Bind dropdown values from mongodb in angular

Question

I am working on a project where i am using mongodb as a backend and angular2 for front end. I have two collections in db as Country{id, countryname} and state{id, userId, statename} I want to find all the states depending on the country , I tried to write a code using $lookup but getting nothing. Also i need to use these two collections in an angular application for cascading dropdown. If i Select "India" only "States" in india should populate.I am new to mongodb. Plz help