Re: [sqlalchemy] eager loading relationships after an initial query?

[Jonathan Vanasco]

On Tuesday, April 25, 2017 at 5:50:48 AM UTC-4, Simon King wrote:
 

>
> def reload(instance, *options): 
> session = saorm.object_session(instance) 
> # TODO: make this generic rather than assuming presence of 'id' column 
> q = (session.query(type(instance)) 
>  .filter_by(id=instance.id) 
>  .options(*options)) 
> return q.one() 
>
> (I don't really understand the concern about grabbing the session from 
> the object) 
>


Thanks, Simon.  That's a smart approach.

I wasn't thinking in terms of re-use, and had a one-off need -- and must 
avoid that `reload` functionality as a one-off part of the code.  A 
function like that works perfectly for my needs.

The concern around this is largely on maintainability.  We have a few 
categories of code that can't explicitly use SA, and this falls into one of 
them.  So the issue isn't as much of 'grabbing the session from the object' 
in general, but doing so in that block of code.  

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Re: [sqlalchemy] eager loading relationships after an initial query?

[Simon King]

On Mon, Apr 24, 2017 at 10:26 PM, Jonathan Vanasco
 wrote:
>
> On Monday, April 24, 2017 at 4:28:22 PM UTC-4, Mike Bayer wrote:
>>
>> yeah just load the object again w/ the eagerloads option you want.
>
>
>
> Thanks.  I was hoping there was a way to just say `Obj.load('foo')` ?
>
> I'll just untangle the code and load the relationship in the first place.
> At this point in the code, I don't have a session handy; grabbing it from
> the object isn't preferred for maintenance/readability.
>

Writing that Obj.load function is pretty simple. If you can make some
assumptions about the sorts of objects you will pass to it (eg. that
they always have an "id" column), it's even simpler:

def reload(instance, *options):
session = saorm.object_session(instance)
# TODO: make this generic rather than assuming presence of 'id' column
q = (session.query(type(instance))
 .filter_by(id=instance.id)
 .options(*options))
return q.one()

(I don't really understand the concern about grabbing the session from
the object)

Here's a script to show it in action:

import sqlalchemy as sa
import sqlalchemy.orm as saorm
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Root(Base):
__tablename__ = 'root'
id = sa.Column(sa.Integer, primary_key=True)
name = sa.Column(sa.String())

class Bar(Base):
__tablename__ = 'bar'
id = sa.Column(sa.Integer, primary_key=True)
rootid = sa.Column(sa.ForeignKey(Root.id))
root = saorm.relationship(Root, backref='bars')

class Widget(Base):
__tablename__ = 'widget'
id = sa.Column(sa.Integer, primary_key=True)
barid = sa.Column(sa.ForeignKey(Bar.id))
bar = saorm.relationship(Bar, backref='widgets')

def reload(instance, *options):
session = saorm.object_session(instance)
# TODO: make this generic rather than assuming presence of 'id' column
q = (session.query(type(instance))
 .filter_by(id=instance.id)
 .options(*options))
return q.one()

def loadchildren(root):
i = 0
for bar in root.bars:
for widget in bar.widgets:
i += 1
print 'loaded %s widgets' % i

if __name__ == '__main__':
engine = sa.create_engine('sqlite:///', echo=True)
Base.metadata.create_all(engine)
Session = saorm.sessionmaker(bind=engine)

s = Session()
root = Root()
for i in range(5):
bar = Bar()
root.bars.append(bar)
for j in range(5):
bar.widgets.append(Widget())
s.add(root)
s.commit()
s.close()

print '### without eagerloading ###'
s = Session()
root = s.query(Root).one()
loadchildren(root)

print ' with eagerloading #'
s = Session()
root = s.query(Root).one()
reload(root, saorm.joinedload('bars').joinedload('widgets'))
loadchildren(root)

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The Python SQL Toolkit and Object Relational Mapper

http://www.sqlalchemy.org/

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Re: [sqlalchemy] eager loading relationships after an initial query?

[Jonathan Vanasco]

On Monday, April 24, 2017 at 4:28:22 PM UTC-4, Mike Bayer wrote:
>
> yeah just load the object again w/ the eagerloads option you want. 
>


Thanks.  I was hoping there was a way to just say `Obj.load('foo')` ?

I'll just untangle the code and load the relationship in the first place. 
 At this point in the code, I don't have a session handy; grabbing it from 
the object isn't preferred for maintenance/readability.

-- 
SQLAlchemy - 
The Python SQL Toolkit and Object Relational Mapper

http://www.sqlalchemy.org/

To post example code, please provide an MCVE: Minimal, Complete, and Verifiable 
Example.  See  http://stackoverflow.com/help/mcve for a full description.
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Re: [sqlalchemy] eager loading relationships after an initial query?

[mike bayer]

yeah just load the object again w/ the eagerloads option you want.

On 04/24/2017 04:12 PM, Jonathan Vanasco wrote:
[I couldn't find any docs on this, and my luck with the list archives 
only showed some potential work for this in 2008.]


Is it currently possible to eager-load a ORM relationship after a query?

For example, I have loaded an instance of `Foo`, but I did not do an 
eager load on `Foo.bars` and 'Foo.bars.widget`.  I'd like to instruct 
Foo to eagerload the `Foo.bars.widget`, instead of lazyloading them as 
iterated.



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SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper

http://www.sqlalchemy.org/

To post example code, please provide an MCVE: Minimal, Complete, and 
Verifiable Example. See http://stackoverflow.com/help/mcve for a full 
description.

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The Python SQL Toolkit and Object Relational Mapper


http://www.sqlalchemy.org/

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[sqlalchemy] eager loading relationships after an initial query?

[Jonathan Vanasco]

[I couldn't find any docs on this, and my luck with the list archives only 
showed some potential work for this in 2008.]

Is it currently possible to eager-load a ORM relationship after a query?

For example, I have loaded an instance of `Foo`, but I did not do an eager 
load on `Foo.bars` and 'Foo.bars.widget`.  I'd like to instruct Foo to 
eagerload the `Foo.bars.widget`, instead of lazyloading them as iterated.


-- 
SQLAlchemy - 
The Python SQL Toolkit and Object Relational Mapper

http://www.sqlalchemy.org/

To post example code, please provide an MCVE: Minimal, Complete, and Verifiable 
Example.  See  http://stackoverflow.com/help/mcve for a full description.
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