diff --git a/migrate/spaces.js b/migrate/spaces.js
new file mode 100644
index 0000000..e2830e3
--- /dev/null
+++ b/migrate/spaces.js
@@ -0,0 +1,12 @@
+import { pool } from '../lib/db/pool.js';
+import * as spaces from '../lib/db/repos/spaces.js';
+
+const SYS = { kind: 'system', id: null };
+
+// Returns the id of the space with `slug`, creating it if absent. Idempotent.
+export async function ensureSpace(slug, name) {
+  const { rows: [r] } = await pool.query(`SELECT id FROM spaces WHERE slug=$1`, [slug]);
+  if (r) return r.id;
+  const created = await spaces.create({ slug, name }, SYS);
+  return created.id;
+}
diff --git a/tests/migrate/spaces.test.js b/tests/migrate/spaces.test.js
new file mode 100644
index 0000000..5609e64
--- /dev/null
+++ b/tests/migrate/spaces.test.js
@@ -0,0 +1,14 @@
+import { describe, it, expect, beforeAll } from 'vitest';
+import { resetDb } from '../helpers/db.js';
+import { migrateUp } from '../../lib/db/migrate.js';
+import { ensureSpace } from '../../migrate/spaces.js';
+
+beforeAll(async () => { await resetDb(); await migrateUp(); });
+
+describe('ensureSpace', () => {
+  it('creates a space once and reuses it by slug', async () => {
+    const a = await ensureSpace('void1', 'Void 1');
+    const b = await ensureSpace('void1', 'Void 1');
+    expect(a).toBe(b);
+  });
+});