Unable to use `db map` pragma for types used for object ids

[karen-arutyunov]

When, for example, we try to use the following mapping for repository_url type

#pragma db map type(repository_url) as(string)   \
    to((?).string ())   \
    from((?).empty () ? bpkg::repository_url () : bpkg::repository_url (?))

and use this type as an id for the pkg_repository_metadata type (url member) in bpkg project, we end up with ODB-generated files which cannot be compiled. The compilation fails for the following reasons:

fetch-cache-data-odb.ixx:

  inline
  access::object_traits< ::bpkg::pkg_repository_metadata >::id_type
  access::object_traits< ::bpkg::pkg_repository_metadata >::
  id (const object_type& o)
  {
    return o.url; // Error: can't convert from repository_url to string.
  }

fetch-cache-data-odb.cxx:

  access::object_traits_impl< ::bpkg::pkg_repository_metadata, id_sqlite >::id_type
  access::object_traits_impl< ::bpkg::pkg_repository_metadata, id_sqlite >::
  id (const image_type& i)
  {
    sqlite::database* db (0);
    ODB_POTENTIALLY_UNUSED (db);

    id_type id;
    {
      ::std::string vt;

      sqlite::value_traits<
          ::std::string,
          sqlite::id_text >::set_value (
        vt,
        i.url_value,
        i.url_size,
        i.url_null);

      // From fetch-cache-data.hxx:68:14
      id = (vt).empty () ? bpkg::repository_url () : bpkg::repository_url (vt); // Error: can't convert from repository_url to string.
    }
  ...

  void access::object_traits_impl< ::bpkg::pkg_repository_metadata, id_sqlite >::
  init (id_image_type& i, const id_type& id)
  {
    bool grew (false);
    {
      // From fetch-cache-data.hxx:68:14
      ::std::string const& vt =
        (id).string (); // Error: std::string has no string() member.
   ...

Note that while the id_type is defined as std::string for the pkg_repository_metadata class, the ODB-generated implementation erroneously treats it as repository_url in a number of places.

Another surprising thing is that comparison of query<pkg_repository_metadata>::url member in query expressions for such a mapping expects the value of std::string type rather than of repository_url type:

db.query<pkg_repository_metadata> (query<pkg_repository_metadata>::url == string (...))         // OK.
db.query<pkg_repository_metadata> (query<pkg_repository_metadata>::url == repository_url (...)) // Error.