Short:
-------
Is there a way to backfill multiple deferred columns in a declarative object
result instance in a dynamic way when groups can't be predicted in the model?
Long:
--------
First, let me just say thanks for SQLAlchemy. This is my first post to this
list and after working with it for quite a while I've found it to be an
excellent tool for working with the database. My previous work (non-Python)
was done with a database abstraction layer that was more relational and less
object-oriented and I've found SQLAlchemy to be amazing for letting me have my
cake (objects) and eat it too (hand-crafted sql optimizations).
Alright, so a few caveats for background:
1) I'm fairly new to Python (~4 months), but not to programming (~10 years)
2) This is the first time I've used an ORM, so my question may be more about
the object-relational mismatch handling, rather than SQLAlchemy directly.
3) I'm using SQLAlchemy with Flask's plug-in flask-sqlalchemy. That may not
have much do with my question, but just in case there is some subtle difference
between declarative's base model and Flask's db.Model
4) The current project is to use Flask and SQLAlchemy to create a web site with
an existing database in a deployed client-server application (96 Tables, can be
anywhere between ~200MB and 30GB)
5) Assumptions abound... this is a fairly complicated/specific case (I think)
so there may be underlying assumptions about how I'm doing things that are
incorrect. If I'm wrong in those underlying assumptions, then feel free to
challenge them.
6) SQLAlchemy 0.7.8
Cool.
So, I see that using declarative objects has a quite a few advantages; you can
easily add attributes(columns, relationships, etc...), validators, and methods
-- all great stuff for keeping things logically grouped. Then when you get to
performance optimizations there is a significant benefit with larger models to
not fetch all the columns for every request (this is a web app after all, so
lower response times are a goal). Great, so deferred looks like the ticket to
be able to handle this particular mis-match in a good enough way. I can defer
any non-essential columns and if I need one or two other columns down the line
then they'll be lazy-loaded as required.
Contrived example:
class User(db.Model, HelperMixin):
__tablename__ = 'user'
id = db.Column(db.Integer, primary_key=True)
password = db.Column(db.String)
type = db.Column(db.Integer)
first_name = db.Column(db.String)
last_name = db.Column(db.String)
title = db.Column(db.String)
birthday = db.Column(db.Date)
height = db.Column(db.Numeric)
width = db.Column(db.Numeric)
# etc...
def is_valid(self, check_password):
# check password in a horribly insecure, but easy way
return True if check_password == self.password else False
So with this model I want to validate a users password on login, but not load
all the other unnecessary stuff, because login probably doesn't need all the
rest of those columns. Because I also want to keep things simple on the model,
I don't use deferred directly, but rather I created a couple helper methods in
a mixin. (Note that other parts of the application may need more columns or
less columns or different columns, depending on context, so putting deferreds
directly in the model would also be impractical)
The mixin looks like this:
from sqlalchemy.orm import defer
from sqlalchemy.orm.properties import ColumnProperty
from sqlalchemy.orm.util import class_mapper
class HelperMixin(object):
@classmethod
def itercolumns(cls):
for prop in class_mapper(cls).iterate_properties:
if isinstance(prop, ColumnProperty):
yield prop.key
@classmethod
def get_deferred_except(cls, *attributes):
attribute_set = set(attributes)
ret = list()
for name in cls.itercolumns():
if name not in attribute_set:
ret.append(defer(name))
return ret
so with this helper I can context sensitively build up a result object with
just the stuff I need (but without losing the benefits of the associated
methods):
deferred = User.get_deferred_except('id', 'password') # Get list of defer()
instances for all columns, but those specified
user = User.query.options(*deferred).first()
# SQL Emitted -- SELECT id, password FROM user
if user.is_valid(the_password):
# Valid stuff
else:
# Invalid stuff
Ok, well that worked great, but now I need to get the patrons name for some
runtime specific reason. So I do this:
full_name = " ".join([user.title, user.first_name, user.last_name])
I now emit:
SELECT title FROM user
SELECT first_name FROM user
SELECT last_name FROM user
When what I really want at this point, and can predictably know in this case,
is:
SELECT title, first_name, last_name FROM user
So, the question is, what is the best way to back-fill an object in a way that
you keep the number of SQL queries low, while also getting the advantages of
using a declarative instance? Additionally, I'd also wonder if it would be
possible to make a 'faulting' type of object; one where requesting any of the
deferred columns would cause all of the deferred columns in that instance to be
loaded.
Thanks for your time!
-Dave
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