> On Thu, May 12, 2011 at 1:52 PM, Andre Nathan <an...@digirati.com.br>
> wrote:
> > Hello

> > Say I have a model with multiple has_many relationships. For example,

> > -module(bookstore, [Id, Name]).
> > -has({employees, many}).
> > -has({books, many}).

> > Now say that when I create a new bookstore, the employes and books are
> > inputs in the bookstore form, so that in my controller I actually have
> > to save instances of the 3 models. I also want to abort the whole
> > process if any of those fail validation.

> > I'm doing something like this:

> > - Create a new bookstore
> > - Create the employees, with bookstore_id undefined
> > - Create the books, with bookstore_id undefined
> > - Call validate() on all those instances; if any of them fails, abort
> > - Save the bookstore
> > - Assign the bookstore ID to each employee and book
> > - Save the employees and books

> > The code for this can become somewhat complicated as more relationships
> > are added (not to mention really boring to write), and it seems wasteful
> > too because I have to call validate() then save(), which calls
> > validate() itself.

> > So, is there a better way do this? It would be cool if there was a way
> > to set related objects directly, something like

> >  B1 = bookstore:new(id, "foo"),
> >  B2 = B1:employees(ListOfEmployees),
> >  B3 = B2:books(ListOfBooks),
> >  B3:save()  % validates and saves each associated object too

> > Is it possible to add something like this to the API?

> The trouble with this approach is that the Bookstore object is really
> just a value-tuple and never includes any information about associated
> books or employees; these are fetched from the database each time
> employees() or books() is called. There's not a clean way that comes
> to mind for keeping track of associated non-saved objects.

> However, your problem might be better solved with database
> transactions.... maybe the API would look like

> ok = boss_db:transaction(fun() ->
>    Bookstore = bookstore:new(id, "foo"),
>    {ok, SavedBookstore} = Bookstore:save(),

>    lists:map(fun(Employee) ->
>         E1 = Employee:bookstore_id(SavedBookstore:id()),
>         {ok, E2} = E1:save()
>    end, ListOfEmployees),

>    lists:map(fun(Book) ->
>          ...
>    end, ListOfBooks)
> end)

> Any run-time error would result in the transaction being rolled back.
> What do you think?

> Not all databases support transactions but we could probably use the
> transaction semantics in Mnesia, MySQL, and Postgres at the very
> least.

> > Thanks,
> > Andre

> --
> Evan Miller
> http://www.evanmiller.org/