I'm starting to understand it! Just a few very quick questions: 1. Is it a good strategy to always use onupdate='CASCADE', ondelete='CASCADE' for all foreign keys if the db supports it (in my case Postgres 9.5 + psycopg2)?
2. If I'd like to use it on an already populated db, can I "alter table" to use these CASCADE options (for example in an alembic migration)? 3. For delete, is this simply an optimisation step and the default behaviour is also perfectly fine? 4. Simply adding the above parameters is not automatically changing SQLAlchemy's delete strategy, I also need to tell it to use passive deletes. Do I also need the cascade="all, delete-orphan" option like in the docs? Zsolt On 15 February 2017 at 17:25, mike bayer <mike...@zzzcomputing.com> wrote: > > > On 02/15/2017 09:45 AM, Zsolt Ero wrote: >> >> 4. An interesting thing is that SQLAlchemy does 3 select calls in the >> delete case, even if 1 would be enough. Can be seen in the logs. > > > the ORM only deletes rows one at a time based on primary key match. So if > you have a relationship() that is configured to cascade deletes, and you > have not instructed the system that "ON DELETE CASCADE" will take care of > those collections, it will need to ensure these collections are present in > memory (e.g. the SELECT) and target each row for deletion individually. You > see only one DELETE statement but you'll note it has multiple parameter sets > to indicate every row being deleted. Background on how to optimize this is > at > http://docs.sqlalchemy.org/en/latest/orm/collections.html#using-passive-deletes > . > > In this specific case there seem to be two SELECT statements but that > appears to be because of the awkward production of a new object that has the > same primary key as another object. In the logs you'll see an UPDATE but > this is actually a special case "hack"; normally, we'd see a DELETE of the > old row and an INSERT of the new one, however the unit of work does not > support this process. There is an option to allow it to work this way in > specific cases, although this feature is not present in SQLAlchemy at this > time. In the absence of that feature, the behavior is that if the same > primary key is present on one object being deleted and another one being > added in the same flush, they are rolled into an UPDATE. Then the > collection is being deleted and re-added again too, so this is a bit of a > crazy example; using a straight UPDATE with correct cascade rules is > obviously much more efficient. > > > >> >> Zsolt >> >> >> >> >> >> On 15 February 2017 at 04:17, mike bayer <mike...@zzzcomputing.com> wrote: >>> >>> >>> >>> On 02/14/2017 08:15 PM, Zsolt Ero wrote: >>>> >>>> >>>> I would like to change a primary key's value, to be deterministic, based >>>> on a multi-to-multi relation. Thus I'm populating the tables with a >>>> temporary ids (just random strings), then calculating the right, unique >>>> id, and changing it afterwards. >>> >>> >>> >>> the examples seem to move "transaction.manager" around, which we assume >>> is >>> the Zope transaction manager and that by using the context manager the >>> Session.commit() method is ultimately called, which raises this error. >>> One >>> guess is that in the second two examples, the Session is not actually >>> getting committed, because no invocation of "dbsession" is present within >>> the "with transaction.manager" block and I have a vague recollection that >>> zope.transaction might work this way. Another guess is that in the >>> second >>> two examples, maybe you already changed the data in the DB and the >>> operation >>> you're doing has no net change to the rows. >>> >>> In any case, all three examples you should echo the SQL emitted so you >>> can >>> see what it's doing. Setting up the onupdate="CASCADE" should fix this >>> problem. As to why that didn't work from you, keep in mind that is a >>> CREATE >>> TABLE directive so if you just changed it in your model and didn't >>> recreate >>> the tables, or at least recreate the foreign key constraints using ALTER >>> to >>> drop and create them again with the CASCADE rule set up; this is a server >>> side rule. >>> >>> Here's the MCVE to demonstrate: >>> >>> from sqlalchemy import * >>> from sqlalchemy.orm import * >>> from sqlalchemy.ext.declarative import declarative_base >>> import md5 as _md5 >>> import random >>> import string >>> >>> >>> def md5(text): >>> return str(_md5.md5(text)) >>> >>> >>> def random_string(num): >>> return ''.join(random.choice( >>> string.ascii_uppercase + string.digits) for _ in range(num)) >>> >>> Base = declarative_base() >>> >>> >>> class Image(Base): >>> __tablename__ = 'images' >>> id = Column(String, primary_key=True, default=lambda: >>> random_string(16)) >>> collections = relationship( >>> 'Collection', secondary='collections_images', >>> back_populates='images') >>> date_created = Column(DateTime, default=func.now()) >>> >>> >>> class Collection(Base): >>> __tablename__ = 'collections' >>> id = Column(String, primary_key=True, default=lambda: >>> random_string(16)) >>> name = Column(String) >>> images = relationship( >>> 'Image', secondary='collections_images', >>> back_populates='collections', >>> order_by='desc(Image.date_created)', >>> lazy='dynamic') >>> >>> >>> collections_images = Table( >>> 'collections_images', Base.metadata, >>> Column('collection_id', >>> ForeignKey('collections.id', onupdate="CASCADE"), >>> primary_key=True), >>> Column('image_id', ForeignKey('images.id'), primary_key=True) >>> ) >>> >>> e = create_engine("postgresql://scott:tiger@localhost/test", echo=True) >>> >>> Base.metadata.drop_all(e) >>> Base.metadata.create_all(e) >>> >>> s = Session(e) >>> >>> with s.transaction: >>> s.add(Collection(name='c1', images=[Image(), Image(), Image()])) >>> >>> with s.transaction: >>> collections = s.query(Collection).all() >>> >>> for collection in collections: >>> image_ids = [i.id for i in collection.images.all()] >>> image_ids_string = ','.join(sorted(image_ids)) + collection.name >>> collection.id = md5(image_ids_string)[:16] >>> >>> >>> >>> >>>> >>>> I have the following models: >>>> >>>> |classImage(Base):id >>>> >>>> >>>> =Column(String,primary_key=True,default=lambda:random_string(16))collections >>>> >>>> >>>> =relationship('Collection',secondary='collections_images',back_populates='images')classCollection(Base):id >>>> =Column(String,primary_key=True,default=lambda:random_string(16))images >>>> >>>> >>>> =relationship('Image',secondary='collections_images',back_populates='collections',order_by='desc(Image.date_created)',lazy='dynamic')Table('collections_images',Base.metadata,Column('collection_id',ForeignKey('collections.id'),primary_key=True),Column('image_id',ForeignKey('images.id'),primary_key=True))| >>>> >>>> My problem is the following: >>>> >>>> 1. >>>> >>>> Out of the 3 examples below, only one triggers an integrity >>>> exception, the other two does not. >>>> >>>> Why? >>>> >>>> In all three I'm trying to write to this primary key, which is >>>> referenced, thus should produce an exception. Yet, in 2. and 3. it >>>> seem nothing is happening when |collection.id =| is set. When I >>>> debug via SQL queries it shows absolutely nothing called for >>>> the |collection.id =| line. >>>> >>>> 2. >>>> >>>> How can I solve this problem? I mean how can I change a primary >>>> key's value which is also used in a multi-to-multi relation? >>>> >>>> The DB is PostgreSQL 9.5 with psycopg2. >>>> >>>> The examples are: >>>> >>>> >>>> A. triggers exception: >>>> >>>> |withtransaction.manager:collections >>>> =dbsession.query(Collection).all()forcollection incollections:image_ids >>>> =[i.id fori incollection.images.all()]image_ids_string >>>> =','.join(sorted(image_ids))+collection.name collection.id >>>> =md5(image_ids_string)[:16]| >>>> >>>> >>>> B. does not trigger exception >>>> >>>> |collections =dbsession.query(Collection).all()# ^ and v only these two >>>> lines are swapped withtransaction.manager:forcollection >>>> incollections:image_ids =[i.id fori >>>> incollection.images.all()]image_ids_string >>>> =','.join(sorted(image_ids))+collection.name collection.id >>>> =md5(image_ids_string)[:16]| >>>> >>>> >>>> C. also does not trigger exception >>>> >>>> |collections =dbsession.query(Collection).all()forcollection >>>> incollections:image_ids =[i.id fori >>>> incollection.images.all()]image_ids_string >>>> =','.join(sorted(image_ids))+collection.name >>>> withtransaction.manager:collection.id =md5(image_ids_string)[:16]| >>>> >>>> >>>> The exception for the first one is: >>>> >>>> | >>>> sqlalchemy.exc.IntegrityError:(psycopg2.IntegrityError)update ordeleteon >>>> table "collections"violates foreign key constraint >>>> "fk_collections_images_collection_id_collections"on table >>>> "collections_images"DETAIL:Key(id)=(jC3sN8952urTGrqz)isstill referenced >>>> fromtable "collections_images". >>>> >>>> | >>>> >>>> I've also tried onupdate='CASCADE' for both columns >>>> in collections_images but didn't change anything. >>>> >>>> -- >>>> 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. >>>> --- >>>> You received this message because you are subscribed to the Google >>>> Groups "sqlalchemy" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to sqlalchemy+unsubscr...@googlegroups.com >>>> <mailto:sqlalchemy+unsubscr...@googlegroups.com>. >>>> To post to this group, send email to sqlalchemy@googlegroups.com >>>> <mailto:sqlalchemy@googlegroups.com>. >>>> Visit this group at https://groups.google.com/group/sqlalchemy. >>>> For more options, visit https://groups.google.com/d/optout. >>> >>> >>> >>> -- >>> 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. >>> --- You received this message because you are subscribed to a topic in >>> the >>> Google Groups "sqlalchemy" group. >>> To unsubscribe from this topic, visit >>> https://groups.google.com/d/topic/sqlalchemy/breBc7iStF0/unsubscribe. >>> To unsubscribe from this group and all its topics, send an email to >>> sqlalchemy+unsubscr...@googlegroups.com. >>> To post to this group, send email to sqlalchemy@googlegroups.com. >>> Visit this group at https://groups.google.com/group/sqlalchemy. >>> For more options, visit https://groups.google.com/d/optout. >> >> > > -- > 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. > --- You received this message because you are subscribed to a topic in the > Google Groups "sqlalchemy" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/sqlalchemy/breBc7iStF0/unsubscribe. > To unsubscribe from this group and all its topics, send an email to > sqlalchemy+unsubscr...@googlegroups.com. > To post to this group, send email to sqlalchemy@googlegroups.com. > Visit this group at https://groups.google.com/group/sqlalchemy. > For more options, visit https://groups.google.com/d/optout. -- 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. --- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. 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