Re: [boost] quince: queries in C++ expressions

17 Jul 2014

      On 2014-07-17 15:34, Michael Shepanski wrote:
...
On 17/07/2014 10:48 PM, Roland Bock wrote:
...
On 2014-07-17 13:11, Michael Shepanski wrote:
...
Every query produces values of its "value type", which can be any type
in this taxonomy:
http://quince-lib.com/mapped_data_types.html#mapped_data_types.definition_ta...
. It is common for a query's value type to be the same as a table's
value type, but this is not the case for queries that use join() or
select().
Shouldn't collections be mentioned on that page, too?
I'm not sure I understand.  Do you mean STL containers?  Then no,
these are not mapped types (except for the special cases std::string
and std::vector).  You can't have a table.
No, sorry for the confusion, I meant the usage of collector classes.
...
...
Sure. Quince's select() is analogous to supplying an SQL "select
list". (I say "analogous", not identical, because here again we work
at the level of C++ types, without regard to whether they are
single-column or multi-column.)
So it's:
struct something {
         metadata meta;  // metadata is some mapped type defined
elsewhere
         vector blob;
         string other;
     };
     QUINCE_MAP_CLASS(something, (meta)(blob)(other))
extern table<something> somethings;
[...]
Hmm. I guess my question was unclear or I misunderstood you answer.
I was thinking something like
auto query = somethings.select(somethings->meta)
if (userWantsBlob)
    query.add_to_select_list(somethings-blob);
for(const auto& row : query)
{
    if (userWantsBlob)
    {
       //somehow access blob here
    }
}
Ah, I see.  No, on two counts.
...
[...]
Iiuc you are starting with a query that gets something narrow, and
then using add_to_select_list() to widen it. With quince, you would
work in the opposite direction: start with a query that gets all the
information anyone needs (or just start with the table itself), and
use select(...) to make narrower queries as needed. Making new queries
in this way is a cheap operation.
Would that cause you any problems? (I'm not sure what you mean by
"Multiplying the code".)
Say I have two conditions, userWantsBlob and userWantsOther. I would
want to have everything in one loop.

In sqlpp11 I could say something like:

auto query = dynamic_select(db, t.meta)...;
if (userWantsBlob)
   query.add_column(t.blob);
if (userWantsOther)
   query.add_column(t.other);

for (const auto& row : db(query))
{
   std::cout << t.meta << '\n';
   if (userWantsBlob)
     std::cout << t.at("blob") << '\n';
   if (userWantsOther)
     std::cout << t.at("other") << '\n';
};

How would you do that in quince?
...
...
If you knew the database type while constructing the query, you could d
those tests during construction. It probably wouldn't even be that hard
with a similar mechanism as used in sqlpp11 (easy for me to say, still
not knowing really much about your code).
As quince builds a query it always "knows" the database type in a
dynamic sense, but not in a static sense.
OK

Cheers,

Roland