2n_relationship` (per D6).
- **Completion:** add `("m", "m:n")` to `COMPOSITE_CANDIDATES`
(`completion.rs`) so Tab on `create m` offers `m:n` as one fluent
piece (exactly as `("1", "1:n")` does for `add`). Identifier slots
for `<T1>`/`<T2>` inherit table-name completion from the walker's
`IdentSource::Tables` automatically.
- **Hints:** set `HintMode`s on the `CREATE_M2N` nodes so the ambient
hint panel guides `from` / table / `to` / table / optional `as`,
matching the `add 1:n relationship` hinting.
- **Highlighting:** automatic — `walker/highlight.rs` is grammar-driven
with no per-command special-casing; verify the line highlights.
- **Help:** `help_id: Some("ddl.create_m2n")` → the command appears in
`help` automatically (REGISTRY iteration) and under `help create`;
add the `help.ddl.create_m2n` catalog string.
- **Usage / parse errors:** `usage_ids: &["parse.usage.create_m2n"]`
→ a malformed `create m:n …` shows the form in the usage block; add
the `parse.usage.create_m2n` catalog string. Add the near-miss cases
to the `parse_error_pedagogy` matrix (ADR-0042) for the `create`
entry word.
- **Teaching echo** (`echo.rs` + `build_schema_echo`): a
`CreateM2nRelationship` arm that echoes the generated junction —
the `create table` it built plus the two `FOREIGN KEY` lines — so the
learner sees the pattern the convenience expanded to.
- **Structure render:** the executor returns the junction's
`TableDescription`; the ADR-0044 render path already draws its
relationships as diagrams on the create echo? No — incidental
`create table` echoes keep prose (ADR-0044 reach); the m:n echo shows
the junction structure with its outbound FKs in the standard prose
form. (A future enhancement could draw both relationships as
diagrams; out of scope here.)
## Genuine forks (escalated, all resolved 2026-06-10)
1. **Junction PK** — compound-over-FKs (chosen) vs surrogate serial +
UNIQUE vs no PK. → D1.
2. **Referential actions**`CASCADE` (chosen) vs `NO ACTION` vs
`RESTRICT`. → D2.
3. **Naming** — auto-name + optional `as` (chosen) vs auto-name only.
→ D3.
4. **Mode** — both (chosen by default, unobjected) vs simple-only. → D5.
## Testing
Integration (`tests/it/`), test-first:
- **Functional:** `create m:n relationship from A to B` creates table
`A_B` with the two FK columns, compound PK over them, and two
enforced FKs; inserting a junction row with a non-existent parent is
refused; the `(fk1, fk2)` pair is unique (duplicate link refused).
- **`as <name>`** overrides the junction name.
- **Compound parent PK:** a parent with a 2-column PK contributes two
FK columns; the junction PK spans all of them; FKs enforce per pair.
- **Cascade:** deleting a parent row removes its junction rows.
- **Undo:** one `create m:n` is exactly one undo step (table + both
relationships gone after `undo`).
- **Persistence round-trip:** the junction + both relationships survive
a save → rebuild-from-text.
- **Errors:** missing parent table; parent without a PK; junction-name
collision with an existing table; (self-m:n → OOS error, below).
Cross-cutting (the surfaces a new command must light up):
- **Completion:** Tab after `create ` surfaces `m:n relationship`;
table-name completion fires at the `<T1>`/`<T2>` slots.
- **Help:** `help` lists the command; `help create` includes the m:n
form; the catalog string renders.
- **Usage / parse pedagogy:** a bare/half `create m:n` shows the usage
block; near-miss matrix entries added (`parse_error_pedagogy`).
- **Hints + highlighting:** ambient hint progression through the form;
the line highlights (snapshot or assertion as the sibling commands
use).
All tiers green, zero skips; clippy clean (nursery).
## Out of scope
- **Self-referential m:n** (`from T to T`) — **refused outright**
(user-confirmed, "full stop"): the two FK column sets would collide
on `{T}_{pkcol}`, and directional disambiguation (`from_*`/`to_*`)
is more complexity than this beginner-facing convenience warrants.
The executor detects `t1 == t2` (on the canonical names) and errors
with a friendly pointer ("an m:n relationship needs two different
tables — to link a table to itself, add the junction by hand").
Not a deferred follow-up; a deliberate non-goal.
- **Per-relationship action overrides** in the command syntax (D2 fixes
`CASCADE`); use a hand-built junction for other actions.
- **Extra junction columns** (payload attributes on the link, e.g. an
enrolment date) — add them afterward with `add column`.
- **m:n visualization as diagrams** on the create echo (ADR-0044 reach
keeps incidental create echoes in prose).
- **Renaming the auto-generated *relationships*** (only the table is
`as`-nameable); drop + re-add covers it.
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- [ADR-0042 — H1a parse-error pedagogy in the grammar-tree era](0042-h1a-parse-error-pedagogy-grammar-tree.md) — **Accepted 2026-06-03.** Continues **H1a** from ADR-0021 against the ADR-0024 grammar tree (ADR-0021's chumsky mechanism is dead). Records the **baseline already shipped** — per-command `usage:` block (38 `parse.usage.*` templates), available-commands fallback, structural "after `…`, expected …" wording, source-derived ident slot labels ("table name"/"column name"), curated `parse.custom.*` near-miss messages, and the ADR-0027/0033/0036 schema-aware `[ERR]` diagnostics — so H1a is *substantially* delivered at the intent level. Defines the remaining work as **(1)** a verified per-command **near-miss matrix** (`tests/typing_surface/` + `tests/it/parse_error_pedagogy.rs`) as the definition of done, test-first; **(2)** **friendlier literal expectation labels** — optional prose glosses on `Word`/`Punct`/`Flag` positions that *add* role context while always keeping the exact literal visible (e.g. "a filter clause: `where …` or `--all-rows`"); **(3)** **advanced-mode SQL** near-miss parity (RETURNING scope, CTE-arity positioning, `CROSS JOIN … ON`, INSERT…SELECT count) — **in scope**, kept distinct from ADR-0019 §OOS-2 which covers advanced-SQL *engine*-error sanitisation, a different layer. Catalog/anchor-phrase discipline (ADR-0019) preserved; no public API change. OOS: I3/I4, spell-correction, multi-error reporting, verbosity-gating the usage block
- [ADR-0043 — Compound-primary-key foreign-key references (T3)](0043-compound-pk-foreign-key-references.md) — **Accepted + implemented 2026-06-09** (all four forks confirmed at the recommended option: full-PK matching, house-style uniform lists, parenthesized DSL syntax, bare-SQL-FK auto-expansion). Closes `requirements.md` **T3** `[x]` — the relationship model went list-based across six layers (single-column preserved, no migration), DSL `from P.(a,b) to C.(x,y)` + SQL `FOREIGN KEY (a,b) REFERENCES P(x,y)` parse/execute/enforce, 12 tests in `tests/it/compound_fk.rs`. Closes the open leg of `requirements.md` **T3**: a foreign key that *references* a parent's compound primary key. A 2026-06-09 audit found single-column FK woven through ~1520 sites (metadata table, `RelationshipSchema`, `project.yaml` `RawEndpoint`, both grammar surfaces, executor FK-DDL emission, per-column type-compat, display) — earns an ADR, not an inline build. **Decision:** reference the parent's **full** compound PK, matched **positionally** to an equal-length child column list, per-pair `fk_target_type` compat (ADR-0011, element-wise); DSL `from <P>.(a, b) to <C>.(x, y)` (single form unchanged), SQL `FOREIGN KEY (x, y) REFERENCES P(a, b)` (extend the existing one-cap lists; bare table-level FK auto-expands to the parent PK when arities match). **Storage — no migration (back-compat not required, user-confirmed 2026-06-09; no installed base):** the relationship endpoint joins the list convention `project.yaml` *already* uses — `columns: [a, b]` like `primary_key: [id]` and index `columns: [...]` (the endpoint was the lone scalar `column:` holdout); the metadata `TEXT` columns are unchanged and store the list **comma-joined** (`a,b`; the bare name for single — safe because identifiers are `[A-Za-z0-9_]+`). No F3 migrator, no version bump; accepted trade-off is that a pre-change `project.yaml` with relationships won't load (clean cutover). In-memory model goes list-based (`Vec<String>`) through all six layers; the enforced FK is the rebuilt child-table DDL (`FOREIGN KEY (a,b) REFERENCES P(x,y)`), one relationship = one undo step (ADR-0013). Genuine forks escalated: matching policy (full-PK vs subset), storage (house-style uniform lists vs normalized table), DSL syntax (parenthesized vs repeated-dotted), bare-SQL-FK auto-expansion. OOS: subset/non-PK (UNIQUE-targeted) FK references; any single-column behaviour change
- [ADR-0044 — Relationship visualization (two-table connector diagrams)](0044-relationship-visualization.md) — **Accepted 2026-06-09; implemented 2026-06-10** (closes `requirements.md` V1; second `/runda` pass over the implementation; §3 last-resort helper line considered and rejected). Resolves **ADR-0016 OOS-1** and closes the open half of `requirements.md` **V1** ("a selected relationship as two tables joined by a line"). Renders a relationship as **Style A** (two structure boxes + connector). **Reach = "relationship-relevant"** (user-chosen over global / show-only): diagrams on the surfaces where the relationship is the *subject*`show relationship <name>` (one full diagram), `show table <T>` (T's structure box then a **Relationships** section of **stacked compact** per-relationship diagrams — chosen over a focal-centred subgraph: no crossing lines, scales via scroll, two-boxes-wide fits any terminal), and relationship DDL echoes (`add`/`drop`/`modify relationship`); incidental DDL echoes (`add column`, `drop index`, `change column`, plain `create table`) keep the terse prose, via a `Diagram`|`Prose` render mode on `render_structure`. Reading convention **child(FK)-left / parent-right, arrow →, `n`…`1` cardinality**, applied uniformly; every box gets a **bold title row + rule** so the name can't read as a column. **Compound FKs** (ADR-0043) route one connector per positional pair + an explicit pairing line. **Width-aware** (first in the codebase) but **App-side**: `render_structure`/diagram rendering runs in `app.rs` (the worker only returns `TableDescription`s), a new `App::last_output_width` (set from `ui.rs`) drives side-by-side vs a **vertical-stack** fallback + last-resort "run `show relationship`" pointer; rendered once at command time, **no live reflow** (V4). `show relationship`'s worker path (`do_show_one`, prose-only) is restructured to return both endpoint `TableDescription`s. Styling reuses **ADR-0028** App-side styled runs (new classes: table-name/key/connector/cardinality/action) — no worker→UI contract change. **Partially supersedes ADR-0016 §5** (prose block replaced on relationship-subject surfaces, retained on incidental ones); extends §4 (layout width-awareness, still no cell truncation) and §6 (per-span theming). Tests: insta snapshots (single, compound, vertical fallback, helper line, self-referential, multi-rel `show table`) + width-threshold/routing unit tests + Tier-3 wiring; enumerated prose-fallout updates (`output_render.rs:121/135/793`, the relationships snapshot, `walking_skeleton.rs:477/530`). A `/runda` DA pass corrected three inverted-architecture claims (App-side rendering, untracked width, prose-in-worker show-relationship) before acceptance. OOS: user-configurable display setting (OOS-7), live reflow (V4), whole-DB ER export (V3), m:n (C4), ASCII fallback (ADR-0016 OOS-5)
- [ADR-0045 — `create m:n relationship` convenience command (C4)](0045-mn-convenience.md) — **Accepted 2026-06-10** (closes `requirements.md` **C4**; all forks user-confirmed + a `/runda` DA pass that verified the `do_create_table` reuse against code and corrected the "no PK-less tables" assumption — advanced SQL `create table t (a int)` has none, so a parent-PK guard is retained). `create m:n relationship from <T1> to <T2> [as <name>]` generates a junction table with one FK column per parent PK column, a **compound PK over all the FK columns** (the textbook junction — the pair is unique, no duplicate links), and **two 1:n relationships**, all in **one transaction = one undo step** (built by reusing `do_create_table`, which already takes `foreign_keys` + writes relationship metadata — no batch bracketing). Forks all user-chosen: junction PK = compound-over-FKs (vs surrogate serial / no PK); referential actions = **`CASCADE`** on delete+update (vs NO ACTION / RESTRICT); naming = auto `{T1}_{T2}` + optional `as` (vs auto-only); available in **both modes** (Simple-category DSL, like the sibling relationship commands). FK columns named `{parent_table}_{pk_column}` (disambiguates shared `id`; generalises to compound parents via ADR-0043), typed via `fk_target_type` (ADR-0011). A distinct `Command::CreateM2nRelationship` (not lowered to `CreateTable`) preserves command identity (X5) and lets the teaching echo speak in m:n terms. Cross-cutting wiring enumerated: separate `CREATE_M2N` `CommandNode` (own `help_id`/`usage_ids`), `("m","m:n")` completion composite, `HintMode`s, grammar-driven highlighting, `help`/`help create`, `parse_error_pedagogy` near-miss matrix, teaching echo. OOS: **self-referential m:n** (`from T to T`) refused outright (user-confirmed "full stop" — directional column-naming is more than this beginner convenience warrants); per-relationship action overrides; extra junction payload columns; m:n diagram echo; renaming the auto-generated relationships