On Thursday, September 27, 2012 9:21:57 AM UTC-4, Shawn Wheatley wrote:
>
>
>
>> the most idiomatic way to handle this is to merge the objects in:
>>
>> obj = session.merge(existing_object)
>>
>> this will emit a SELECT for the existing row, then copy the state of
>> "existing_object" to an object located for that primary key, if found.
>> It ensures that the correct choice of "pending" or "persistent" is made
>> depending on if the row already exists.
>>
>
> Thanks for your response Michael. It wasn't clear from my original post,
> but I am using merge to copy from PROD to DEV. My merge function looks
> something like this (simplified, but I'm copying multiple entities)
>
> session_dest.merge(entity)
> session_dest.commit()
> session_dest.expunge_all() # large object graphs were causing me to run
> low on memory, so I merge them one at a time and then clear the local cache.
>
> So, assuming DEV has a single record {acct_id: 1, env_id: 1} and I'm
> copying a record {acct_id: 1, env_id: 4} from PROD, it incorrectly thinks
> that this record should be INSERTed, when in fact there is a constraint
> (acct_id must be unique) that prevents this.
>
> The more I evaluate this, the more I think that correctly modeling the
> unique constraint will fix my problem. Then my before_update handler would
> function but would properly UPDATE the record.
>
I updated my class to include a UniqueConstraint on acct_id:
class CiAcctK(DeclarativeBase):
__tablename__ = 'ci_acct_k'
__table_args__ = {}
#column definitions
acct_id = Column(u'acct_id', CHAR(length=10),
ForeignKey('ci_acct.acct_id'), primary_key=True, nullable=False)
env_id = Column(u'env_id', NUMERIC(precision=6, scale=0),
primary_key=True, nullable=False)
uix_1 = UniqueConstraint(u'acct_id')
I also separated out my event handlers:
@event.listens_for(CiAcctK, "before_update")
def CiAcctK_gen_default_upd(mapper, connection, instance):
print "got here! update"
instance.env_id = ENV_ID
@event.listens_for(CiAcctK, "before_insert")
def CiAcctK_gen_default_ins(mapper, connection, instance):
print "got here! insert"
instance.env_id = ENV_ID
Sure enough, when executing the session.merge, SA is trying to INSERT the
record {acct_id: 1, env_id: 4} even though acct_id: 1 already exists. Any
thoughts on what I'm doing wrong?
Shawn
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