examples.postgis.postgis — SQLAlchemy 1.2 Documentation

Release: 1.2.19 | Release Date: April 15, 2019

# SQLAlchemy 1.2 Documentation

# Source code for examples.postgis.postgis

    import binascii

    from sqlalchemy import event
    from sqlalchemy import Table
    from sqlalchemy.sql import expression
    from sqlalchemy.sql import type_coerce
    from sqlalchemy.types import UserDefinedType

    # Python datatypes

    class GisElement(object):
        """Represents a geometry value."""

        def __str__(self):
            return self.desc

        def __repr__(self):
            return "<%s at 0x%x; %r>" % (
                self.__class__.__name__,
                id(self),
                self.desc,
            )

    class BinaryGisElement(GisElement, expression.Function):
        """Represents a Geometry value expressed as binary."""

        def __init__(self, data):
            self.data = data
            expression.Function.__init__(
                self, "ST_GeomFromEWKB", data, type_=Geometry(coerce_="binary")
            )

        @property
        def desc(self):
            return self.as_hex

        @property
        def as_hex(self):
            return binascii.hexlify(self.data)

    class TextualGisElement(GisElement, expression.Function):
        """Represents a Geometry value expressed as text."""

        def __init__(self, desc, srid=-1):
            self.desc = desc
            expression.Function.__init__(
                self, "ST_GeomFromText", desc, srid, type_=Geometry
            )

    # SQL datatypes.

    class Geometry(UserDefinedType):
        """Base PostGIS Geometry column type."""

        name = "GEOMETRY"

        def __init__(self, dimension=None, srid=-1, coerce_="text"):
            self.dimension = dimension
            self.srid = srid
            self.coerce = coerce_

        class comparator_factory(UserDefinedType.Comparator):
            """Define custom operations for geometry types."""

            # override the __eq__() operator
            def __eq__(self, other):
                return self.op("~=")(other)

            # add a custom operator
            def intersects(self, other):
                return self.op("&&")(other)

            # any number of GIS operators can be overridden/added here
            # using the techniques above.

        def _coerce_compared_value(self, op, value):
            return self

        def get_col_spec(self):
            return self.name

        def bind_expression(self, bindvalue):
            if self.coerce == "text":
                return TextualGisElement(bindvalue)
            elif self.coerce == "binary":
                return BinaryGisElement(bindvalue)
            else:
                assert False

        def column_expression(self, col):
            if self.coerce == "text":
                return func.ST_AsText(col, type_=self)
            elif self.coerce == "binary":
                return func.ST_AsBinary(col, type_=self)
            else:
                assert False

        def bind_processor(self, dialect):
            def process(value):
                if isinstance(value, GisElement):
                    return value.desc
                else:
                    return value

            return process

        def result_processor(self, dialect, coltype):
            if self.coerce == "text":
                fac = TextualGisElement
            elif self.coerce == "binary":
                fac = BinaryGisElement
            else:
                assert False

            def process(value):
                if value is not None:
                    return fac(value)
                else:
                    return value

            return process

        def adapt(self, impltype):
            return impltype(
                dimension=self.dimension, srid=self.srid, coerce_=self.coerce
            )

    # other datatypes can be added as needed.

    class Point(Geometry):
        name = "POINT"

    class Curve(Geometry):
        name = "CURVE"

    class LineString(Curve):
        name = "LINESTRING"

    # ... etc.

    # DDL integration
    # PostGIS historically has required AddGeometryColumn/DropGeometryColumn
    # and other management methods in order to create PostGIS columns.  Newer
    # versions don't appear to require these special steps anymore.  However,
    # here we illustrate how to set up these features in any case.

    def setup_ddl_events():
        @event.listens_for(Table, "before_create")
        def before_create(target, connection, **kw):
            dispatch("before-create", target, connection)

        @event.listens_for(Table, "after_create")
        def after_create(target, connection, **kw):
            dispatch("after-create", target, connection)

        @event.listens_for(Table, "before_drop")
        def before_drop(target, connection, **kw):
            dispatch("before-drop", target, connection)

        @event.listens_for(Table, "after_drop")
        def after_drop(target, connection, **kw):
            dispatch("after-drop", target, connection)

        def dispatch(event, table, bind):
            if event in ("before-create", "before-drop"):
                regular_cols = [
                    c for c in table.c if not isinstance(c.type, Geometry)
                ]
                gis_cols = set(table.c).difference(regular_cols)
                table.info["_saved_columns"] = table.c

                # temporarily patch a set of columns not including the
                # Geometry columns
                table.columns = expression.ColumnCollection(*regular_cols)

                if event == "before-drop":
                    for c in gis_cols:
                        bind.execute(
                            select(
                                [
                                    func.DropGeometryColumn(
                                        "public", table.name, c.name
                                    )
                                ],
                                autocommit=True,
                            )
                        )

            elif event == "after-create":
                table.columns = table.info.pop("_saved_columns")
                for c in table.c:
                    if isinstance(c.type, Geometry):
                        bind.execute(
                            select(
                                [
                                    func.AddGeometryColumn(
                                        table.name,
                                        c.name,
                                        c.type.srid,
                                        c.type.name,
                                        c.type.dimension,
                                    )
                                ],
                                autocommit=True,
                            )
                        )
            elif event == "after-drop":
                table.columns = table.info.pop("_saved_columns")

    setup_ddl_events()

    # illustrate usage
    if __name__ == "__main__":
        from sqlalchemy import (
            create_engine,
            MetaData,
            Column,
            Integer,
            String,
            func,
            select,
        )
        from sqlalchemy.orm import sessionmaker
        from sqlalchemy.ext.declarative import declarative_base

        engine = create_engine(
            "postgresql://scott:tiger@localhost/test", echo=True
        )
        metadata = MetaData(engine)
        Base = declarative_base(metadata=metadata)

        class Road(Base):
            __tablename__ = "roads"

            road_id = Column(Integer, primary_key=True)
            road_name = Column(String)
            road_geom = Column(Geometry(2))

        metadata.drop_all()
        metadata.create_all()

        session = sessionmaker(bind=engine)()

        # Add objects.  We can use strings...
        session.add_all(
            [
                Road(
                    road_name="Jeff Rd",
                    road_geom="LINESTRING(191232 243118,191108 243242)",
                ),
                Road(
                    road_name="Geordie Rd",
                    road_geom="LINESTRING(189141 244158,189265 244817)",
                ),
                Road(
                    road_name="Paul St",
                    road_geom="LINESTRING(192783 228138,192612 229814)",
                ),
                Road(
                    road_name="Graeme Ave",
                    road_geom="LINESTRING(189412 252431,189631 259122)",
                ),
                Road(
                    road_name="Phil Tce",
                    road_geom="LINESTRING(190131 224148,190871 228134)",
                ),
            ]
        )

        # or use an explicit TextualGisElement
        # (similar to saying func.GeomFromText())
        r = Road(
            road_name="Dave Cres",
            road_geom=TextualGisElement(
                "LINESTRING(198231 263418,198213 268322)", -1
            ),
        )
        session.add(r)

        # pre flush, the TextualGisElement represents the string we sent.
        assert str(r.road_geom) == "LINESTRING(198231 263418,198213 268322)"

        session.commit()

        # after flush and/or commit, all the TextualGisElements
        # become PersistentGisElements.
        assert str(r.road_geom) == "LINESTRING(198231 263418,198213 268322)"

        r1 = session.query(Road).filter(Road.road_name == "Graeme Ave").one()

        # illustrate the overridden __eq__() operator.

        # strings come in as TextualGisElements
        r2 = (
            session.query(Road)
            .filter(Road.road_geom == "LINESTRING(189412 252431,189631 259122)")
            .one()
        )

        r3 = session.query(Road).filter(Road.road_geom == r1.road_geom).one()

        assert r1 is r2 is r3

        # core usage just fine:

        road_table = Road.__table__
        stmt = select([road_table]).where(
            road_table.c.road_geom.intersects(r1.road_geom)
        )
        print(session.execute(stmt).fetchall())

        # TODO: for some reason the auto-generated labels have the internal
        # replacement strings exposed, even though PG doesn't complain

        # look up the hex binary version, using SQLAlchemy casts
        as_binary = session.scalar(
            select([type_coerce(r.road_geom, Geometry(coerce_="binary"))])
        )
        assert as_binary.as_hex == (
            "01020000000200000000000000b832084100000000"
            "e813104100000000283208410000000088601041"
        )

        # back again, same method !
        as_text = session.scalar(
            select([type_coerce(as_binary, Geometry(coerce_="text"))])
        )
        assert as_text.desc == "LINESTRING(198231 263418,198213 268322)"

        session.rollback()

        metadata.drop_all()