How should I struct my Firebase data (base- & movement-data)?

Question

I've created an android app with Java and I use a Firebase realtime-database. Via REST, I get JSON-data for cities. "Data" means:

1. base data like the city-id, name or population. They rarely change. This is represented in my model class "BaseData".
2. (movement) data which changes every day. The class is called "CityData".

BaseData is currently part of the CityData :

public class CityData
{
    private BaseData baseData;
    private int cases;
    //...
}

At this time, I can only request data for today. When I save a CityData-object in my db, it looks like this:

My next step: Save the data for the last 7 days. But: Firebase would also save the base data every day. Can I prevent this (=> only save when not exists or update when something changes)? Otherwise, I have to change the data struct. But which is the best possibility? My first idea: Create two seperate paths like this:

But then, I have to extract BaseData from CityData and join them every time, for example when I need the city name, which is often. Is this the best way? It's my first time with NoSql, so every help would be nice