It sounds to me like the problem you're having has to do with how you are
getting the reference to the package, which isn't shown in your example.
How are you getting it?
The session doesn't do anything "by magic", even if it seems that way
sometimes. It just manages things behind the scenes.
If you want a new Package, create a new Package object. If you want an
exisiting package, query for it. Just like you would in bare SQL code.
If you don't know which you need, try searching and create it if it isn't
found. I usually do something like:
try:
pkg = session.query(Package).filter(<condition-goes-here>)
except sa.orm.exc.NotFound:
pkg = Package()
# populate attributes and add to session
This is off the top of my head and from memory, so beware of possible
(probable) errors, but hopefully you get the idea.
Unfortunately (fortunately?) the SQLAlchemy docs are good enough that there
isn't a huge impetus for people outside the project to write tutorials,
blog posts, etc. I'd like to see more of that kind of thing as well, but
everyone has limited time, I guess.
Good luck!
On Fri, Jan 10, 2014 at 7:44 AM, Paul Moore <p.f.mo...@gmail.com> wrote:
> I'm developing an application using the ORM, and I am getting into all
> sorts of trouble with what I think should be a pretty simple data model.
> I've tried following the ORM tutorial from the docs, but it seems to get me
> confused every time I try. So I'm looking for something else that maybe
> takes a different approach. Or maybe someone can point out what's wrong for
> me in the following - but even then, pointers to other tutorial material
> would be useful, as I don't really want to end up pestering the list every
> time I hit an issue :-)
>
> My data model is fairly straightforward. I have a Package class, with a
> name. I then have a Release class - each Release is associated with a
> single package, and has a unique version. Releases have a few children -
> Classifiers, Dependencies, and URLs. All of these are multi-valued with no
> natural key (at least, not one worth worrying about). There is some extra
> data associated with Releases and URLs, but that's not too important.
> (People may recognise this as the PyPI data model). This is a pretty
> trivial parent/child one-to-many hierarchy, and I didn't expect it to be
> hard to model.
>
> The obvious (to me!) model is basically (trimmed down a bit):
>
> class Package(Base):
> __tablename__ = 'packages'
> # Use a synthetic key, as package names can change
> id = Column(Integer, primary_key=True)
> name = Column(String, unique=True, nullable=False)
> releases = relationship("Release", backref="package",
> cascade="all, delete-orphan")
>
> class Release(Base):
> __tablename__ = 'releases'
> id = Column(Integer, primary_key=True)
> package_id = Column(Integer, ForeignKey('packages.id'),
> nullable=False)
> version = Column(String, nullable=False) # Unique within package
> classifiers = relationship("Classifier", backref="release",
> cascade="all, delete-orphan")
>
> class Classifier(Base):
> __tablename__ = 'classifiers'
> id = Column(Integer, primary_key=True)
> release_id = Column(Integer, ForeignKey('releases.id'),
> nullable=False)
> classifier = Column(String, nullable=False)
>
> So far, so good. But if I want to create a new Release, things get messy.
> This is my basic function:
>
> def new_release(package, version, data):
> r = Release(version)
> r.package = Package(package)
> # Populate the data in r, and create child items
> return r
>
> It's that package reference that messes me up. If the release is for a new
> package, then when I merge the release into the session, the package is
> created. But if it's for an existing package, a new package is created
> (which gives a constraint error if the package name is unique, and
> duplicates if it's not) rather than the session recognising that it's an
> existing package and linking the release to it.
>
> If I was working at the SQL core level, I'd expect to have to query for
> the package and create it if it wasn't there. But I'm not sure I can do
> that with a session, as things get cached in memory by the "unit of work"
> stuff, and I don't know if a query for the release could miss a package
> that's pending insertion, or if the query could cause auto-flushing which
> might cause other issues (either with performance or integrity). Because
> the session is managing the in-memory caching and the transaction
> management "by magic", I don't want to interfere with its mechanisms any
> more than I have to. If I have to keep track of what's gone to the
> database, and query for existing instances and manage the transactions, I
> probably should just use the SQL layer directly (I have a lot of
> experiences with databases, but very little with ORMs, so pure DB code
> isn't too scary for me, but on the other hand I don't know what benefits
> the ORM should be giving me that I'm not seeing).
>
> Is this an application that doesn't actually benefit from the ORM? Or am I
> using it wrongly, and my problems come from misunderstanding the way it
> should be used? As I say, I've a lot of database experience but very little
> with ORMs, so maybe I have an unbalanced view of how much data management
> the ORM should be able to handle for me.
>
> The particular problem here is what's affecting me right now - but I'd be
> even more interested in a good "ORM for experienced SQL developers"
> tutorial that tells me how the ORM differs from the core level (and where
> its benefits lie).
>
> Thanks,
> Paul
>
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