you can load just primary key attributes, sure, or you can load batches of objects in that correspond to the field of identifiers you're working with. Sort the input in PK order so that you only need to fetch a chunk at a time.
Mapped objects can be hashed normally too, though the default hash isn't on primary key, it's just on object identity in memory. you can override __hash__() if you want. On Jul 29, 2014, at 7:32 AM, Milind Vaidya <kava...@gmail.com> wrote: > > With reference to you comment above "personally I prefer just to emit a > SELECT first, which may be a per-row SELECT or may be a prefetch of all the > rows from the table in question." > > This approach is faster as it allows me to create a bunch of objects that are > new and use add_all() to push them to DB. > > But peers are unhappy about the fact of fetching ALL records every time, > which may also be a performance hit. I had to add the logic of extracting > name from the object selected from DB and get the difference (as in sets) > with new names that were supplied and then create object for all the NEW > names and them push. I > > Can the __hash__ be used for mapper classes or is there any way to take > advantage of PK to check for new objects only in efficient manner ? > > > On Wednesday, July 23, 2014 6:19:16 PM UTC-5, Michael Bayer wrote: > > On Jul 23, 2014, at 4:52 PM, Milind Vaidya <kav...@gmail.com> wrote: > >> Well only caveat here is that I am inserting a list of objects in one go >> using session.add_all() > > > if you do an add_all(), then say flush(), it will emit INSERT statements for > all of them. If any INSERT fails, the whole operation is rolled back. > > there is a tradeoff here between atomicity and ability to skip failures (as > opposed to preventing them ahead of time). you have to make that choice. > > > > >> >> On Wednesday, July 23, 2014 2:01:18 PM UTC-5, Michael Bayer wrote: >> use a savepoint: >> >> from sqlalchemy import exc >> try: >> with session.begin_nested(): >> session.add(MyObject()) >> session.flush() >> except exc.IntegrityError: >> pass >> session.commit() >> >> >> personally I prefer just to emit a SELECT first, which may be a per-row >> SELECT or may be a prefetch of all the rows from the table in question. >> >> >> >> On Jul 23, 2014, at 2:15 PM, Milind Vaidya <kav...@gmail.com> wrote: >> >>> I have a script which updates DB periodically. There are some base table >>> which may undergo some change but very rarely. In such case if this script >>> tries to insert the already present value it will throw integrity error. >>> Is there any way of dealing with this other that first fetch the value >>> manually and then only insert if not present ? >>> >>> -- >>> 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+...@googlegroups.com. >>> To post to this group, send email to sqlal...@googlegroups.com. >>> Visit this group at http://groups.google.com/group/sqlalchemy. >>> For more options, visit https://groups.google.com/d/optout. >> >> >> -- >> 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 tosqlalchemy+...@googlegroups.com. >> To post to this group, send email to sqlal...@googlegroups.com. >> Visit this group at http://groups.google.com/group/sqlalchemy. >> For more options, visit https://groups.google.com/d/optout. > > > -- > 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 tosqlalchemy+unsubscr...@googlegroups.com. > To post to this group, send email to sqlalchemy@googlegroups.com. > Visit this group at http://groups.google.com/group/sqlalchemy. > For more options, visit https://groups.google.com/d/optout. -- 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. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/d/optout.
