Some PostGIS functions behave differently based on the arguments types.

Example: ST_Distance will calculate projected units distances for geometry arguments and meters distances for geography arguments.

In PostgreSQL you can easily cast with the :: syntax, but what if you are using sqlalchemy objects or geoalchemy?


1 Answer 1


Here are two ways of casting in python, when a table has a Geometry column.


from geoalchemy2 import Geometry
from sqlalchemy import create_engine
from sqlalchemy import (Column, Integer)
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

SRS_WGS_84 = 4326

ClassBase = declarative_base()

class MyTable(ClassBase):
    id = Column(Integer, primary_key=True)
    geometry = Column(Geometry(srid=SRS_WGS_84, management=True))

LOCAL_TEST_DB = "postgresql+psycopg2:///my_test_db"
DB_ECHO = False

engine = create_engine(LOCAL_TEST_DB, echo=DB_ECHO)
Session = sessionmaker(bind=engine)

1. Casting with Geoalchemy

from geoalchemy2.types import Geography
from sqlalchemy.sql import cast

record1_geography = session.query(cast(MyTable.geometry, Geography(srid=SRS_WGS_84))
                                 ).filter(MyTable.id == 1).one()

2. Casting with PostGIS support function

from sqlalchemy.sql import func

cast(models.Task.geometry, Geography(srid=SRID))

# ref: https://postgis.net/docs/ST_GeogFromWKB.html
record1_geography_2 = session.query(func.ST_GeogFromWKB(MyTable.geometry)
                                   ).filter(MyTable.id == 1).one()

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