On 7/29/15 11:30 AM, Morgan McClure wrote:
I’ve been looking to make a simple typeadapter based on the HSTORE
data type in postgres to do some value coercion and I have been having
some problems.
I want to coerce the individual values inside my dictionary before
they are put into the postgres HSTORE column and when I select keys, I
want to coerce them back.
What methods do I need to subclass to intercept subkey/indexing
operations as well as full assignment operations?
Normally for a simpler datatype I’d just implement process_bind_param,
process_result_value but that hasn't been working the way I expect it
to with HSTORE.
If it makes the problem simpler I don’t need the MutableExtension to
work so I only need to coerce data on the first assignment to the column.
I'll primarily be selecting data like
Query(MyObjectClass.MyHSTOREType['somekey'])
but occasionally, I may be doing
Query(MyObjectClass.MyHSTOREType)
and would like my postgres->python coercion to work in both scenarios.
the issue here is that for PG's special types like HSTORE, JSON, ARRAY,
JSONB, those objects are of those types sure, but when we use the
special "index" operators, we *dont* get that type back; e.g. for an
HSTORE, myobject['foo'] returns an expression that is of type Text:
>>> from sqlalchemy.dialects import postgresql
>>> from sqlalchemy import column
>>> print column('x', postgresql.HSTORE())['foo'].type
TEXT
The issue of these return types for the PG indexing types has been
coming up a lot
(https://bitbucket.org/zzzeek/sqlalchemy/issues/3499/flip-off-the-hashable-flag-for-pg,
https://bitbucket.org/zzzeek/sqlalchemy/issues/3487/support-direct-mult-dimensional-array).
Something will have to improve in general for this kind of issue.
For now you can apply your type using either cast() or coerce_type:
from sqlalchemy import coerce_type
Query(coerce_type(MyObjectClass.MyHSTOREType['somekey'], MySpecialType))
https://bitbucket.org/zzzeek/sqlalchemy/issues/3503/add-full-control-for-return-type-under-pg
is added as a catchall to look into this.
Thanks in advance
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