fix(populate): avoid setting empty array on lean document when populate result is undefined

Fix #10599

Author: vkarpov15

lib/helpers/populate/assignVals.js

@@ -43,6 +43,9 @@ module.exports = function assignVals(o) {
     if (val instanceof SkipPopulateValue) {
       return val.val;
     }
+    if (val === void 0) {
+      return val;
+    }
 
     const _allIds = o.allIds[i];
 

lib/helpers/populate/lookupLocalFields.js

@@ -13,6 +13,9 @@ module.exports = function lookupLocalFields(cur, path, val) {
     if (typeof cur !== 'object') {
       return void 0;
     }
+    if (val === void 0) {
+      return void 0;
+    }
     cur[path] = val;
     return val;
   }

test/model.populate.test.js

@@ -10565,4 +10565,41 @@ describe('model: populate:', function() {
       assert.equal(populatedBooks[0].author.name, 'Author1');
     });
   });
+
+  it('avoids setting empty array on lean document when populate result is undefined (gh-10599)', function() {
+    return co(function*() {
+      const ImageSchema = Schema({ imageName: String }, { _id: false });
+      const TextSchema = Schema({
+        textName: String,
+        attached: [{ type: 'ObjectId', ref: 'Test2' }]
+      }, { _id: false });
+
+      const ItemSchema = Schema({ objectType: String }, {
+        discriminatorKey: 'objectType',
+        _id: false
+      });
+
+      const ExampleSchema = Schema({ test: String, list: [ItemSchema] });
+
+      ExampleSchema.path('list').discriminator('Image', ImageSchema);
+      ExampleSchema.path('list').discriminator('Text', TextSchema);
+      const Example = db.model('Test', ExampleSchema);
+      const Another = db.model('Test2', Schema({ test: 'String' }));
+
+      yield Another.create({ _id: '61254490ea89de0004c8f2a0', test: 'test' });
+
+      const example = yield Example.create({
+        test: 'example',
+        list: [
+          { imageName: 'an image', objectType: 'Image' },
+          { textName: 'this is a text', attached: ['61254490ea89de0004c8f2a0'], objectType: 'Text' }
+        ]
+      });
+      const query = Example.findById(example._id).populate('list.attached').lean();
+
+      const result = yield query.exec();
+      assert.strictEqual(result.list[0].attached, void 0);
+      assert.equal(result.list[1].attached[0].test, 'test');
+    });
+  });
 });