On Jun 3, 2008, at 1:13 PM, Gary Poster wrote:
Uh-oh, I'm implicated!
(see below)
On Jun 3, 2008, at 12:53 PM, Sean Allen wrote:
...
If you do that in gemstone, there is only one copy of Order B, no
matter
what variable in what dictionary you come at it from. And its
drop dead
simple.
I looked at implementing that with zodb and moved along.
I'm confused. This has been the way the ZODB worked for a long time,
unless I'm really missing something in your description.
i tried to do this:
create customer that has order
so that i can have different extents type situations...
store customer in one dictionary.
store order in another.
if i pulled the order back out from the order dictionary and
modified it
then pulled the customer out, the customers order was no longer in
sync
with what came out of the order dictionary.
the reference was lost on serialization. original in memory objects
were fine,
those that came back out from zodb werent.
i'm going to quote the initial email i sent with the idea in
general and the followup i got
and i then tried it out to make sure i hadnt asked the question
wrong, and yeah...
what i wanted to do, wasnt easily done.
the quotes:
The biggest concern I have is how do to the layout/storage so that
this slightly contrived example works:
Product has a brand.
There are many brands.
How do I store so that I can find all products simply and all
brands simply and also so that changes in a brand instance are
reflected when
the product instance is deserialized. By 'simply' I mean that it
doesnt really work on our end to have to walk all Products looking
for unique brands. Should just be able to go directly in and get
said brands ( using keys() or similar call ).
If I create 'brand' and 'product' as btrees, then if i do
something like
some_product.brand.name = 'something entirely different'
and that brand already exists in 'brand', would it be updated? are
references maintained in that fashion?
do we have to handle manually on update and creation?
Note that we would just be using ZODB not Zope in this scenario.
Back references are not maintained automatically.
I'd identify two classic solutions to this sort of thing.
One is to make a custom mapping (using a BTree as the inner data
structure) that maintains back-references when objects are placed
in them or removed. zope.app(.container? .folder? I'd have to
look) has code that does this, along with firing events. For
simple stories like the one you describe here, that's what I'd
probably recommend. It works to the strengths of the ZODB, which
particularly shines in terms of readability when you just need to
walk a tree of attributes to get what you want.
The other is to keep an external index, a la zc.extrinsicreference
or zc.relation.
zc.extrinsicreference does not have too many dependencies beyond
ZODB, and as long as zope.app.keyreference doesn't drag much along
with it, might be usable as a library. That said, it's also very
simple, and could be used as a model for you, even if you don't use
it directly. It would also be a reasonable choice for a simple
situation like the one you describe. It relies on events to update
its data structures.
zc.relation an almost-released-revision of zc.relationship that
drastically reduces dependencies--actually, it has no additional
dependencies to ZODB, as you can see at http://svn.zope.org/zc.relation/trunk/setup.py?view=markup
. It's also a bit overwhelming and low-level: see the README:http://svn.zope.org/zc.relation/trunk/src/zc/relation/README.txt?view=auto
. It doesn't hook anything up for you: you set the relationship
catalog up and you arrange for it to be updated, via events or
direct messages. That said, if you need its power, it is well-
tested and would be a good choice for some jobs from at least some
perspectives (caveat read-or: I'm the author).
Now in the context of this discussion, I see that I misread you. I
apologize.
This works out of the box:
You have a Product class and a Brand class. Both inherit from
persistent.Persistent.
You have a persistent-aware mapping such as a BTree.
In the root of your ZODB, put two BTrees, one for your products and
one for your brands.
Create some Brand instances and put them in the brand mapping.
Create some Product instances and put them in the product mapping.
Assign some of the brands you have made as attributes of the products.
Now, product.brand.foo = 'bar' (in any thread or any ZEO client)
will be changing the same effective object (let's call it a record)
as the one in the brand mapping you have in the root of your db.
I believe that this is what you are talking about.
Again, I apologize for not reading your original question closely
enough.
What you *can't* do out of the box is ask "hey, what products have
an attribute that points to this brand?". That's a back-reference,
and that needs solutions like the ones to which I was referring.
Gary
That means when I tested it, I did something horribly wrong in my code.
Damn.
I'm going to have to dig my test code up and see what I really screwed
up.
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