Filter results from dropdown

Hi, now after three years, I have similiar problem with filtering results. I use MODx Evo 1.0.10, not Revo.

I have list of countries and I want to filter results from a list of cities, each city is different in various country. And the city is written in document.

The main difference is the name of the city doesn't exist in TV as static value, but it is written by the editor in document and safe in db in the proper position.

I have tv with name "city" which appears in the edit mode of the document.

The question is how to get value of this from db, perhaps what I need to put in tv as the value to collect this data?