Hello Michael,
thank you (all) for sqlalchemy :)
The "uniquifying" had me spooked, and i am afraid it got me confused
completely about connections, sessions, autoflushing and all the
wonderful toys. Still don't know what a good pattern would be. I now
even flush() and commit() directly after every add ... (not that it
helped, ofc). So, lots of gratitude here for answering me on a sunday
evening (for me). Ppl here all of a sudden thought i was going nuts.
Back to it, thank you very much.
cheers
-----Original Message-----
From: Michael Bayer <[email protected]>
To: [email protected]
Sent: Sun, Feb 20, 2011 7:30 pm
Subject: Re: [sqlalchemy] difference between query.count() and
query.all(), concurrency?
On Feb 20, 2011, at 8:40 AM, [email protected] wrote:
Hi listmembers,
i have a problem with a difference i find playing with sqla between
query.count and query.all results.
I have a parent class declaratively mapped to a table in MySQL, there
is a child class with a relationship.
I'm not sure how to create a quick session (i suspect a problem lies
there, but i have no clue) - my usecase is very general, but i am not
sure if i have the lingo right (been reading the docs till blue in the
face, didn't know using this wonderful stuff is so intricate).
In short i use the following, (productdescriptions depending on
languages):
Session = scoped_session(sessionmaker())
session = Session(autoflush=True)
class Products(Base):
products_id = Column(Integer, primary_key=True)
products_loc = Column(String)
product_descriptions = relationship("Products_description",
cascade="all",
backref=backref('product'))
class Products_description(Base):
products_description_id = Column(Integer, primary_key=True)
products_id = Column(Integer, ForeignKey('products.products_id'),
nullable=False)
language_id = Column(Integer, ForeignKey('languages.languages_id'),
nullable=False)
class Languages(Base):
languages_id = Column(Integer, primary_key=True)
etc..
I would expect the product_descriptions to hold two objects if there
are two records in de database (for two languages, but that is
irrelevant here .. or is it?), related via the products_id attribute.
When i query the database like so:
qry =
session.query(Products_description).join(Products).filter(Products_descri
ption.products_id == :someniceproducts_id)
with some nice existing products_id
qry.count() results in 2L (like i would expect, there being two
description records in the db for the product)
but when i issue:
qry.all()
i only get one object as a result.
I clearly - after studying the docs - don't understand sqlalchemy at
all :)
What am i doing wrong? it looks so easy and straightforward that i
find myself checking my chair twice to be sure it's there before
sitting down (..).
count() is going to count rows, not necessarily distinct entities. You
have a join in your query so you're going to get multiple
Products_description rows with the same identifier, for each related
Products. all() gets two rows as well internally, which you'll see if
you enable echo='debug' on your engine, but because they have the same
primary key identifier for Products_description they are "uniqueified"
before being returned.
If you queried for both, query(Products_description, Products), the
"uniquifying" step is removed and you'd get two entries.
Can somebody please HELP me :)
Cheers
Freddy
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