How to map page unique identifier with a custom table as foreign key

I have a page type associated with pages. In the Form Tab of a page, I want to select multiple cities using checkbox dropdown(custom form control) and store it in page_cities table (ResourceID, CityID and other default columns)

I handled a SaveVersion.After event and able to store the data in page_cities table but I'm confused about what should I store in ResourceID:

private void SaveVersion_After(object sender, WorkflowEventArgs e)
{
    var resourceID = e.Document.DocumentID
    or
    var resourceID = e.Document.NodeID
    or
    var resourceID = e.Document.DocumentNodeID

    .. insert into page_cities logic ..

While displaying the page on site, I want to use a query something like this:

   LEFT JOIN page_cities on page_cities.ResourceID = View_CMS_Tree_Joined.NodeID

I hope I was able to clarify my problem.