Storing prepared statements about sqlpp11 HOT 4 CLOSED

Hmm, good question.

Sometimes, you just don't need to do that.

{
auto s =m_db->prepare...;
// use s a number of times and then discard it
// this would be useful for inserts, for instance
}

Otherwise its tough, I think.

Some brainstorming:

With C++14, you could wrap the preparation into a function with
automatic return type deduction. And then you could use that function in
decltype. I am really looking forward to C++14...

Oh, with C++11 you might be able to use a lambda to do the same. This
should work:

namespace
{
auto prepareCountId = [](connection& db) { return db.prepare(...); }
}

int main()
{
connection db;
using CountId = decltype(prepareCountId(db));
CountId countId = prepareCountId(db);
}

As a sidenote:
As of now, the prepared_* have the database and the statement as
template parameters. Especially in case of a select, it would be
possible to have a slightly less specific class by using only those
parts of the select which are actually needed by the prepared select.
But that would still be a typename you would not want to write by hand...

This might be helpful though to be able to have just one
prepared_statement type if result-type and parameters are the same.

from sqlpp11.

Using the lambda to avoid duplication is a good idea.

I think a simpler prepared statement type is also a good idea because storing the prepared statement in a member seems like a pretty common use case to me, and obtaining the prepared statement type is not not really trivial and user friendly at the moment.

For reference, this is my current solution:

namespace
{
// query lambda and type alias
auto childCountQuery = [] () {
    return select(count(objects.Id).as(numObjects))
           .from(objects)
           .where(objects.ParentId == parameter(objects.ParentId));
};

using ChildCountQuery = decltype(childCountQuery());

// ... other queries

// type alias for the prepared statement types
template <typename SelectType>
using PreparedStatement = decltype(((sql::connection*)nullptr)->prepare(*((SelectType*)nullptr)));

}

// member structure containing the prepared statements (used as pimpl data member)
struct Database::PreparedStatements
{
    std::unique_ptr<PreparedStatement<ObjectCountQuery>> objectCount;
    std::unique_ptr<PreparedStatement<ChildCountQuery>> childCount;
};

// called at construction time to prepare the statements
void Database::prepareStatements()
{
    m_statements->objectCount.reset(new PreparedStatement<ObjectCountQuery>(m_db->prepare(objectCountQuery())));
    m_statements->childCount.reset(new PreparedStatement<ChildCountQuery>(m_db->prepare(childCountQuery())));
}

It would be nice if the prepared statement would allow default construction which would avoid having to use the unique_ptr, making the code a little more elegant

from sqlpp11.

On 2014-08-10 22:47, dirkvdb wrote:

Using the lambda to avoid duplication is a good idea.

:-)

I think a simpler prepared statement type is also a good idea because
storing the prepared statement in a member seems like a pretty common
use case to me, and obtaining the prepared statement type is not not
really trivial and user friendly at the moment.

As stated before, it won't be that much simpler (unless you want to
sacrifice the type safety in setting parameters and obtaining results).

For reference, this is my current solution:

Thanks!

It would be nice if the prepared statement would allow default
construction which would avoid having to use the unique_ptr, making
the code a little more elegant

I was conservative about the default constructor because a default
constructed prepared statement is unusable. But yes, that unique_ptr
code is ugly. I therefore added default constructibility for sqlite3
(develop branch). Others will follow soon.

Please note again that you must not use the default-constructed prepared
statement for anything but assigning it a "real" prepared statement.

Best,

Roland

from sqlpp11.

As stated before, it won't be that much simpler (unless you want to
sacrifice the type safety in setting parameters and obtaining results).

You are right, reducing some of the template arguments doesn't help

I therefore added default constructibility for sqlite3
(develop branch). Others will follow soon.

Great, looks much nicer now:

struct Database::PreparedStatements
{
    PreparedStatement<ObjectCountQuery> objectCount;
    PreparedStatement<ChildCountQuery> childCount;
};

void Database::prepareStatements()
{
    m_statements->objectCount = m_db->prepare(objectCountQuery());
    m_statements->childCount = m_db->prepare(childCountQuery());
}

from sqlpp11.