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# PhoenixKit.Migrations.UUIDFKColumns (phoenix_kit v1.7.87)

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Adds UUID FK columns alongside integer FKs across PhoenixKit tables.

Called from V56 migration. This helper module keeps V56 manageable by extracting the UUID FK column creation logic (~80 columns across ~40 tables).

## How It Works

For each FK column, three operations:

1. `ALTER TABLE ... ADD COLUMN IF NOT EXISTS {uuid_fk} UUID`
2. Backfill via JOIN: `UPDATE t SET {uuid_fk} = s.uuid FROM source s WHERE s.id = t.{int_fk}`
3. `CREATE INDEX IF NOT EXISTS ... ON table({uuid_fk})`

Large tables use batched backfills (PL/pgSQL DO block, 10k rows/batch) to avoid long-running transactions.

After columns are created and backfilled, `add_constraints/1` adds:

- NOT NULL constraints where the integer FK is NOT NULL
- FK constraints where the integer FK has an explicit DB-level FK constraint

## Safety

- All operations wrapped in `table_exists?` + `column_exists?` checks
- FK constraint creation uses `pg_constraint` existence check (idempotent)
- NOT NULL uses `ALTER COLUMN SET NOT NULL` (idempotent in PostgreSQL)
- Idempotent — safe to run multiple times

## Summary

### Functions

add_constraints(opts)

Adds NOT NULL constraints and FK constraints to UUID FK columns.

down(opts)
drop_constraints(opts)

Drops FK constraints and NOT NULL from UUID FK columns.

up(opts)

## Functions

# add_constraints(opts)

Adds NOT NULL constraints and FK constraints to UUID FK columns.

Must be called AFTER `up/1` so that columns exist and are backfilled.

Order: NOT NULL first (data already backfilled), then ensure unique indexes on all FK-target tables, then FK constraints.

# down(opts)

# drop_constraints(opts)

Drops FK constraints and NOT NULL from UUID FK columns.

Must be called BEFORE `down/1` so constraints are removed before columns are dropped.

Order: FK constraints first (unblocks column removal), then NOT NULL.

# up(opts)