Using Shapley types with geoalchemy2 / sqlalchemy

shapley_sqla.py

import binascii
import sqlalchemy.types
import geoalchemy2.types
from geoalchemy2.shape import WKTElement
import shapely.wkb
import shapely.geometry


class ShapelyGeo(sqlalchemy.types.TypeDecorator):
    """Geometry that speaks in Shapely objects"""

    impl = geoalchemy2.types.Geometry

    def result_processor(self, dialect, coltype):
        def process(value):
            if value is not None:
                return shapely.wkb.loads(binascii.a2b_hex(value))
        return process

    def bind_processor(self, dialect):
        def process(bindvalue):
            if isinstance(bindvalue, WKTElement):
                return 'SRID=%d;%s' % (bindvalue.srid, bindvalue.data)
            if isinstance(bindvalue, shapely.geometry.base.BaseGeometry):
                return bindvalue.to_wkt()
            else:
                return bindvalue
        return process



def an_example():

    class Shape(Base):
        id = Column(Integer, primary_key=True)
        lines = Column(ShapelyGeo('LINESTRING'))


    points_on_line = [(0, 0), (10,343), (323,34)]

    s = Shape(lines=shapely.geometry.LineString(points_on_line))

    session.add(s)
    session.commit()
    session.expunge_all()
    
    s = session.query(Shape).order_by(Shape.id.desc()).first()
    
    type(s.lines)  # shapely.geometry.linestring.LineString
    
    list(s.lines.coords)  # [(0, 0), (10,343), (323,34)]

I couldn't find a simple example of extending geoalchemy2 to use shapley types anywhere. This seems to work ok for me.

ps: I haven't tested the example code