[Bug] populate with clone: true throws an error with lean

[AbdelrahmanHafez]

What is the current behavior?
Can not use clone: true in populate along with lean(), it throws an error:

TypeError: doc.constructor.hydrate is not a function
    at assignRawDocsToIdStructure (node_modules\mongoose\lib\helpers\populate\assignRawDocsToIdStructure.js:58:29)
    at assignRawDocsToIdStructure (node_modules\mongoose\lib\helpers\populate\assignRawDocsToIdStructure.js:41:7)
    at assignVals (node_modules\mongoose\lib\helpers\populate\assignVals.js:26:3)
    at _assign (node_modules\mongoose\lib\model.js:4564:3)
    at _done (node_modules\mongoose\lib\model.js:4411:7)
    at _next (node_modules\mongoose\lib\model.js:4403:7)
    at node_modules\mongoose\lib\model.js:4837:16
    at node_modules\mongoose\lib\helpers\promiseOrCallback.js:24:16
    at node_modules\mongoose\lib\model.js:4860:21
    at node_modules\mongoose\lib\query.js:4366:11
    at node_modules\kareem\index.js:135:16
    at processTicksAndRejections (internal/process/task_queues.js:79:11)
Emitted 'error' event on Function instance at:
    at node_modules\mongoose\lib\model.js:4839:13
    at node_modules\mongoose\lib\helpers\promiseOrCallback.js:24:16
    [... lines matching original stack trace ...]
    at processTicksAndRejections (internal/process/task_queues.js:79:11)

If the current behavior is a bug, please provide the steps to reproduce.

const mongoose = require('mongoose');
const { Schema } = mongoose;
const assert = require('assert');

mongoose.connect('mongodb://localhost:27017/test', { useNewUrlParser: true, useUnifiedTopology: true });

const blogPostSchema = new Schema({
  title: { type: String },
  commentsIds: [{ type: Schema.ObjectId, ref: 'Comment' }]
});

const commentSchema = new Schema({
  content: String
});

const BlogPost = mongoose.model('BlogPost', blogPostSchema);
const Comment = mongoose.model('Comment', commentSchema);

async function run () {
  await BlogPost.deleteMany();

  const blogPost = new BlogPost({ title: 'Important post' });
  const comment = new Comment({ content: 'Cool post.' });

  blogPost.commentsIds = [comment._id];

  await Promise.all([
    blogPost.save(),
    comment.save()
  ]);

  const foundBlogPost = await BlogPost.findOne({ _id: blogPost._id }).populate({
    path: 'commentsIds',
    options: { clone: true }
  }).lean();

  assert.equal(foundBlogPost.commentsIds[0].content, 'Cool post.');

  console.log('Done.');
}


run().catch(console.error);

What is the expected behavior?

To populate successfully.

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

mongoose v5.9.7
MongoDB v4.0.4
Node v12.0

Will be putting a PR to solve this sometime soon.