On Thu, Nov 19, 2020, at 6:15 PM, Vinit Shah wrote:
> I posted this on StackOverflow a few days ago, but I haven't been able to
> figure this one out yet. The original post can be found here: StackOverflow:
> object "is already present in this session"
> <https://stackoverflow.com/questions/64897088/sqlalchemy-object-is-already-present-in-this-session-when-using-a-new-sessio>.
>
> I'm seeing the below error:
> sqlalchemy.exc.InvalidRequestError: Can't attach instance <ObjectType at
> 0x10592fe50>; another instance with key ( <class '__main__.ObjectType'>,
> (1,), None) is already present in this session.
>
> I'm seeing this issue when I try to instantiate a new object that has a
> foreign key relationship with an existing object that was created in another
> session.
>
> This happens in a few different cases in my actual code, but in the provided
> sample it occurs with the following steps:
> 1. Add a new object into a new session
> 2. Close session and remove() from scoped_session
> 3. Reference the object in two newly constructed ones via their relationship
> 4. Error appears on the second object
This can be worked around using the cascade_backrefs option which in 1.4 will
default to the better setting. you can see what's happening here in the stack
trace where you will see some of this:
File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/orm/attributes.py", line
1017, in set
value = self.fire_replace_event(state, dict_, value, old, initiator)
File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/orm/attributes.py", line
1039, in fire_replace_event
value = fn(
File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/orm/attributes.py", line
1472, in emit_backref_from_scalar_set_event
child_impl.append(
File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/orm/attributes.py", line
1244, in append
collection.append_with_event(value, initiator)
File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/orm/collections.py", line
654, in append_with_event
see the "backref" ? your program seeks to add objects to the session using
session.merge() exclusively. but above you can see the offending object is
being "added", that's wrong, it's being "added" due to an "append" to your
collection, which is from a backref, all of that is what you don't want and
it's because of "origin=tokyo, destinationInput=luggage" are reverse-cascading
your object directly into the session including the "origin" which you'd prefer
is merged.
this is all described at
https://docs.sqlalchemy.org/en/13/orm/cascades.html#controlling-cascade-on-backrefs
, however the example there assumes the relationship is configured on the
one-to-many side. so to set this for your example, it's a little more verbose:
class Gate(Thing):
__tablename__ = "gates"
id = Column(Integer, ForeignKey("things.id"), primary_key=True)
originId = Column(Integer, ForeignKey("locations.id"))
origin = relationship(
Location,
foreign_keys=[originId],
backref=backref("originGates", cascade_backrefs=False),
cascade_backrefs=False,
)
originInputId = Column(Integer, ForeignKey("locationinputs.id"))
originInput = relationship(
LocationInput,
foreign_keys=[originInputId],
backref=backref("originInputGates", cascade_backrefs=False),
cascade_backrefs=False,
)
destinationId = Column(Integer, ForeignKey("locations.id"))
destination = relationship(
Location,
foreign_keys=[destinationId],
backref=backref("destinationGates", cascade_backrefs=False),
cascade_backrefs=False,
)
destinationInputId = Column(Integer, ForeignKey("locationinputs.id"))
destinationInput = relationship(
LocationInput,
foreign_keys=[destinationInputId],
backref=backref("destinationInputGates", cascade_backrefs=False),
cascade_backrefs=False,
)
in 1.4, cascade_backrefs defaults to False so you won't need to do this anymore.
> # typeDict just contains a pre-fetched ObjectTypes
> tokyo = Location(name="tokyo", objectType=typeDict['location'])
> tokyo = write(tokyo)
>
> # If I clear out the current session here, the error will occur
> scopedSessionFactory().close()
> scopedSessionFactory.remove()
>
> westGate = Gate(name="westGate", destination=tokyo,
> objectType=typeDict['gate'])
> westGate = write(westGate)
>
> luggage = LocationInput(name="luggage",
> objectType=typeDict['locationinput'])
> luggage = write(luggage)
>
> # This is the line where the error occurs
> eastGate = Gate(name="eastGate", origin=tokyo, destinationInput=luggage,
> objectType=typeDict['gate'])
> eastGate = write(eastGate)
>
> I'm not sure what exactly causes this or way. For this example, I could just
> reuse the same session, but I'd like to be able to take an object from one
> closed session and add as a relationship field to another.
>
> Full code sample available here:
> https://gist.github.com/funseiki/a73424bebfb0d809e6d934f699a725bf
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
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