[odb-users] About using odb in same program to connect to mysql/sqlite/pgsql at same time

[Scott Zhang]

Hello.
    I am reading code to try to add pagination to odb. For orderby and
groupby etc, as they are trial in most case, so append sql string is
acceptable. Because I am going to replace my 2 projects DB access (one
mysql, another oracle) library with odb, so I think add pagination will
worth that.

    Well. odb in my view is a really complex and heavy used template
design. I am not sure the benefit of this. If the template was used to
improve the flexibly to support static compiling check etc, because of so
many different data types. I do think we have easier design. If you could
look at the C# subsonic's activerecord template, from a "ODB/Subsonice
compiler" level, the generate code can itself be static typed so could need
less or none template. Just subsonic can reverse generate class from
database. odb can generate persistent class from transient class.

   And from the code flow. Looks I only need to add a Take(int num_),
Offset(int offset_), (maybe OrderBy(string, enum orderby?)  and more ), in
the "query" class then we can support "pagination". Because
  for the query to run:
eg:
  result r (db->query<person> (query::age > 30));
although the first sight to convert "query::age>30" or even more complex
expression into a where clause looks mysterious. But after reading code,
query::age itself is a query_column, which can parse ">30" and return a
query. Then by the override operators, query can parse the left expressions
itself. Then the place for the Offset, Take(orderby, groupby ....) could be
after all where clause processed. We just need to call on  query object like

   result r(db->query<person>(
(query::aget>30).orderby(query::aget::column(), DESC).offset(10).take(10)) )

then we can get the 10 - 20 rows.

    And another question, from the code, looks odb::query linked with its
mysql implementation  so the query it returned will be a mysql query. But
if we link with both mysql and oracle at same time, eg for an application
need to use mysql and oracle at same time. Can odb::query work correctly?


Thanks.
Regards
Scott