On Sep 26, 2008, at 9/2610:04 AM , Andrus Adamchik wrote:
I can check into how they handle the non-explicit-ordering-column
case if you'd like.
I would appreciate that. Maybe that join is their answer. Not
pretty at all.
Nope, it's not. But it is their answer, apparently (note that they
documentation on hibernate core doesn't seem to mention this; they say
you have to provide an explicit "index" for list ordering). I took a
working hibernate mapping/project and stepped through the sql events.
Test looks something like (pseudocode)
Parent p = new Parent();
p.getChildren().add(new Child(0));
p.getChildren().add(new Child(1));
session.save(p);
assert children in correct order
p.getChildren().add(1,new Child(2));
assert children in correct order (child(0), child(2), child(1))
save(p);
p = refetch p from database using a new session
assert children in correct order (child(0), child(2), child(1))
And the test passes.
So, looking at the sql events, the table structure comes out like:
Parent table one to many Parent_Child (join table) many to one Child
Now when looking at the sql log, the set of events is:
insert parent
insert child0
insert child1
insert parent1_child0 join row
insert parent1_child1 join row
insert child2
delete from parent_child where parent is 1
insert parent1_child0 join row
insert parent1_child2 join row
insert parent1_child1 join row
So they achieve the ordering by relying on the default returned order
from the database of the join table.
At least, that's what hibernate does when working with hsqldb. It may
be different for other database types.
If you were going to map the relationship w/out the join table, then I
imagine hibernate would complain and make you specify an index, but I
haven't tried that.
Robert
Interestingly, by default, a one-to-many relationship in hibernate
is still handled through an intermediary join table.
You can override that, of course, but I thought it a curious default.
Ugh, I am sure it sucks for the (unsuspecting) users.
Andrus
On Sep 26, 2008, at 6:01 PM, Robert Zeigler wrote:
HIbernate actually handles this.
When you define a relationship as a list, hibernate ensures that
the items are always fetched in the same order.
I haven't dug into the details of how to do this. I know its
possible to explicitly declare a "sort column", but generally
unnecessary;
I assume that in the absence of an explicit sort column, hibernate
(silently) adds a sort column for you.
I can check into how they handle the non-explicit-ordering-column
case if you'd like.
Interestingly, by default, a one-to-many relationship in hibernate
is still handled through an intermediary join table.
You can override that, of course, but I thought it a curious
default. That said, by silently handling relationships that way,
it would allow them to add the sort information to the join table,
which has no object corollary so your object model is uncluttered.
Robert
On Sep 26, 2008, at 9/269:48 AM , Andrus Adamchik wrote:
On Sep 26, 2008, at 2:41 AM, Chris Murphy wrote:
Wouldn't it be a good idea for the generated methods to have the
extra int argument?
It is a bit more involved than that. The problem with including
this in Cayenne is that it won't work in a more general case. E.g.
if you add an object at a particular index, and the master object
is later invalidated and refetched, the order will be lost. Or if
it is refetched by another user. So Scott's answer was essentially
correct.
We tried to solve it from another angle, by defining a certain
column as the "ordering" column to instruct Cayenne to order
fetched relationship lists. It is still on the table, but it is
also hairy...
For now I can't think of a clean generic solution that would map
to a DB. The ordering column is the closest I can think of.
Thanks,
Andrus
