I found the following code in org.javalite.activejdbc.Model.parent method:
List<Map> results = new DB(getMetaModelLocal().getDbName()).findAll(query, Integer.parseInt(fkValue));
Does it mean that we can't use Long FK in Active JDBC? We will have NumberFormatException if it's more than Integer.MAX_VALUE = 2147483647.
No, this is an omission on our part. We filed an issue: https://github.com/javalite/activejdbc/issues/375 to fix it. Usually things like that are fixed within day/two. Thanks for reporting!
UPDATE TO THE ANSWER: You are looking at old code. This commit: https://github.com/javalite/activejdbc/commit/d183cb3ac8c567504d4d5a47986fe549dd1aebca fixed this issue on May 31, 2014.