[sqlalchemy] Cascade Delete, session confusion

[Dmitry Berman]

Hi all,

I just started using SQLA, and I am confused by some cascade delete 
behaviour I am seeing. Please see the code and tests below which show that 
the session is seeing table rows in one area and not in the other:

CODE:

print Product1Mod3
__tablename__ = 'products'
id = Column(Integer, primary_key=True)
product_name = Column(String(250), unique=True)
vendor_id = Column(Integer, ForeignKey('vendors.id'), nullable=False)

vendor = relationship('Vendor', backref = backref('products', 
order_by=id, cascade=all, delete-orphan))

def __init__(self, product_name, vendor_id):
self.product_name = product_name
self.vendor_id = vendor_id

def __repr__(self):
return 'Product: %r Product ID: %r Vendor ID: %r' % 
(self.product_name, self.id, self.vendor_id)


class Module(Base):
__tablename__ = 'modules'
id = Column(Integer, primary_key=True)
module_name = Column(String(250), unique=True)
product_id = Column(Integer, ForeignKey('products.id'), nullable=False)

product = relationship('Product', backref = backref('modules', 
order_by=id, cascade=all, delete-orphan))

def __init__(self, module_name, product_id):
self.module_name = module_name
self.product_id = product_id


def __repr__(self):
return 'Module: %r Module ID: %r Product ID: %r' % 
(self.module_name, self.id ,self.product_id)

TESTING:

msg('Module Tests')
Product2Mod1 = Module('Product2Mod1',1)
Product2Mod2 = Module('Product2Mod2',1)
Product1Mod1 = Module('Product1Mod1',2)
Product1Mod2 = Module('Product1Mod2',2)
Product1Mod3 = Module('Product1Mod3',2)
db_session.add(Product2Mod1)
db_session.add(Product2Mod2)
db_session.add(Product1Mod1)
db_session.add(Product1Mod2)
db_session.add(Product1Mod3)
db_session.commit()
msg(Product2Mod1 Product:)
print Product2Mod1.product

msg('delete tests')

print Query to show all products: \n
print Product.query.all()

print \nQuery to show all modules: \n
print Module.query.all()

print \ndeleting product 1: db_session.delete(Product1) -- 
db_session.commit()
db_session.delete(Product1)
db_session.commit()

print \nQuery to check for changes with products and modules, shows that 
the modules and product are gone:\n
print Product.query.all()
print Module.query.all()

print \nThe modules below belong to the deleted product, they should have 
disappeared, but do not: -- NOT SURE WHY THIS IS HAPPENING
print Product1Mod1
print Product1Mod2
print Product1Mod3

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[sqlalchemy] Re: Cascade Delete, session confusion

[Dmitry Berman]

I also looked at this through SQLite database browser, and the database is 
correct, so is this a Python side error?

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Re: [sqlalchemy] Cascade Delete, session confusion

[Dmitry Berman]

This makes a lot of sense, I just didn't realize how it worked... 

*When I did the following:*
print inspect(Product1Mod1).deleted
print inspect(Product1Mod2).deleted
print inspect(Product1Mod3).deleted

*Instead of just:*
print Product1Mod1 
print Product1Mod2 
print Product1Mod3 

This returned:
True
True
True

And this makes total sense now. Thanks.


On Sunday, March 9, 2014 7:22:49 PM UTC-4, Michael Bayer wrote:


 On Mar 9, 2014, at 3:18 PM, Dmitry Berman dmik...@gmail.com javascript: 
 wrote: 

  
  print \nQuery to check for changes with products and modules, shows 
 that the modules and product are gone:\n 
  print Product.query.all() 
  print Module.query.all() 
  
  print \nThe modules below belong to the deleted product, they should 
 have disappeared, but do not: -- NOT SURE WHY THIS IS HAPPENING 
  print Product1Mod1 
  print Product1Mod2 
  print Product1Mod3 


 not sure what you’re expecting there, are you expecting that the 
 “Product1Mod1” symbol would be modified within your interpreter to be None? 
  Python can’t do that under print inspect(Product1Mod1).deleted
 print inspect(Product1Mod2).deleted
 print inspect(Product1Mod3).deletednormal circumstances.  A variable 
 always points to the thing that it was assigned to, there’s no (normal, 
 non-hacky) mechanism by which variables change into “None” without 
 explicitly being reassigned. 

 Those product objects represent what used to be in those rows.  They have 
 a “deleted” flag you can see: 

  from sqlalchemy import inspect 
  inspect(deleted_product).deleted 
 True 



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