fix populate called with lean (gh-14794)

lib/model.js

@@ -4468,7 +4468,7 @@ function _assign(model, vals, mod, assignmentOpts) {
  rawOrder[key].push(i);
  } else if (isVirtual ||
  rawDocs[key].constructor !== val.constructor ||
- String(rawDocs[key]._doc._id) !== String(val._doc._id)) {
+ (rawDocs[key] instanceof Document ? String(rawDocs[key]._doc._id) : String(rawDocs[key]._id)) !== (val instanceof Document ? String(val._doc._id) : String(val._id))) {
  // May need to store multiple docs with the same id if there's multiple models
  // if we have discriminators or a ref function. But avoid converting to an array
  // if we have multiple queries on the same model because of `perDocumentLimit` re: gh-9906

test/model.populate.test.js

@@ -11048,4 +11048,52 @@ describe('model: populate:', function() {
  assert.equal(pet1.owner.name, 'Alice');
  assert.equal(pet2.owner.name, 'Alice');
  });
+
+ it('makes sure that populate works correctly with duplicate foreignField with lean(); (gh-14794)', async function() {
+ const authorSchema = new mongoose.Schema({
+ group: String,
+ name: String
+ });
+
+ const postSchema = new mongoose.Schema({
+ authorGroup: String,
+ title: String,
+ content: String
+ });
+
+ const Author = db.model('Author', authorSchema);
+ const Post = db.model('Post', postSchema);
+
+ await Author.create({ group: 'AUTH1', name: 'John Doe' });
+ await Author.create({ group: 'AUTH2', name: 'Jane Smith' });
+ await Author.create({ group: 'AUTH2', name: 'Will Jons' });
+
+ await Post.create({
+ authorGroup: 'AUTH1',
+ title: 'First Post',
+ content: 'Content 1'
+ });
+
+ await Post.create({
+ authorGroup: 'AUTH2',
+ title: 'Second Post',
+ content: 'Content 2'
+ });
+
+ const posts = await Post.find()
+ .populate({
+ path: 'authorGroup',
+ model: 'Author',
+ select: { _id: 1, name: 1 },
+ foreignField: 'group'
+ })
+ .select({ _id: 1, authorGroup: 1, title: 1 })
+ .lean();
+
+ for (const post of posts) {
+ assert.ok(post.authorGroup._id instanceof mongoose.Types.ObjectId);
+ assert.ok(typeof post.authorGroup.name === 'string');
+ }
+ assert.equal(posts.length, 2);
+ });
 });