[odb-users] one-to-many relationship question

[Lidia Kalinovsky]

ok, thanks.

waiting for support for containers in queries.




On Wed, Aug 13, 2014 at 2:50 PM, Boris Kolpackov <boris at codesynthesis.com>
wrote:

> Hi Lidia,
>
> Lidia Kalinovsky <lidia at lemur-soft.com> writes:
>
> > #pragma db...
> > class A
> > {
> >      bool hidden;
> > };
> >
> > #pragma db...
> > class B
> > {
> >     bool hidden;
> >
> >     #pragma db unordered
> >     std::vector<A>  manyA;
> > }
> >
> > to load objects of B, i use
> >
> > typedef odb::query<T> queryT;
> > queryT q;
> > if (!fullLoading)
> > q = queryT(queryT::hidden != true );
> >
> > typename odb::core::result<T>  r (m_DB->query<T>(q,true));
> >
> > This way i have only Bs with hidden=0.
> >
> > Is it possible to have some condition for loaded A's objects inside manyA
> > vector inside pragma (means, at compilation) ?
>
> No, and I doubt we will support something like this. Seems very
> inflexible.
>
>
> > inside dynamic query ?
>
> Also not, but this is something on our TODO list. That is, support for
> containers in queries.
>
> What you can do as a workaround in the meantime is this (I am using
> raw pointers below but you will want to replace them with smart
> pointers):
>
> 1. Add the "back" pointer to A:
>
> class B;
>
> #pragma db...
> class A
> {
>   bool hidden;
>
>   #pragma db inverse(manyA)
>   odb::lazy_ptr<B> oneB; // Use the lazy (and weak) pointer corresponding
>                          // to the A's object pointer.
> };
>
> This doesn't cost anything on the database side. If you want, you can
> also avoid loading it completely on the C++ side (see lazy-loaded
> sections).
>
> 2. Re-implement your query in terms of querying for A, not for B:
>
> typedef odb::query<A> queryA;
> typedef odb::result<A> resultA;
>
> queryA q (!queryA::hidden && !queryA::oneB->hidden)
>
> resultA r (db.query<A> (q));
>
> // Create an object cache so that Bs points to As that we have already
> // loaded.
> //
> odb::session s;
>
> std::setM<B*> bs;
>
> for (resultA::iterator i (r.begin ()); i != r.end (); ++i)
> {
>   bs.insert (i->oneB.load ());
> }
>
> // bs now contains all the Bs that are not hidden and only containing
> // non-hidden As.
>
> Boris
>



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