Hello Sergey,
Sorry for the very late reply; it wasn't until now that I stumbled across your post.
Re 1: My recommendation would be NOT to use unmanaged associations in the import statement. And is there a reason you defined the association to country differently from the association to rating? Plain 1:1 associations should be imported just fine, without any need for additional annotations in the import statement.
Re 2: BIGINTs are returned as ctypes.Int64 types, which are not rendered properly by JSON.stringify. Try calling toString on those objects directly, or use one of the available conversion functions.
Hope that helps,
Ralph