rdbms-playground/docs/adr/0043-compound-pk-foreign-key-references.md

# ADR-0043: Compound-primary-key foreign-key references (T3)

## Status

**Accepted** — 2026-06-09. All four genuine forks confirmed by the
user at the recommended option: **F-A** full PK in order, **F-B**
house-style uniform column lists (no migration; back-compat not
required), **F-C** parenthesized DSL lists, **F-D** bare table-level
SQL FK auto-expands to the parent's full PK. Closes the one open
leg of
`requirements.md` **T3** ("compound primary keys handled
end-to-end (DSL, storage, display, **FK reference**)"): a foreign
key that *references* a compound (multi-column) primary key.

Cross-references **ADR-0011** (FK column type compatibility —
`Type::fk_target_type`), **ADR-0013** (relationships, naming, the
rebuild-table strategy, and the `__rdbms_playground_relationships`
metadata table), **ADR-0035 §4b** (the SQL `FOREIGN KEY` surface),
**ADR-0004 / ADR-0015** (`project.yaml` as the authoritative
format; `playground.db` is a derived artifact), and **ADR-0009**
(DSL surface conventions).

## Context

Compound PRIMARY KEYs are declared, stored, and displayed today
(`create table T with pk a(int), b(int)` → `primary_key:
Vec<String>`). The missing leg is the *reference*: a child table
whose foreign key points at a parent's compound PK. A 2026-06-09
codebase audit found single-column FK is a pervasive assumption —
~15–20 sites across 6+ files:

- **Metadata** — `__rdbms_playground_relationships` stores scalar
  `parent_column TEXT` / `child_column TEXT`
  (`PRIMARY KEY (child_table, child_column)`).
- **Persistence** — `RelationshipSchema { parent_column: String,
  child_column: String }`; `project.yaml` `RawEndpoint { table,
  column }`.
- **Grammar** — `add 1:n relationship … from <P>.<col> to
  <C>.<col>` (one ident per side); SQL `FOREIGN KEY (<col>)
  REFERENCES <P>(<col>)` (parens that hold exactly one ident).
- **AST** — `Command::AddRelationship { parent_column: String,
  child_column: String }`; `SqlForeignKey { child_column: String,
  parent_column: Option<String> }`.
- **Executor** — `schema_to_ddl` emits a single-column
  `FOREIGN KEY (c) REFERENCES P(p)`; `check_fk_type_compat`
  compares one parent type to one child type; bare
  `REFERENCES <P>` on a compound-PK parent is refused as
  ambiguous (`resolve_create_table_fks`,
  `do_alter_add_foreign_key`).
- **Display** — `RelationshipEnd { other_column: String,
  local_column: String }`.

This is not a sweep-sized change, which is why it earns an ADR
rather than an inline build. The decisions below also turn the
audit's worst-case framing (a metadata-schema + yaml-format
migration via the F3 framework) into a **no-migration** change.

### Why no migration is needed

**Decision input (user, 2026-06-09): back-compatibility with
existing saved projects is not required.** The project is
pre-release; there is no installed base of `project.yaml` /
`playground.db` files to preserve. This removes the only force
that would have demanded an F3 migrator or a version bump, and —
more importantly — it lets the representation be chosen for
*cleanliness and consistency* rather than for byte-identical
back-compat. The consequence is explicit and accepted: a
`project.yaml` written before this change that contains
relationships will not load under the new format.

Freed of back-compat, the storage follows the convention the file
**already uses** for ordered column lists rather than inventing a
new one:

- `project.yaml` already writes `primary_key: [id]` (a compound PK
  is `primary_key: [a, b]`) and index `columns: [a, b]`
  (`RawIndex { columns: Vec<String> }`). The relationship endpoint
  is the lone multi-column-capable slot still using a scalar
  `column:`. It joins the house style (D5).
- The metadata columns are `TEXT`; SQLite has no array type, so a
  list lives in a text cell as JSON regardless. That JSON is now a
  *uniform* encoding (a one-element array for the single-column
  case), not a "bare-name-or-JSON, sniff which" fallback — the
  fallback only existed to keep old rows identical, which is no
  longer a goal.

So this is not a clever back-compat dodge; it is "use the existing
list convention, uniformly." No version bump, no F3 migrator.

## Decision

Support a foreign key that references a parent's **full** compound
primary key, matched **positionally** to an equal-length child
column list, with per-pair type compatibility — across both the
DSL and SQL surfaces — using format-flexible storage that needs no
migration.

### D1 — Matching policy: the full PK, in order

A compound-PK FK references **all** columns of the parent's
primary key, in PK declaration order, matched 1:1 to the child's
column list (same length). Referencing a *subset* of a compound PK
is **out of scope**: SQL/SQLite require FK parent columns to form a
PK or UNIQUE key, and a strict subset of a compound PK is not
itself unique unless separately constrained. Teaching-clean rule:
*a foreign key to a compound key names every column of that key.*

A length mismatch (child supplies N columns, parent PK has M ≠ N)
is a friendly error naming both counts.

### D2 — Type compatibility: per pair, positional

Each child column's type must satisfy
`parent_pk_col.fk_target_type() == child_col` for the
corresponding pair (the existing ADR-0011 rule, applied
element-wise in order). `check_fk_type_compat` generalises to walk
the pairs and report the **first** offending pair with the same
wording it uses today.

### D3 — DSL syntax: parenthesized column lists

`add 1:n relationship [as <name>]
   from <P>.(<a>, <b>) to <C>.(<x>, <y>)
   [on delete …] [on update …] [--create-fk]`

The single-column form `from <P>.<col> to <C>.<col>` is unchanged
(no parens) — back-compatible and the common case. The
parenthesized list is the multi-column form. Both sides must use
the same arity (enforced as a D1 length check). Parentheses mirror
the existing compound-PK *declaration* syntax (`with pk a(int),
b(int)` uses parens around the per-column type; the FK list uses
parens around the column names) and the SQL `FOREIGN KEY (…)`
shape, so the surface stays internally consistent.

### D4 — SQL syntax: extend the existing lists

`FOREIGN KEY (<x>, <y>) REFERENCES <P> (<a>, <b>)` — the grammar's
child and parent column slots become comma-separated **lists**
(today capped at one). Inline `<col> <type> REFERENCES <P>(<a>,
<b>)` stays single-child-column (one inline column can't match a
2-column key) — a compound FK uses the table-level form. Bare
table-level `FOREIGN KEY (x, y) REFERENCES <P>` (no parent
columns) **auto-expands to the parent's full PK** when the arities
match; bare inline `<col> REFERENCES <P>` on a compound-PK parent
keeps today's friendly refusal, with the message pointing at the
table-level multi-column form.

### D5 — Storage: uniform column lists, matching the house style

Both stores hold an **ordered column list**, uniformly (a
one-element list for the single-column case), following the
convention `project.yaml` already uses for `primary_key` and index
`columns`.

- **`project.yaml`**: `RawEndpoint` becomes `{ table, columns:
  Vec<String> }` and writes `columns: [a, b]` (single-column →
  `columns: [id]`), exactly parallel to `primary_key: [id]`. No
  scalar `column:` form, no dual-shape reader.
- **Metadata** (`__rdbms_playground_relationships`): no
  `CREATE TABLE` change (the `TEXT` columns and
  `PRIMARY KEY (child_table, child_column)` are untouched).
  `parent_column` / `child_column` store the list as a JSON array
  string — uniformly, including `["id"]` for a single column
  (SQLite has no array type, so a text cell is where a list lives).
  The actual enforced FK lives on the rebuilt child table's DDL
  (`FOREIGN KEY (a, b) REFERENCES P(x, y)`), emitted by
  `schema_to_ddl`, exactly as the single-column FK is today via the
  rebuild-table primitive (ADR-0013) — one relationship, one undo
  step.

### D6 — In-memory model: `Vec<String>` column lists

`Command::AddRelationship`, `SqlForeignKey`, `RelationshipSchema`,
the internal `ReadForeignKey`, and `RelationshipEnd` (display) all
carry `parent_columns: Vec<String>` / `child_columns: Vec<String>`
(or `Option<Vec<String>>` for the bare-SQL parent case). A
one-element vec is the single-column case; nothing about the
single-column UX changes.

## Genuine forks (escalated for sign-off)

These are decisions, not facts. Recommendations are marked; the
user confirms before this ADR moves to Accepted.

- **F-A — matching policy.** Full PK only (D1, *recommended*) vs.
  allow a subset (needs a separate UNIQUE key; larger, less
  teaching-clean).
- **F-B — storage encoding.** Uniform column lists in the existing
  house style — `columns: [a, b]` in yaml (like `primary_key`),
  JSON-array in the unchanged metadata `TEXT` columns; no
  back-compat, no migration (D5, *recommended*) vs. a normalized
  relationship-columns child table (more "correct" but a schema
  change with joins on read, no learner-visible payoff). Premise:
  no existing projects to preserve (confirmed).
- **F-C — DSL multi-column syntax.** `from P.(a, b) to C.(x, y)`
  parenthesized (D3, *recommended*) vs. a repeated-dotted form
  (`from P.a, P.b to C.x, C.y`, more ambiguous to parse and read).
- **F-D — bare table-level SQL FK auto-expansion.** Auto-expand
  `FOREIGN KEY (x,y) REFERENCES P` to P's full PK when arities
  match (D4, *recommended*) vs. always require explicit parent
  columns.

## Implementation sketch (change sites)

Grouped; each lands behind tests. No migration step.

1. **AST** — `AddRelationship` + `SqlForeignKey` column fields →
   `Vec<String>` / `Option<Vec<String>>` (`command.rs`).
2. **Grammar** — DSL endpoint column slot → optional
   parenthesized list (`ddl.rs`); SQL child/parent column slots →
   comma lists (`sql_create_table.rs`). Builders collect lists.
3. **Metadata** — `insert_relationship_metadata` /
   `read_all_relationships` encode/decode bare-or-JSON
   (`db.rs`); no `CREATE TABLE` change.
4. **Persistence** — `RelationshipSchema` → `Vec<String>`;
   `RawEndpoint` becomes `{ table, columns: Vec<String> }`, written
   `columns: [a, b]` like `primary_key`
   (`persistence/mod.rs`, `persistence/yaml.rs`).
5. **Executor** — `do_add_relationship` /
   `resolve_create_table_fks` / `do_alter_add_foreign_key` walk
   column lists; `schema_to_ddl` emits multi-column `FOREIGN KEY
   (…) REFERENCES P(…)`; `check_fk_type_compat` loops pairs;
   bare-reference paths auto-expand to the full PK (D4) or refuse
   with the improved message (`db.rs`).
6. **Display** — `RelationshipEnd` → column lists; `describe` /
   echo render `(a, b) → (x, y)` (`db.rs`, `echo.rs`).
7. **Tests** — parse (DSL + SQL, single still works, multi parses,
   arity mismatch errors); worker round-trip (declare a 2-col FK,
   rebuild, FK enforced, type-mismatch refused); persistence
   round-trip (yaml `columns:` reads + writes; a legacy
   single-column yaml still loads); display.

## Consequences

- T3 closes; a learner can model a real composite-key relationship
  end to end.
- No migration, and the on-disk representation gets *more*
  consistent: the relationship endpoint joins the `primary_key:
  [...]` / index `columns: [...]` list convention. The in-app
  single-column UX is untouched (one-element vecs).
- Accepted trade-off (user, 2026-06-09): a `project.yaml` written
  before this change that contains relationships will not load
  under the new format. There is no installed base to preserve, so
  this is a clean cutover, not data loss.
- The relationship model becomes list-based throughout, which is
  the natural foundation if subset/UNIQUE-targeted FKs are ever
  wanted (explicitly OOS here).
- A modest, broad refactor (the `Vec` field change ripples through
  the 6 layers) — methodical, not deep; locked by tests at each
  layer.

## Out of scope

- Subset/non-PK FK targets (referencing a UNIQUE key that isn't
  the PK) — possible later on this list-based foundation.
- Any change to single-column behaviour, the rebuild-table
  primitive, or the undo model (one relationship = one undo step
  stands).
- A `project.yaml` version bump or F3 migrator (not needed —
  no installed base to migrate; clean cutover per D5).