[odb-users] Persisting objects after ID change

[Erez Pics]

 Hi Boris,

Thank you for your reply, your are right, with regards to the object
id being immutable, this is a very special behavior we need.

This is not the main flow of our framework, however we need the temp
identifiers to change to permanent ones seamlessly and once made
permanent the ODB objects need to work regularly.

I thought that ODB behaves in a similar way regarding the update
query, to solve it I execute a small update query that directly
changes the object id
and then I change the object id in the ODB object, now that the new id
is persisted it seems ODB behaves great and all update operations
afterwards work :-)

Do you think that there is a problem with my update query and ODB
using the new object id after the change ?

Thank you and Happy New Year,
Erez.

On Tue, Dec 30, 2014 at 3:40 PM, Boris Kolpackov
<boris at codesynthesis.com> wrote:
> Hi Erez,
>
> Erez Pics <picserez at gmail.com> writes:
>
>> The flow: I have an ODB object that has been persisted before with a
>> temp identifier, I set a new value to it's identity field and call
>> data_base->update(object), the result "object not persistent"
>> exception, ODB refuses to update an existing object to use the new
>> identifier.
>
> The whole ODB architecture assumes that object identifiers are
> immutable. In fact, conceptually, saying something like "I set
> a new value to it's identity field" doesn't make sense since
> if you change the object's identity, it is no longer the same
> object. Or, to put it in more technical terms, when you call
> update(), underneath, ODB executes "UPDATE ... WHERE id = ?"
> with the value for id coming from the object identifier member.
> But, since its value you have changed, ODB will not be able to
> find the original row in the database.
>
> This is why in my original reply to your idea of using negative
> id values to indicate a special object state I suggested that
> a separate flag might be a cleaner solution.
>
> But, if you still want to update the object id, there is a way,
> not a very elegant one, though. The idea is to map another special
> class to the same table that would allow you to change the id.
> Here is an outline:
>
> #pragma db object table("my_object")
> class my_object // You original object.
> {
>   ...
>
>   #pragma db id column("id")
>   unsigned int id;
> };
>
> #pragma db object table("my_object")
> struct my_object_id
> {
>   my_object_id (unsigned int o, unsigned int n): old_id (o), new_id (n) {}
>
>   #pragma db id column("id")
>   unsigned int old_id;
>
>   #pragma db column("id")
>   unsigned int new_id;
> };
>
> my_object o = ...;
>
> db.update (my_object_id (o.id, -o.id));
>
> Note that my_object_id should come after my_object in the header file
> in order to get the correct table definition in the generated schema.
>
> Boris