Thanks. That could work. The reason we need specific relationships to the child class types is for our front end when it queries the data, which we use sqlalchemy-graphene to do, if we query the base class relationship it returns empty rows and makes pagination impossible as the front end is unable to know how many rows containing the desired type are present when the mixer table contains references to several different classes by using the id shared on the base class.
Mark Aquino ________________________________ From: [email protected] <[email protected]> on behalf of Mike Bayer <[email protected]> Sent: Thursday, December 17, 2020 9:32:40 PM To: noreply-spamdigest via sqlalchemy <[email protected]> Subject: Re: [sqlalchemy] Re: Can't delete cascade many-to-many with polymorphic relationships unless using lazy=dynamic perhaps you are looking to set viewonly=True on this second relationship? that will exclude it from any kind of persistence operation. it will only be used to load things in from the database but not accept or honor any mutations of data. not sure why you need to have two relationships that are identical, but if you do, then yes you want only one of them to be the one that writes changes to the DB. On Thu, Dec 17, 2020, at 7:40 PM, [email protected]<mailto:[email protected]> wrote: My operating assumption is that sqlalchemy looks at each relationship and tries to delete it, but since the previous relationship to the same base class was already deleted, it throws the exception and the session rolls back. The error from above is essentially the same as the actual error in my code base. I guess ideally there would be a way to just disable that functionality but my solution works. Just will take several hours to change everything correctly like this because we have about 200 models that are all interconnected like this. I apologize for not giving a proper working example I would have needed to create a brand new project unfortunately because the code base is so complicated On Thursday, December 17, 2020 at 7:32:43 PM UTC-5 [email protected] wrote: 1. target database = postgres, example queries and stacktrace: >>> from webapp.database.orm.models import ParentClass, ChildClass, >>> ChildChildClass >>> p = ParentClass() >>> c = ChildClass() >>> cc = ChildChildClass() >>> c.children.append(cc) >>> p.children.append(c) >>> session.add(p) >>> session.commit() >>> p = session.query(ParentClass).one() >>> [cc for c in p.children for cc in c.children] [ChildChildClass(id=UUID('1253a435-3330-4e36-bafc-ad8ff5176c4d'))] >>> session.delete(p) >>> session.flush() Traceback (most recent call last): File "<console>", line 1, in <module> File "/Users/aquinmx3/venv/espresso3.8/lib/python3.8/site-packages/sqlalchemy/orm/session.py", line 2496, in flush self._flush(objects) File "/Users/aquinmx3/venv/espresso3.8/lib/python3.8/site-packages/sqlalchemy/orm/session.py", line 2637, in _flush transaction.rollback(_capture_exception=True) File "/Users/aquinmx3/venv/espresso3.8/lib/python3.8/site-packages/sqlalchemy/util/langhelpers.py", line 68, in __exit__ compat.raise_( File "/Users/aquinmx3/venv/espresso3.8/lib/python3.8/site-packages/sqlalchemy/util/compat.py", line 178, in raise_ raise exception File "/Users/aquinmx3/venv/espresso3.8/lib/python3.8/site-packages/sqlalchemy/orm/session.py", line 2597, in _flush flush_context.execute() File "/Users/aquinmx3/venv/espresso3.8/lib/python3.8/site-packages/sqlalchemy/orm/unitofwork.py", line 422, in execute rec.execute(self) File "/Users/aquinmx3/venv/espresso3.8/lib/python3.8/site-packages/sqlalchemy/orm/unitofwork.py", line 538, in execute self.dependency_processor.process_deletes(uow, states) File "/Users/aquinmx3/venv/espresso3.8/lib/python3.8/site-packages/sqlalchemy/orm/dependency.py", line 1104, in process_deletes self._run_crud( File "/Users/aquinmx3/venv/espresso3.8/lib/python3.8/site-packages/sqlalchemy/orm/dependency.py", line 1201, in _run_crud raise exc.StaleDataError( sqlalchemy.orm.exc.StaleDataError: DELETE statement on table 'mix_child_class_child_child_class' expected to delete 1 row(s); Only 0 were matched. >>> KeyboardInterrupt Works when ChildClass is declared as follows instead of as written in original question: class ChildClass(Base): __tablename__ = "child_class" id = Column("id", UUID(as_uuid=True), primary_key=True, default=uuid4()) parents = relationship( "ParentClass", secondary="mix_parent_class_child_class", passive_deletes=True, ) children = relationship( "ChildChildClass", secondary="mix_child_class_child_child_class", cascade="all, delete", ) children2 = relationship( "ChildChildClass", secondary="mix_child_class_child_child_class", passive_deletes=True ) >>> from webapp.database.orm.models import ParentClass, ChildClass, >>> ChildChildClass >>> p = ParentClass() >>> c = ChildClass() >>> cc = ChildChildClass() >>> c.children.append(cc) >>> p.children.append(c) >>> session.add(p) >>> session.commit() >>> p = session.query(ParentClass).one() >>> session.delete(p) >>> session.commit() On Thursday, December 17, 2020 at 7:25:58 PM UTC-5 Mike Bayer wrote: On Thu, Dec 17, 2020, at 7:15 PM, Mark Aquino wrote: They seem to be runnable except the base should be= DeclarativeMeta OK, runnable example is below. how about: 1. target database type (Postgresql?) 2. sample data, inserts, etc. 3. sample queries 4. operation that fails 5. stack trace, etc thanks! from uuid import uuid4 from sqlalchemy import Column from sqlalchemy.dialects.postgresql.base import UUID from sqlalchemy.orm import relationship from sqlalchemy.ext.declarative import declarative_base Base = declarative_base() class ChildClass(Base): __tablename__ = "child_class" id = Column("id", UUID(as_uuid=True), primary_key=True, default=uuid4()) parents = relationship( "ParentClass", secondary="mix_parent_class_child_class", passive_deletes=True, ) children = relationship( "ChildChildClass", secondary="mix_child_class_child_child_class", cascade="all, delete", ) children2 = relationship( "ChildChildClass", secondary="mix_child_class_child_child_class", cascade="all, delete", ) class ChildChildClass(Base): __tablename__ = "child_child_class" id = Column("id", UUID(as_uuid=True), primary_key=True, default=uuid4()) parents = relationship( "ChildClass", secondary="mix_child_class_child_child_class", passive_deletes=True, ) class ParentClass(Base): __tablename__ = "parent_class" id = Column("id", UUID(as_uuid=True), primary_key=True, default=uuid4()) children = relationship( "ChildClass", secondary="mix_parent_class_child_class", cascade="all, delete", ) from sqlalchemy.sql.schema import Table, ForeignKey, Column, UniqueConstraint mix_parent_class_child_class = Table( "mix_parent_class_child_class", Base.metadata, Column( "parent_class_id", ForeignKey("parent_class.id<http://parent_class.id>", ondelete="CASCADE"), nullable=False, ), Column("child_class_id", ForeignKey("child_class.id<http://child_class.id>"), nullable=False), UniqueConstraint( "parent_class_id", "child_class_id", name="uix_parent_class_child_class", ), ) mix_child_class_child_child_class = Table( "mix_child_class_child_child_class", Base.metadata, Column( "child_class_id", ForeignKey("child_class.id<http://child_class.id>", ondelete="CASCADE"), nullable=False, ), Column( "child_child_class_id", ForeignKey("child_child_class.id<http://child_child_class.id>"), nullable=False, ), UniqueConstraint( "child_class_id", "child_child_class_id", name="uix_child_class_child_child_class", ), ) The relationships aren’t to the same target in real life, It’s like this: Class BaseClass: ... Class SubClassA(BaseClass) ... Class SubclassB(BaseClass): ... (Plus Many other subclasses) A Mixer “BaseClassBaseClass” such that child subclassA and subclassB are both present in mix_base_class_bass_class BaseClass has relationships to BaseClass, SubClassA and SubClassB, so you can access only SubClassAs and SubClassBs or all SubClassXs that could be filtered to whatever subclass you need. All these relationships use the same mixer table, because they all share a primary key with BaseClass (and removing the need to make explicit mixed tables for every subclass). Mark Aquino ________________________________ From: [email protected] <[email protected]> on behalf of Mike Bayer <[email protected]> Sent: Thursday, December 17, 2020 7:08:58 PM To: noreply-spamdigest via sqlalchemy <[email protected]> Subject: Re: [sqlalchemy] Re: Can't delete cascade many-to-many with polymorphic relationships unless using lazy=dynamic your examples aren't complete or runnable so I don't really know what the issue is, although having two relationships to the same target class seems a little unusual and I'm not sure why you'd need that. On Thu, Dec 17, 2020, at 6:01 PM, [email protected] wrote: I think I may have just found a solution? Not sure if this is correct but it looks like it worked when i changed the "extra" relationship to passive_deletes=True instead of cascade class ChildClass(XPressoBase): __tablename__ = "child_class" id = Column("id", UUID(as_uuid=True), primary_key=True, default=uuid4()) parents = relationship("ParentClass", secondary="mix_parent_class_child_class", passive_deletes=True) children = relationship("ChildChildClass", secondary="mix_child_class_child_child_class", cascade="all, delete") children2 = relationship("ChildChildClass", secondary="mix_child_class_child_child_class", passive_deletes=True) On Thursday, December 17, 2020 at 5:50:06 PM UTC-5 [email protected] wrote: I have a polymorphic data model where association tables are to base classes and some relationships that link to different child classes to filter out the non-matching base classes, however these seem to result in sqlalchemy being unable to delete cascade properly In a real case lets say i have class Base(): children = relationship("Base", secondary="mix_base_base", cascade="all, delete") someTypes = relationship("SomeType", secondary="mix_base_base", cascade="all, delete") other_types = relationship("OtherType", secondary="mix_base_base", cascade="all") class SomeType(Base): parents = relationship("Base", secondary="mix_base_base", passive_deletes=True) class OtherType(Base): parents = relationship("Base", secondary="mix_base_base", passive_deletes=True) if I delete a base that doesn't have relationships to SomeType and OtherType, then everything works great. However once I add those extra relationships sqlalchemy no longer deletes the children relationships. This can be overcome by using lazy="dynamic" on the relationships, but at GREAT performance cost (about 100%), which isn't good. Does anyone know how to get around this without using dynamic loading? The specific relationships are important for front end pagination of data, but the performance cost is too great for the amount of data in this system to double the query times. a simplified and stupid workable example where I just add a redundant children2 relationship to ChildClass, resulting in the same problem: from uuid import uuid4 from sqlalchemy import Column from sqlalchemy.dialects.postgresql.base import UUID from sqlalchemy.orm import relationship from webapp.database.orm.base import XPressoBase class ChildClass(Base): __tablename__ = "child_class" id = Column("id", UUID(as_uuid=True), primary_key=True, default=uuid4()) parents = relationship("ParentClass", secondary="mix_parent_class_child_class", passive_deletes=True) children = relationship("ChildChildClass", secondary="mix_child_class_child_child_class", cascade="all, delete") children2 = relationship("ChildChildClass", secondary="mix_child_class_child_child_class", cascade="all, delete") class ChildChildClass(Base): __tablename__ = "child_child_class" id = Column("id", UUID(as_uuid=True), primary_key=True, default=uuid4()) parents = relationship("ChildClass", secondary="mix_child_class_child_child_class", passive_deletes=True) class ParentClass(Base): __tablename__ = "parent_class" id = Column("id", UUID(as_uuid=True), primary_key=True, default=uuid4()) children = relationship("ChildClass", secondary="mix_parent_class_child_class", cascade="all, delete") from sqlalchemy.sql.schema import Table, ForeignKey, Column, UniqueConstraint from webapp.database.orm.base import XPressoBase mix_parent_class_child_class = Table( "mix_parent_class_child_class", XPressoBase.metadata, Column("parent_class_id", ForeignKey("parent_class.id<http://parent_class.id>", ondelete="CASCADE"), nullable=False), Column("child_class_id", ForeignKey("child_class.id<http://child_class.id>"), nullable=False), UniqueConstraint( "parent_class_id", "child_class_id", name="uix_parent_class_child_class" ), ) mix_child_class_child_child_class = Table( "mix_child_class_child_child_class", XPressoBase.metadata, Column("child_class_id", ForeignKey("child_class.id<http://child_class.id>", ondelete="CASCADE"), nullable=False), Column("child_child_class_id", ForeignKey("child_child_class.id<http://child_child_class.id>"), nullable=False), UniqueConstraint( "child_class_id", "child_child_class_id", name="uix_child_class_child_child_class" ), ) -- 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. 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