On 10/29/2010 01:23 PM, Mark Erbaugh wrote:
> I have a table (T) that has a many-to-one relationship (via foreign key
> inclusion) to a category table (C). Let's name the relationship category.
> When I retrieve an existing T record, SA populates the category field with an
> instance of C. I can change the category to a different value by storing a
> different instance of C into category.
Technically, SQLAlchemy by default does not populate the "category"
relation until you access it the first time (i.e. it is a lazy load).
> My question is does it matter if the various instances of C are associated
> with the same session as the T instance?
I'm not sure what you mean here. There is no problem having multiple C
instances in the same session as the T instance. SQLAlchemy will
complain (hopefully; I haven't tried it) if multiple C instances in the
same session share a primary key.
> Can the C instances come from a different session, or can the be expunge'd
> from the session that retrieved them?
The only restriction is, if the T instance is part of session S, then
the C instance that you assign to T.category must be part of S or not be
part of any session. It is allowed if you first expunge the C instance
from another session and merge it into S before assigning it to T.category.
Note that there are only a few use cases for moving instances from one
session to another that I am aware of:
* Caching query results. You expunge the instances from the original
session, keep them around in memory (or serialized to disk,
memcache, etc.). Then you can merge them back into subsequent
sessions to avoid repeated DB queries.
* Passing objects between threads. Sessions are not thread-safe, so
if you want to pass objects from thread A to thread B, you have to
merge session A's objects into session B before thread B can use them.
> Can I store an entirely new C instance, and if so, will SA do an insert into
> the C table?
Yes, this is part of the "save-update" cascade[1] which is enabled by
default on a relationship. Generally you only worry about cascade for
one-to-many or many-to-many relationships. At least that's my experience.
-Conor
[1]
http://www.sqlalchemy.org/docs/orm/relationships.html#the-relationship-api
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