fix(document): correct context for default functions in subdocuments …

…with init

Fix #12328

lib/schema/SubdocumentPath.js

@@ -170,7 +170,7 @@ SubdocumentPath.prototype.cast = function(val, doc, init, priorVal, options) {
   }, null);
   options = Object.assign({}, options, { priorDoc: priorVal });
   if (init) {
-    subdoc = new Constructor(void 0, selected, doc);
+    subdoc = new Constructor(void 0, selected, doc, false, { defaults: false });
     subdoc.$init(val);
   } else {
     if (Object.keys(val).length === 0) {

test/document.test.js

@@ -11833,6 +11833,28 @@ describe('document', function() {
 
     assert.ok(doc.nestedPath1.mapOfSchema);
   });
+
+  it('correct context for default functions in subdocuments with init (gh-12328)', async function() {
+    const subSchema = new mongoose.Schema({
+      propertyA: { type: String },
+      propertyB: { type: String, default: function() {
+        return this.propertyA;
+      } }
+    });
+
+    const testSchema = new mongoose.Schema(
+      {
+        name: String,
+        sub: { type: subSchema, default: () => ({}) }
+      }
+    );
+
+    const Test = db.model('Test', testSchema);
+
+    await Test.collection.insertOne({ name: 'test', sub: { propertyA: 'foo' } });
+    const doc = await Test.findOne({ name: 'test' });
+    assert.strictEqual(doc.sub.propertyB, 'foo');
+  });
 });
 
 describe('Check if instance function that is supplied in schema option is availabe', function() {