Hi,
I'm using SQLAlchemy (0.7.4) with GeoAlchemy to map tables containing
geographic entities from a PostgreSQL/PostGIS database.
As I have to handle data from several parts of the world, each part using
it's own geographic projection, my testing model which I'll try to describe
is based on:
- a parent class ("TestIMG"), containing common attributes and a shape in
"world-wide" WGS84 coordinates system
- for each projection system, a child class, inheriting from TestIMG and
containing only the shapes in the given projection system.
TestIMG can be seen as an abstract class; discrimination between child
classes is based on a common "SRID" attribute.
I've mapped my classes as follow:
class TestIMG(Base):
__tablename__ = 'test_img'
id = Column('id', Integer, primary_key=True)
data = Column('data', Unicode(20))
wgs_shape = GeometryColumn('wgs_shape', Point(srid=4326))
srid = Column('srid', Integer)
__mapper_args__ = { 'concrete': False,
'polymorphic_on': srid }
class TestIMG_france(TestIMG):
__tablename__ = 'test_img_france'
__mapper_args__ = { 'polymorphic_identity': 2154,
'concrete': True }
id = Column('id', Integer, ForeignKey(TestIMG.id), primary_key=True)
shape = GeometryColumn('shape', Point(srid=2154))
class TestIMG_guyane(TestIMG):
__tablename__ = 'test_img_guyane'
__mapper_args__ = { 'polymorphic_identity': 32622,
'concrete': True }
id = Column('id', Integer, ForeignKey(TestIMG.id), primary_key=True)
shape = GeometryColumn('shape', Point(srid=32622))
TestIMG_union = polymorphic_union({ 'test_img_france':
TestIMG_france.__table__,
'test_img_guyane':
TestIMG_guyane.__table__ },
'projection', 'test_img')
When I make a query in a child class, everything is OK; but when I try to
query the main class, I get an error:
>>> session.query(TestIMG).all()
Traceback (most recent call last):
File "<console>", line 1, in <module>
File
"/var/local/eggs/SQLAlchemy-0.7.4-py2.7-linux-x86_64.egg/sqlalchemy/orm/query.py",
line 1947, in all
return list(self)
File
"/var/local/eggs/SQLAlchemy-0.7.4-py2.7-linux-x86_64.egg/sqlalchemy/orm/query.py",
line 2178, in instances
rows = [process[0](row, None) for row in fetch]
File
"/var/local/eggs/SQLAlchemy-0.7.4-py2.7-linux-x86_64.egg/sqlalchemy/orm/mapper.py",
line 2614, in _instance
return _instance(row, result)
File
"/var/local/eggs/SQLAlchemy-0.7.4-py2.7-linux-x86_64.egg/sqlalchemy/orm/mapper.py",
line 2627, in _instance
tuple([row[column] for column in pk_cols])
File
"/var/local/eggs/SQLAlchemy-0.7.4-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py",
line 2654, in _key_fallback
expression._string_or_unprintable(key))
NoSuchColumnError: "Could not locate column in row for column
'test_img_france.id'"
I've tried to test several configurations but everything fails until now.
I'll also add that I **have to** use declarative form of SQLAlchemy,
because GeoAlchemy doesn't seem to handle anything else :-/
Any help would be greatly welcome !
Best regards,
Thierry
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