On 9/6/15 11:28 AM, Demitri Muna wrote:
Hi Ian,
Thanks for the reply.
On Saturday, September 5, 2015 at 11:53:39 AM UTC-4, Ian McCullough
wrote:
Is there some compelling reason you wouldn't just install the
PostGIS extensions? Assuming there is...
There is; I can't use any of the functionality. My use case is
astronomical data where all values are points on a sphere. I can't use
the various projections that GIS provides. I investigated the
possibility of using GIS tools for astronomical data a few years back,
but found it wasn't really appropriate. I actually came across this
which I found amusing:
http://lists.osgeo.org/pipermail/postgis-devel/2009-March/005184.html
Actually, now that I think about it, since you won't be able to
use them in query expressions anyway, there's probably no reason
to go to the extra effort of hybrid properties; just make a pure
python property to do the conversion between the mapped TEXT|BLOB
column and the Geometry types.
PostrgreSQL has a native "polygon" data type that I'd like to use:
http://www.postgresql.org/docs/current/static/datatype-geometric.html
There is a library that is specifically designed for astronomical data
(which indexes points on a sphere):
https://github.com/segasai/q3c
and uses the polygon data type in this function:
q3c_poly_query(ra, dec, poly) -- returns true if ra, dec is within
the postgresql polygon poly.
This is all I really need. I imagine my class will look something like
this:
class Field(Base):
__tablename__ = 'field'
__table_args__ = {'autoload' : True}
polygon_column = ???
Where I can do:
f = Field()
f.polygon = ???
I just don't know what to put into the '???'s.
SQLAlchemy doesn't do much else with types at the most basic level other
than pass the data through to the DBAPI, in this case hopefully
psycopg2. Feel free to set the column type to NullType and just pass
through strings, assuming that's what psycopg2 does here by default, or
if you'd like to define your own type that translates some Python value
to what psycopg2 expects here, there is UserDefinedType as well as
TypeDecorator:
http://docs.sqlalchemy.org/en/rel_1_0/core/custom_types.html#types-custom
qc3_poly_query would be a SQL function; any function name is available
from func:
http://docs.sqlalchemy.org/en/rel_1_0/core/sqlelement.html?highlight=func#sqlalchemy.sql.expression.func
Thanks,
Demitri
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