On May 6, 2010, at 3:35 AM, George V. Reilly wrote:
> Our production database uses MySQL. For various reasons, the schema
> often uses MySQL-specific DDL. We want that represented in our
> mapper classes. We also awant to be able to exercise our SQLAlchemy
> unit tests against both local MySQL databases and SQLite databases.
>
> Using the MySQL-specific types wasn't a problem with SA 0.5.x, but
> it is with 0.6.
would be curious to know why that is.
> I've started working around it like this:
>
> class MSBigInteger(sqltypes.TypeDecorator):
> impl = sqltypes.Integer
>
> def load_dialect_impl(self, dialect):
> return dialect.type_descriptor(
> mysql.base.MSBigInteger if dialect.name == 'mysql'
> else sqltypes.INTEGER)
>
> This approach falls apart when the type's __init__ takes custom
> parameters, such as collation or charset for the strings.
>
> class MSLongText(sqltypes.TypeDecorator):
> impl = sqltypes.String
>
> def load_dialect_impl(self, dialect):
> self.dialect = dialect
> return dialect.type_descriptor(
> mysql.base.MSLongText if dialect.name == 'mysql' else
> sqltypes.TEXT)
>
> def __init__(self, **kwargs):
> # FIXME: figure out what the dialect is in __init__
> (somehow) so can call
> # correct dialect_impl
> if 'collation' in kwargs: del kwargs['collation']
> if 'charset' in kwargs: del kwargs['charset']
> super(MSLongText, self).__init__(**kwargs)
>
> What's a good way to fix this? I'm not wedded to the TypeDecorator
> for MSBigInteger either, if there's a better approach.
your load_dialect_impl() should be returning the fully constructed instance of
MSBigInteger, there's no need to call dialect.type_descriptor here. If the
type did have some DBAPI-specific subclass that needs to be invoked there,
type_descriptor should be handed the fully constructed MSBigInteger, not the
class.
ultimately I'd like to add a new type to SQLAlchemy core which serves the
purpose of "delegating" some type to different backend implementations. It
would look like this:
Column('foo', CompositeType(default=Integer(),
mysql=mysql.INTEGER(precision=12))
CompositeType is a subclass of TypeDecorator and only needs to store those
initial types, and return them within load_dialect_impl(). Since you've
already written most of that logic yourself you can try going with that
approach.
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