Appstore proj: Calculate an App's Top-5 related apps(2)

Save data into MongoDB

instead of print, we do persist

persist top 5 recommended apps in app_info

1. Persist top 5 apps in MongoDB

2. Do it for all apps 

loop through all apps, get the list from collection app_info

add function in DataService

add for loop in main function

call the same function calculate_top_5(app, user_download_history)