Re: [sqlalchemy] Re: many-to-many orm warnings
I'm sorry you're getting bit by this messaging - but also glad that I'm not
the only one. This got me a while ago too.
SqlAlchemy just uses a bare field name when emitting the warning and
accepting the `overlaps` arguments. In more complex models with 3+ tables
that have standardize relationship names, it's hard to tell what caused the
issue and fixing one relationship can unknowingly affect others.
There is a related ticket/PR. I'm not sure if you can pull it against the
current main branch, but you can do a manual patch of the warnings code
locally to make the output better:
https://github.com/sqlalchemy/sqlalchemy/issues/7309 - Make the
overlaps arguments use fully-qualified names
There's also a related ticket to improve the errors when not calling
`configure_mappers` as
above: https://github.com/sqlalchemy/sqlalchemy/issues/7305
On Thursday, March 10, 2022 at 12:27:33 PM UTC-5 Michael Merickel wrote:
> Thank you Mike. Really appreciate you unpacking my rambling. This works
> for me. I found a few spots in our codebase where we were relying on
> append() working because it really was a simple link table but I rewrote
> them to just create the link manually and add it to the session which also
> causes them to appear in the lists.
>
> On Thu, Mar 10, 2022 at 9:17 AM Mike Bayer
> wrote:
>
>> hey there.
>>
>> The warnings go away entirely by making Parent.children viewonly=True,
>> which for this type of mapping is recommended:
>>
>> class Parent(Base):
>> __tablename__ = "left"
>> id = Column(Integer, primary_key=True)
>> children = relationship(
>> "Child", secondary=Association.__table__, backref="parents",
>> viewonly=True
>>
>> )
>>
>>
>> you wouldn't want to append new records to Parent.children because that
>> would create invalid Association rows (missing extra_data).
>>
>> The warning box at the end of
>> https://docs.sqlalchemy.org/en/14/orm/basic_relationships.html#association-object
>>
>> discusses this situation and the desirability of making the relationship
>> which includes "secondary" as viewonly=True.
>>
>> hope this helps
>>
>>
>> On Wed, Mar 9, 2022, at 8:09 PM, Michael Merickel wrote:
>>
>> Sorry for the rambling, it's been difficult for me to figure out what
>> question to ask because I'm so confused. Below is the minimum viable
>> example that produces no warnings with respect to the overlaps flags and I
>> cannot explain hardly any of them. For example, why does Child.parents
>> require "child_links,parent,child"? 3 values that seem to be somewhat
>> unrelated and are at the very least definitely on different models?
>>
>> class Association(Base):
>> __tablename__ = 'association'
>> left_id = Column(ForeignKey('left.id'), primary_key=True)
>> right_id = Column(ForeignKey('right.id'), primary_key=True)
>> extra_data = Column(String(50))
>>
>> parent = relationship('Parent', back_populates='child_links')
>> child = relationship('Child', back_populates='parent_links')
>>
>> class Parent(Base):
>> __tablename__ = 'left'
>> id = Column(Integer, primary_key=True)
>>
>> children = relationship(
>> 'Child',
>> secondary=Association.__table__,
>> back_populates='parents',
>> overlaps='child,parent',
>> )
>> child_links = relationship(
>> 'Association',
>> back_populates='parent',
>> overlaps='children',
>> )
>>
>> class Child(Base):
>> __tablename__ = 'right'
>> id = Column(Integer, primary_key=True)
>>
>> parents = relationship(
>> 'Parent',
>> secondary=Association.__table__,
>> back_populates='children',
>> overlaps='child_links,parent,child',
>> )
>> parent_links = relationship(
>> 'Association',
>> back_populates='child',
>> overlaps='children,parents',
>> )
>>
>>
>> On Wed, Mar 9, 2022 at 4:50 PM Michael Merickel
>> wrote:
>>
>> I think ultimately I want the overlaps config but reading through
>> https://docs.sqlalchemy.org/en/14/errors.html#relationship-x-will-copy-column-q-to-column-p-which-conflicts-with-relationship-s-y
>>
>> it doesn't make any sense to me what the values in the overlaps= argument
>> are referring to. For example in last snippet that was simpler, what is
>> overlaps='parent' referring to? Neither the Parent object, nor the Child
>> object has something named "parent" so other than blinding trusting the
>> warning I'm unclear how to see what the mapper is building that conflicts
>> here.
>>
>> On Wed, Mar 9, 2022 at 4:33 PM Michael Merickel
>> wrote:
>>
>> It's probably worth noting I can narrow it down to a single warning with
>> the following snippet and it's still unclear to me how to resolve this:
>>
>> class Association(Base):
>> __tablename__ = 'association'
>> left_id = Column(ForeignKey('left.id'), primary_key=True)
>> right_id = Column(ForeignKey('right.id'), primary_key=True)
>>
Re: [sqlalchemy] Re: many-to-many orm warnings
Thank you Mike. Really appreciate you unpacking my rambling. This works for
me. I found a few spots in our codebase where we were relying on append()
working because it really was a simple link table but I rewrote them to
just create the link manually and add it to the session which also causes
them to appear in the lists.
On Thu, Mar 10, 2022 at 9:17 AM Mike Bayer wrote:
> hey there.
>
> The warnings go away entirely by making Parent.children viewonly=True,
> which for this type of mapping is recommended:
>
> class Parent(Base):
> __tablename__ = "left"
> id = Column(Integer, primary_key=True)
> children = relationship(
> "Child", secondary=Association.__table__, backref="parents",
> viewonly=True
>
> )
>
>
> you wouldn't want to append new records to Parent.children because that
> would create invalid Association rows (missing extra_data).
>
> The warning box at the end of
> https://docs.sqlalchemy.org/en/14/orm/basic_relationships.html#association-object
> discusses this situation and the desirability of making the relationship
> which includes "secondary" as viewonly=True.
>
> hope this helps
>
>
> On Wed, Mar 9, 2022, at 8:09 PM, Michael Merickel wrote:
>
> Sorry for the rambling, it's been difficult for me to figure out what
> question to ask because I'm so confused. Below is the minimum viable
> example that produces no warnings with respect to the overlaps flags and I
> cannot explain hardly any of them. For example, why does Child.parents
> require "child_links,parent,child"? 3 values that seem to be somewhat
> unrelated and are at the very least definitely on different models?
>
> class Association(Base):
> __tablename__ = 'association'
> left_id = Column(ForeignKey('left.id'), primary_key=True)
> right_id = Column(ForeignKey('right.id'), primary_key=True)
> extra_data = Column(String(50))
>
> parent = relationship('Parent', back_populates='child_links')
> child = relationship('Child', back_populates='parent_links')
>
> class Parent(Base):
> __tablename__ = 'left'
> id = Column(Integer, primary_key=True)
>
> children = relationship(
> 'Child',
> secondary=Association.__table__,
> back_populates='parents',
> overlaps='child,parent',
> )
> child_links = relationship(
> 'Association',
> back_populates='parent',
> overlaps='children',
> )
>
> class Child(Base):
> __tablename__ = 'right'
> id = Column(Integer, primary_key=True)
>
> parents = relationship(
> 'Parent',
> secondary=Association.__table__,
> back_populates='children',
> overlaps='child_links,parent,child',
> )
> parent_links = relationship(
> 'Association',
> back_populates='child',
> overlaps='children,parents',
> )
>
>
> On Wed, Mar 9, 2022 at 4:50 PM Michael Merickel
> wrote:
>
> I think ultimately I want the overlaps config but reading through
> https://docs.sqlalchemy.org/en/14/errors.html#relationship-x-will-copy-column-q-to-column-p-which-conflicts-with-relationship-s-y
> it doesn't make any sense to me what the values in the overlaps= argument
> are referring to. For example in last snippet that was simpler, what is
> overlaps='parent' referring to? Neither the Parent object, nor the Child
> object has something named "parent" so other than blinding trusting the
> warning I'm unclear how to see what the mapper is building that conflicts
> here.
>
> On Wed, Mar 9, 2022 at 4:33 PM Michael Merickel
> wrote:
>
> It's probably worth noting I can narrow it down to a single warning with
> the following snippet and it's still unclear to me how to resolve this:
>
> class Association(Base):
> __tablename__ = 'association'
> left_id = Column(ForeignKey('left.id'), primary_key=True)
> right_id = Column(ForeignKey('right.id'), primary_key=True)
> extra_data = Column(String(50))
>
> parent = relationship('Parent')
>
> class Parent(Base):
> __tablename__ = 'left'
> id = Column(Integer, primary_key=True)
> children = relationship('Child', secondary=Association.__table__)
>
> class Child(Base):
> __tablename__ = 'right'
> id = Column(Integer, primary_key=True)
>
> foo.py:24: SAWarning: relationship 'Parent.children' will copy column
> left.id to column association.left_id, which conflicts with
> relationship(s): 'Association.parent' (copies left.id to
> association.left_id). If this is not the intention, consider if these
> relationships should be linked with back_populates, or if viewonly=True
> should be applied to one or more if they are read-only. For the less common
> case that foreign key constraints are partially overlapping, the
> orm.foreign() annotation can be used to isolate the columns that should be
> written towards. To silence this warning, add the parameter
> 'overlaps="parent"' to the 'Parent.children' relationship. (Background on
> this error at:
Re: [sqlalchemy] Re: many-to-many orm warnings
hey there.
The warnings go away entirely by making Parent.children viewonly=True, which
for this type of mapping is recommended:
class Parent(Base):
__tablename__ = "left"
id = Column(Integer, primary_key=True)
children = relationship(
"Child", secondary=Association.__table__, backref="parents",
viewonly=True
)
you wouldn't want to append new records to Parent.children because that would
create invalid Association rows (missing extra_data).
The warning box at the end of
https://docs.sqlalchemy.org/en/14/orm/basic_relationships.html#association-object
discusses this situation and the desirability of making the relationship which
includes "secondary" as viewonly=True.
hope this helps
On Wed, Mar 9, 2022, at 8:09 PM, Michael Merickel wrote:
> Sorry for the rambling, it's been difficult for me to figure out what
> question to ask because I'm so confused. Below is the minimum viable example
> that produces no warnings with respect to the overlaps flags and I cannot
> explain hardly any of them. For example, why does Child.parents require
> "child_links,parent,child"? 3 values that seem to be somewhat unrelated and
> are at the very least definitely on different models?
>
> class Association(Base):
> __tablename__ = 'association'
> left_id = Column(ForeignKey('left.id'), primary_key=True)
> right_id = Column(ForeignKey('right.id'), primary_key=True)
> extra_data = Column(String(50))
>
> parent = relationship('Parent', back_populates='child_links')
> child = relationship('Child', back_populates='parent_links')
>
> class Parent(Base):
> __tablename__ = 'left'
> id = Column(Integer, primary_key=True)
>
> children = relationship(
> 'Child',
> secondary=Association.__table__,
> back_populates='parents',
> overlaps='child,parent',
> )
> child_links = relationship(
> 'Association',
> back_populates='parent',
> overlaps='children',
> )
>
> class Child(Base):
> __tablename__ = 'right'
> id = Column(Integer, primary_key=True)
>
> parents = relationship(
> 'Parent',
> secondary=Association.__table__,
> back_populates='children',
> overlaps='child_links,parent,child',
> )
> parent_links = relationship(
> 'Association',
> back_populates='child',
> overlaps='children,parents',
> )
>
>
> On Wed, Mar 9, 2022 at 4:50 PM Michael Merickel wrote:
>> I think ultimately I want the overlaps config but reading through
>> https://docs.sqlalchemy.org/en/14/errors.html#relationship-x-will-copy-column-q-to-column-p-which-conflicts-with-relationship-s-y
>> it doesn't make any sense to me what the values in the overlaps= argument
>> are referring to. For example in last snippet that was simpler, what is
>> overlaps='parent' referring to? Neither the Parent object, nor the Child
>> object has something named "parent" so other than blinding trusting the
>> warning I'm unclear how to see what the mapper is building that conflicts
>> here.
>>
>> On Wed, Mar 9, 2022 at 4:33 PM Michael Merickel wrote:
>>> It's probably worth noting I can narrow it down to a single warning with
>>> the following snippet and it's still unclear to me how to resolve this:
>>>
>>> class Association(Base):
>>> __tablename__ = 'association'
>>> left_id = Column(ForeignKey('left.id'), primary_key=True)
>>> right_id = Column(ForeignKey('right.id'), primary_key=True)
>>> extra_data = Column(String(50))
>>>
>>> parent = relationship('Parent')
>>>
>>> class Parent(Base):
>>> __tablename__ = 'left'
>>> id = Column(Integer, primary_key=True)
>>> children = relationship('Child', secondary=Association.__table__)
>>>
>>> class Child(Base):
>>> __tablename__ = 'right'
>>> id = Column(Integer, primary_key=True)
>>>
foo.py:24: SAWarning: relationship 'Parent.children' will copy column
left.id to column association.left_id, which conflicts with
relationship(s): 'Association.parent' (copies left.id to
association.left_id). If this is not the intention, consider if these
relationships should be linked with back_populates, or if viewonly=True
should be applied to one or more if they are read-only. For the less
common case that foreign key constraints are partially overlapping, the
orm.foreign() annotation can be used to isolate the columns that should be
written towards. To silence this warning, add the parameter
'overlaps="parent"' to the 'Parent.children' relationship. (Background on
this error at: https://sqlalche.me/e/14/qzyx)
>>>
>>> On Wed, Mar 9, 2022 at 4:31 PM Michael Merickel wrote:
I have looked at the couple examples in the docs (many-to-many, and
association table) and have noticed that my codebase has a slightly
different pattern which is causing warnings when upgrading to 1.4. I'm
trying
[sqlalchemy] Re: many-to-many orm warnings
Sorry for the rambling, it's been difficult for me to figure out what
question to ask because I'm so confused. Below is the minimum viable
example that produces no warnings with respect to the overlaps flags and I
cannot explain hardly any of them. For example, why does Child.parents
require "child_links,parent,child"? 3 values that seem to be somewhat
unrelated and are at the very least definitely on different models?
class Association(Base):
__tablename__ = 'association'
left_id = Column(ForeignKey('left.id'), primary_key=True)
right_id = Column(ForeignKey('right.id'), primary_key=True)
extra_data = Column(String(50))
parent = relationship('Parent', back_populates='child_links')
child = relationship('Child', back_populates='parent_links')
class Parent(Base):
__tablename__ = 'left'
id = Column(Integer, primary_key=True)
children = relationship(
'Child',
secondary=Association.__table__,
back_populates='parents',
overlaps='child,parent',
)
child_links = relationship(
'Association',
back_populates='parent',
overlaps='children',
)
class Child(Base):
__tablename__ = 'right'
id = Column(Integer, primary_key=True)
parents = relationship(
'Parent',
secondary=Association.__table__,
back_populates='children',
overlaps='child_links,parent,child',
)
parent_links = relationship(
'Association',
back_populates='child',
overlaps='children,parents',
)
On Wed, Mar 9, 2022 at 4:50 PM Michael Merickel wrote:
> I think ultimately I want the overlaps config but reading through
> https://docs.sqlalchemy.org/en/14/errors.html#relationship-x-will-copy-column-q-to-column-p-which-conflicts-with-relationship-s-y
> it doesn't make any sense to me what the values in the overlaps= argument
> are referring to. For example in last snippet that was simpler, what is
> overlaps='parent' referring to? Neither the Parent object, nor the Child
> object has something named "parent" so other than blinding trusting the
> warning I'm unclear how to see what the mapper is building that conflicts
> here.
>
> On Wed, Mar 9, 2022 at 4:33 PM Michael Merickel
> wrote:
>
>> It's probably worth noting I can narrow it down to a single warning with
>> the following snippet and it's still unclear to me how to resolve this:
>>
>> class Association(Base):
>> __tablename__ = 'association'
>> left_id = Column(ForeignKey('left.id'), primary_key=True)
>> right_id = Column(ForeignKey('right.id'), primary_key=True)
>> extra_data = Column(String(50))
>>
>> parent = relationship('Parent')
>>
>> class Parent(Base):
>> __tablename__ = 'left'
>> id = Column(Integer, primary_key=True)
>> children = relationship('Child', secondary=Association.__table__)
>>
>> class Child(Base):
>> __tablename__ = 'right'
>> id = Column(Integer, primary_key=True)
>>
>> foo.py:24: SAWarning: relationship 'Parent.children' will copy column
>>> left.id to column association.left_id, which conflicts with
>>> relationship(s): 'Association.parent' (copies left.id to
>>> association.left_id). If this is not the intention, consider if these
>>> relationships should be linked with back_populates, or if viewonly=True
>>> should be applied to one or more if they are read-only. For the less common
>>> case that foreign key constraints are partially overlapping, the
>>> orm.foreign() annotation can be used to isolate the columns that should be
>>> written towards. To silence this warning, add the parameter
>>> 'overlaps="parent"' to the 'Parent.children' relationship. (Background on
>>> this error at: https://sqlalche.me/e/14/qzyx)
>>
>>
>> On Wed, Mar 9, 2022 at 4:31 PM Michael Merickel
>> wrote:
>>
>>> I have looked at the couple examples in the docs (many-to-many, and
>>> association table) and have noticed that my codebase has a slightly
>>> different pattern which is causing warnings when upgrading to 1.4. I'm
>>> trying to figure out the best pattern to accomplish what I've been doing
>>> which doesn't match the docs exactly.
>>>
>>> In the below example you can see that there are backrefs on all of the
>>> links, and that there are backrefs from the link table to the related
>>> objects, as well as a secondary link from Parent to Child via
>>> Parent.children and Child.parents.
>>>
>>> There seem to be several options and I'm struggling to figure out what
>>> the solution should be to maintain the behavior with all of the following
>>> relationships working:
>>>
>>> - Parent.children
>>> - Parent.child_links
>>> - Child.parents
>>> - Child.parent_links
>>> - Association.parent
>>> - Association.child
>>>
>>> Code and warnings are below:
>>>
>>> from sqlalchemy import Column, ForeignKey, String, Integer
>>> from sqlalchemy.orm import configure_mappers, relationship
>>> from sqlalchemy.ext.declarative import declarative_base
>>>
>>> Base = declarative_base()
>>>
[sqlalchemy] Re: many-to-many orm warnings
I think ultimately I want the overlaps config but reading through
https://docs.sqlalchemy.org/en/14/errors.html#relationship-x-will-copy-column-q-to-column-p-which-conflicts-with-relationship-s-y
it doesn't make any sense to me what the values in the overlaps= argument
are referring to. For example in last snippet that was simpler, what is
overlaps='parent' referring to? Neither the Parent object, nor the Child
object has something named "parent" so other than blinding trusting the
warning I'm unclear how to see what the mapper is building that conflicts
here.
On Wed, Mar 9, 2022 at 4:33 PM Michael Merickel wrote:
> It's probably worth noting I can narrow it down to a single warning with
> the following snippet and it's still unclear to me how to resolve this:
>
> class Association(Base):
> __tablename__ = 'association'
> left_id = Column(ForeignKey('left.id'), primary_key=True)
> right_id = Column(ForeignKey('right.id'), primary_key=True)
> extra_data = Column(String(50))
>
> parent = relationship('Parent')
>
> class Parent(Base):
> __tablename__ = 'left'
> id = Column(Integer, primary_key=True)
> children = relationship('Child', secondary=Association.__table__)
>
> class Child(Base):
> __tablename__ = 'right'
> id = Column(Integer, primary_key=True)
>
> foo.py:24: SAWarning: relationship 'Parent.children' will copy column
>> left.id to column association.left_id, which conflicts with
>> relationship(s): 'Association.parent' (copies left.id to
>> association.left_id). If this is not the intention, consider if these
>> relationships should be linked with back_populates, or if viewonly=True
>> should be applied to one or more if they are read-only. For the less common
>> case that foreign key constraints are partially overlapping, the
>> orm.foreign() annotation can be used to isolate the columns that should be
>> written towards. To silence this warning, add the parameter
>> 'overlaps="parent"' to the 'Parent.children' relationship. (Background on
>> this error at: https://sqlalche.me/e/14/qzyx)
>
>
> On Wed, Mar 9, 2022 at 4:31 PM Michael Merickel
> wrote:
>
>> I have looked at the couple examples in the docs (many-to-many, and
>> association table) and have noticed that my codebase has a slightly
>> different pattern which is causing warnings when upgrading to 1.4. I'm
>> trying to figure out the best pattern to accomplish what I've been doing
>> which doesn't match the docs exactly.
>>
>> In the below example you can see that there are backrefs on all of the
>> links, and that there are backrefs from the link table to the related
>> objects, as well as a secondary link from Parent to Child via
>> Parent.children and Child.parents.
>>
>> There seem to be several options and I'm struggling to figure out what
>> the solution should be to maintain the behavior with all of the following
>> relationships working:
>>
>> - Parent.children
>> - Parent.child_links
>> - Child.parents
>> - Child.parent_links
>> - Association.parent
>> - Association.child
>>
>> Code and warnings are below:
>>
>> from sqlalchemy import Column, ForeignKey, String, Integer
>> from sqlalchemy.orm import configure_mappers, relationship
>> from sqlalchemy.ext.declarative import declarative_base
>>
>> Base = declarative_base()
>>
>> class Association(Base):
>> __tablename__ = 'association'
>> left_id = Column(ForeignKey('left.id'), primary_key=True)
>> right_id = Column(ForeignKey('right.id'), primary_key=True)
>> extra_data = Column(String(50))
>>
>> parent = relationship('Parent', backref='child_links')
>> child = relationship('Child', backref='parent_links')
>>
>> class Parent(Base):
>> __tablename__ = 'left'
>> id = Column(Integer, primary_key=True)
>> children = relationship('Child', secondary=Association.__table__,
>> backref='parents')
>>
>> class Child(Base):
>> __tablename__ = 'right'
>> id = Column(Integer, primary_key=True)
>>
>> configure_mappers()
>>
>> foo.py:25: SAWarning: relationship 'Child.parents' will copy column
>>> right.id to column association.right_id, which conflicts with
>>> relationship(s): 'Association.child' (copies right.id to
>>> association.right_id), 'Child.parent_links' (copies right.id to
>>> association.right_id). If this is not the intention, consider if these
>>> relationships should be linked with back_populates, or if viewonly=True
>>> should be applied to one or more if they are read-only. For the less common
>>> case that foreign key constraints are partially overlapping, the
>>> orm.foreign() annotation can be used to isolate the columns that should be
>>> written towards. To silence this warning, add the parameter
>>> 'overlaps="child,parent_links"' to the 'Child.parents' relationship.
>>> (Background on this error at: https://sqlalche.me/e/14/qzyx)
>>> configure_mappers()
>>> foo.py:25: SAWarning: relationship 'Child.parents' will copy column
>>> left.id to column association.left_id, which conflicts
[sqlalchemy] Re: many-to-many orm warnings
It's probably worth noting I can narrow it down to a single warning with
the following snippet and it's still unclear to me how to resolve this:
class Association(Base):
__tablename__ = 'association'
left_id = Column(ForeignKey('left.id'), primary_key=True)
right_id = Column(ForeignKey('right.id'), primary_key=True)
extra_data = Column(String(50))
parent = relationship('Parent')
class Parent(Base):
__tablename__ = 'left'
id = Column(Integer, primary_key=True)
children = relationship('Child', secondary=Association.__table__)
class Child(Base):
__tablename__ = 'right'
id = Column(Integer, primary_key=True)
foo.py:24: SAWarning: relationship 'Parent.children' will copy column
> left.id to column association.left_id, which conflicts with
> relationship(s): 'Association.parent' (copies left.id to
> association.left_id). If this is not the intention, consider if these
> relationships should be linked with back_populates, or if viewonly=True
> should be applied to one or more if they are read-only. For the less common
> case that foreign key constraints are partially overlapping, the
> orm.foreign() annotation can be used to isolate the columns that should be
> written towards. To silence this warning, add the parameter
> 'overlaps="parent"' to the 'Parent.children' relationship. (Background on
> this error at: https://sqlalche.me/e/14/qzyx)
On Wed, Mar 9, 2022 at 4:31 PM Michael Merickel wrote:
> I have looked at the couple examples in the docs (many-to-many, and
> association table) and have noticed that my codebase has a slightly
> different pattern which is causing warnings when upgrading to 1.4. I'm
> trying to figure out the best pattern to accomplish what I've been doing
> which doesn't match the docs exactly.
>
> In the below example you can see that there are backrefs on all of the
> links, and that there are backrefs from the link table to the related
> objects, as well as a secondary link from Parent to Child via
> Parent.children and Child.parents.
>
> There seem to be several options and I'm struggling to figure out what the
> solution should be to maintain the behavior with all of the following
> relationships working:
>
> - Parent.children
> - Parent.child_links
> - Child.parents
> - Child.parent_links
> - Association.parent
> - Association.child
>
> Code and warnings are below:
>
> from sqlalchemy import Column, ForeignKey, String, Integer
> from sqlalchemy.orm import configure_mappers, relationship
> from sqlalchemy.ext.declarative import declarative_base
>
> Base = declarative_base()
>
> class Association(Base):
> __tablename__ = 'association'
> left_id = Column(ForeignKey('left.id'), primary_key=True)
> right_id = Column(ForeignKey('right.id'), primary_key=True)
> extra_data = Column(String(50))
>
> parent = relationship('Parent', backref='child_links')
> child = relationship('Child', backref='parent_links')
>
> class Parent(Base):
> __tablename__ = 'left'
> id = Column(Integer, primary_key=True)
> children = relationship('Child', secondary=Association.__table__,
> backref='parents')
>
> class Child(Base):
> __tablename__ = 'right'
> id = Column(Integer, primary_key=True)
>
> configure_mappers()
>
> foo.py:25: SAWarning: relationship 'Child.parents' will copy column
>> right.id to column association.right_id, which conflicts with
>> relationship(s): 'Association.child' (copies right.id to
>> association.right_id), 'Child.parent_links' (copies right.id to
>> association.right_id). If this is not the intention, consider if these
>> relationships should be linked with back_populates, or if viewonly=True
>> should be applied to one or more if they are read-only. For the less common
>> case that foreign key constraints are partially overlapping, the
>> orm.foreign() annotation can be used to isolate the columns that should be
>> written towards. To silence this warning, add the parameter
>> 'overlaps="child,parent_links"' to the 'Child.parents' relationship.
>> (Background on this error at: https://sqlalche.me/e/14/qzyx)
>> configure_mappers()
>> foo.py:25: SAWarning: relationship 'Child.parents' will copy column
>> left.id to column association.left_id, which conflicts with
>> relationship(s): 'Association.parent' (copies left.id to
>> association.left_id), 'Parent.child_links' (copies left.id to
>> association.left_id). If this is not the intention, consider if these
>> relationships should be linked with back_populates, or if viewonly=True
>> should be applied to one or more if they are read-only. For the less common
>> case that foreign key constraints are partially overlapping, the
>> orm.foreign() annotation can be used to isolate the columns that should be
>> written towards. To silence this warning, add the parameter
>> 'overlaps="child_links,parent"' to the 'Child.parents' relationship.
>> (Background on this error at: https://sqlalche.me/e/14/qzyx)
>> configure_mappers()
>> foo.py:25:
