On 08/04/2009, at 11:03 AM, Joseph Schmidt wrote:
* write the record to database, fetch it back again and
then you'll have the primary key
Than this is not the same transaction :(.
Correct. This is a limitation of how databases work, not Cayenne. You
can't have a primary key until you write the record, unless you do
something tricky like using a nanosecond timestamp hashed with the MAC
address of your machine and the number you first thought of.
* create a relationship between A and B
In this particular case, there's no relationship because the
'id' from entity A is written to a text field in entity B together
with other strings, e.g. "Entity A with #id added at #date..."
So it's impossible to map here a relationship :(.
Therein lies the danger of exposing primary keys to the user. I'd
recommend you keep keys for relating data and have another generated
number for this sort of thing. Or else, map a relationship and have
your text message:
Entity A added at #date...
Then put the related id into another attribute.
In other usage scenarios I have, e.g. for entity C, there's no
Cayenne relationship because it would mean to connect it with every
other table.
That might be a good case for using inheritance.
If you find yourself using the primary keys directly very
often, you are probably not using Cayenne in the manner it
was intended. The idea is that it removes you from thinking
about database-centric issues like primary keys and just
think about objects and how they relate to each other.
Mostly.
Well, I'm not sure about Cayenne, but the entire RESt architecture
is based around the idea of using 'id's to identify entities (but
not just RESt).
If Cayenne can't play simply with 'id's (and it's supposed to be
used without them) than I think it automatically excludes itself
from most web applications :(, since most of them rely on sending
the 'id's back and forth.
Cayenne can quite happily play with ids if that is what you need. My
point was that if you are accessing the ids directly and manually
joining up records with them, then you may be missing some of the
power of Cayenne. But without knowing the details, I couldn't say for
sure.
Is there no other way to get those IDs?
How to do e.g. audit with Cayenne? Mostly the audit string or record
must be in the same transaction with with what they log.
Look at the documentation for lifecycle events. That is probably the
best solution for this.
Ari Maniatis
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