fix: 🐛 handle missing host_ids array in host sets

addons/api/addon/models/host-set.js

@@ -9,7 +9,10 @@ export default class HostSetModel extends GeneratedHostSetModel {
    * Grants is read-only _most_ under normal circumstances.  But grants can
    * be persisted via a dedicated call to `saveGrants()`.
    */
-  @fragmentArray('fragment-string', {readOnly: true}) host_ids;
+  @fragmentArray('fragment-string', {
+    readOnly: true,
+    emptyArrayIfMissing: true
+  }) host_ids;
 
   // =methods
 

addons/api/tests/unit/serializers/host-set-test.js

@@ -74,4 +74,36 @@ module('Unit | Serializer | host set', function(hooks) {
       },
     });
   });
+
+  test('it normalizes missing host_ids to empty array', function (assert) {
+    assert.expect(2);
+    const store = this.owner.lookup('service:store');
+    const serializer = store.serializerFor('host-set');
+    const hostSet = store.createRecord('host-set').constructor;
+    const payload = {
+      id: '1',
+      name: 'Host Set 1',
+      scope: { id: 'o_123' }
+    };
+    const normalized = serializer.normalizeSingleResponse(
+      store,
+      hostSet,
+      payload
+    );
+    assert.notOk(payload.bogusArrayField);
+    assert.deepEqual(normalized, {
+      included: [],
+      data: {
+        id: '1',
+        type: 'host-set',
+        attributes: {
+          name: 'Host Set 1',
+          scope: { scope_id: 'o_123' },
+          host_ids: []
+        },
+        relationships: {},
+      },
+    });
+  });
+
 });

ui/core/mirage/serializers/host-set.js

@@ -12,8 +12,10 @@ export default ApplicationSerializer.extend({
 
   _hashForModel(model) {
     const json = ApplicationSerializer.prototype._hashForModel.apply(this, arguments);
-    json.host_ids = model.hostIds;
     json.host_catalog_id = model.hostCatalogId;
+    if (model.hostIds?.length) {
+      json.host_ids = model.hostIds;
+    }
     return json;
   }