fix: JSONAPISerializer should not reify empty records

packages/legacy-compat/src/legacy-network-handler/snapshot.ts

@@ -148,8 +148,13 @@ export default class Snapshot implements Snapshot {
    @type {Model}
    @public
    */
-  get record(): RecordInstance {
-    return this._store._instanceCache.getRecord(this.identifier);
+  get record(): RecordInstance | null {
+    const record = this._store.peekRecord(this.identifier);
+    assert(
+      `Record ${this.identifier.type} ${this.identifier.id} (${this.identifier.lid}) is not yet loaded and thus cannot be accessed from the Snapshot during serialization`,
+      record !== null
+    );
+    return record;
   }
 
   get _attributes(): Record<string, unknown> {

tests/main/tests/integration/relationships/has-many-test.js

@@ -191,6 +191,64 @@ module('integration/relationships/has_many - Has-Many Relationships', function (
     }, /Assertion Failed: Encountered a relationship identifier without a type for the hasMany relationship 'comments' on <post:2>, expected an identifier with type 'comment' but found/);
   });
 
+  test('A record with an async hasMany relationship can safely be saved and later access the relationship', async function (assert) {
+    const store = this.owner.lookup('service:store');
+    this.owner.register(
+      'adapter:application',
+      class extends JSONAPIAdapter {
+        updateRecord(store, schema, snapshot) {
+          assert.step('updateRecord');
+          const data = store.serializerFor(schema.modelName).serialize(snapshot, { includeId: true });
+
+          return Promise.resolve(data);
+        }
+        findRecord(store, schema, id, snapshot) {
+          assert.step('findRecord');
+          return Promise.resolve({
+            data: { id, type: 'chapter', attributes: { title: `Chapter ${id}` } },
+          });
+        }
+      }
+    );
+    this.owner.register(
+      'serializer:application',
+      class extends JSONAPISerializer {
+        serialize(snapshot, options) {
+          assert.step('serialize');
+          return super.serialize(snapshot, options);
+        }
+      }
+    );
+    const book = store.push({
+      data: {
+        type: 'book',
+        id: '1',
+        relationships: {
+          chapters: {
+            data: [
+              { type: 'chapter', id: '1' },
+              { type: 'chapter', id: '2' },
+            ],
+          },
+        },
+      },
+    });
+    book.title = 'The Book of Foo';
+
+    await book.save();
+
+    assert.verifySteps(['updateRecord', 'serialize']);
+
+    const chapters = await book.chapters;
+
+    assert.verifySteps(['findRecord', 'findRecord']);
+    assert.strictEqual(chapters.length, 2);
+    assert.deepEqual(
+      chapters.map((v) => v.id),
+      ['1', '2']
+    );
+  });
+
   test("When a hasMany relationship is accessed, the adapter's findMany method should not be called if all the records in the relationship are already loaded", async function (assert) {
     assert.expect(1);