Deploy heroku app from a Git subfolder

I have an existing Git repo on Github, with an example app inside the sub-folder. That looks like

dj-mongo-reader
├── dist
├── dj_mongo_reader.egg-info
├── djmongoreader
│   ├── static
│   │   └── dj-mongo-reader
│   │       └── js
│   └── templates
│       └── dj-mongo-reader
└── example
    └── *****sampleapp****
        ├── sampleapp
        └── templates
            └── dj-mongo-reader

The sampleapp folder under example is the app which I wanna push to Heroku.

Here's how to archive the goal:
1, enter git root folder

cd dj-mongo-reader 

2, register app on Hoerku (add remote url)

heroku git:remote -a dj-mongo-reader

3, push sub folder to heroku

git subtree push --prefix example/sampleapp heroku master

4, to force push sub folder to heroku

git push heroku `git subtree split --prefix example/sampleapp  master`:master --force