(the primary key should consist of only one column at the moment)
git-svn-id: svn://ttmath.org/publicrep/morm/branches/join_models@1186 e52654a7-88a9-db11-a3e9-0013d4bc506e
added: using ModelData in BaseExpression so Model::to_text() functions can use a ModelData object now
git-svn-id: svn://ttmath.org/publicrep/morm/trunk@1175 e52654a7-88a9-db11-a3e9-0013d4bc506e