如何为geoalchemy2类型使用SQLAlchemy@compiles decorator

Question

我正在尝试使用最初为Postgres数据库设计的SQLAlchemy ORMs创建一个内存中的SQLite数据库。为了允许SQLite引擎转换postgres特定的数据类型，我依赖于SQLAlchemy @compiles装饰器，它与sqlalchemy.dialects.postgresql命名空间中的类型一样工作

如何将geoalchemy2.Raster数据类型转换为引擎无关的数据类型（我所关心的可能是sqlalchemy.BLOB类型。重要的是，我可以创建类似的表，它们不需要精确）

这个示例代码按照预期转换了DOUBLE_PRECISION类型，但是我不知道如何修改geoalchemy2.Raster数据类型，它是UserDefinedType。我希望能够为compile_DOUBLE_PRECISION()编写一个类似的方法，将Raster更改为BLOB类型

import sqlalchemy
from sqlalchemy import Column, Integer
from sqlalchemy.dialects.postgresql import DOUBLE_PRECISION
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.dialects.sqlite.base import SQLiteTypeCompiler
from geoalchemy2 import Raster

Base = declarative_base()

class SomeModel(Base):
    __tablename__ = "some_model"
    id = Column(Integer, primary_key=True)
    double_precision = Column(DOUBLE_PRECISION)
    raster = Column(Raster)

# The following @compiles works as intended
@compiles(DOUBLE_PRECISION, 'sqlite')
def compile_DOUBLE_PRECISION(element, compiler: SQLiteTypeCompiler, **kw):
    """ Handles postgres DOUBLE_PRECISION datatype as REAL in sqlite. """
    return compiler.visit_real(element, **kw)

def test_throws_error_due_to_raster_data_type():
    engine = sqlalchemy.create_engine("sqlite:///:memory:")
    Base.metadata.create_all(engine)

Answer 1

我对postgresql不太熟悉，但也许类似的东西可以用

import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base
from geoalchemy2 import Raster

Base = declarative_base()


def raster_if_postgresql_else_blob():
    return sa.BLOB().with_variant(Raster, "postgresql")


class SomeModel(Base):
    __tablename__ = "some_model"
    id = sa.Column(sa.Integer, primary_key=True)
    raster = sa.Column(raster_if_postgresql_else_blob())


lite_engine = sa.create_engine("sqlite:///:memory:", echo=True)
Base.metadata.create_all(lite_engine)