Symphony.

While new API's documentation is in progress I'd like to make clear basic concepts for myself. The example may be a bit complicated for a start. But it's "from a real life" - I made this in Symphony 1.7.

So...
I have a list of regions (about 20 items).
I have a list of streets (about 3000 items).
Each street belong to one of the regions.

I have "Apartments" section.
For entry (flat) in this section I can select region from a select box. When I choose region another select box appears with list of streets from only that region (Ajax).

It's a long story how I make this in Symphony 1.7 =).
In S2 first steps looks easier - I make "Regions" and "Streets" sections and binding each street entry to region.
But then - Ajax part in "Apartment" section. It's quite difficult to understand where to start with this.

Could you explain briefly what should be my following steps?
And, maybe in S2 I should use some absolutely another method?