ST_Union on Rasters in Geoalchemy2 using ORM

Question

I am attempting to perform a union on two rasters I pick from my postgis database using GeoAlchemy2.

I have a Raster table defined:

from geoalchemy2 import Raster
from sqlalchemy.sql import func

Base = declarative_base()

class RasterData(Base):
'''
Raster class
'''
__tablename__ = 'images'
__table_args__ = {"schema": "public"}
raster = Column(Raster)

Then I attempt to use ORM to union two raster tiles using

engine = create_engine(db_str, echo=False)
Session = sessionmaker(bind=engine)
session = Session(expire_on_commit=False)

rasters = session.query(RasterData.raster).limit(2).all()
merged = session.query(func.ST_Union(rasters)).all()

This unfortunately errors out with:

psycopg2.ProgrammingError: can't adapt type 'RasterElement'

The above exception was the direct cause of the following exception:

It seems I might need to cast this as something else but I am unsure what. Any ideas what the code above is missing to complete the union?

Answer 1

I needed to pass the type_ kwarg to the function ST_Union.

E.g.

merged = session.query(func.ST_Union(RasterData.raster, type_=Raster)).all()

Thanks to the developers of Geoalchemy2 who pointed this out.

Example shown in their gallery: Reproject A Raster using ST_Transform usage on Line 54