Hi Adam,
What you want to do cannot be done using just one data source.
Here is a solution you can try, taking country - city data as example:
1. Create a data source 1 with unique country name in it; Create another data source 2 with country - city (its corresponding relationship) in it
2. List 1 use data source 1 and List 2 use data source 2
3. Add a trigger in List 1
a. For Action, select "Filter Data Records"
b. For Action data, select data souce 2
c. For Conditions, enable "Trigger / action connection", and select "Country" for trigger and anction fields
And the action setting will look like this:
If you are familiar with database, this works very similar to foreign key.
This is how it works:
Shengdi