HSTORE adapter registration in psycopg2 conflicts with SQLAlchemy

[gthb]

If both django_hstore and SQLAlchemy are used in the same Python process with psycopg2, then django_hstore's adapter registration will cause psycopg2 to return HStoreDict instances to SQLAlchemy, which expects str. This causes errors like AttributeError: 'HStoreDict' object has no attribute 'decode' in sqlalchemy/dialects/postgresql/hstore.py:

    def process(value):
        if value is not None:
            return _parse_hstore(value.decode(encoding))

Some solution for the two to coexist would be good, for people who are using both Django and SQLAlchemy together.

One way might be to register the adapter only per connection/cursor, rather than globally (probably per cursor would be required for people using https://github.com/kennethreitz/django-postgrespool since the connections get shared).

[niwinz]

Hi!

Registering in each connection seems to be much overhead for normal use case, in my opinion, and per connection registering works well with connection pools because it register hstore on each connection retrieval from pool.

(With https://github.com/djangonauts/djorm-ext-pool works well and it not requires special backend...)

[gthb]

“Registering in each connection seems to be much overhead” — did you mean “in each cursor”?

I agree (if that's what you meant) that per-connection makes more sense. The main thing is to get rid of globally=True, to allow django_hstore to coexist with SQLAlchemy.

[nemesifier]

Another settings like: 02d398c ?

[niwinz]

Yes @gthb! I was wrong!

Additional config with good entry on documentation for notify about it for any sqlalchemy users seems a good solution...

I'm currently very busy and I can't find time for it en almost three weeks :(

[nemesifier]

I'm terribly busy too. @gthb pheraphs you could send a pull request with a patch similar to the one added by @niwibe: 02d398c

[DataMarket]

[Edit: This comment was actually from @gthb, mistakenly logged in as DataMarket.]

Me too : ) ... so I'll probably only do this if it becomes necessary to us to use HSTORE fields via Django. We don't need that (yet) so I just reported this and then backed out of using django-hstore for now.

[nemesifier]

I was wondering... the settings instroduced in 02d398c is called HSTORE_GLOBAL_REGISTER, how would this setting be called?

[nemesifier]

also, shouldn't this line https://github.com/djangonauts/django-hstore/blob/master/django_hstore/apps.py#L59 pass unique=HSTORE_GLOBAL_REGISTER too as in line https://github.com/djangonauts/django-hstore/blob/master/django_hstore/apps.py#L75?

[nemesifier]

discussion should continue on #50