On May 23, 2013, at 1:29 AM, Anthony Kong <[email protected]> wrote:
> Hi, all,
>
> We are upgrading our application to use SA0.8.0. For reason outside our
> control, some XP workstations are using old sybase ODBC driver and we cannot
> upgrade them as of yet.
>
> The sybase driver will cause this problem in the following situation:
>
> 1) we have a table that have a field of type 'bit' to hold boolean value
> (i.e. 0 for false and 1 for true)
>
> 2) we need to select subset data out of this table based on this flag
>
> When we run something like
>
> session.query(SubProduct).filter(Product.isTradeable==True)
>
> we get this error message:
>
> DatabaseError: (DatabaseError) Msg 257, Level 16, State 1, Line 1
> Implicit conversion from datatype 'CHAR' to 'BIT' is not allowed. Use the
> CONVERT function to run this query.
> Msg 257, Level 16, State 1, Line 1
> Implicit conversion from datatype 'CHAR' to 'BIT' is not allowed. Use the
> CONVERT function to run this query.
> 'SELECT SUBPRODUCT ... WHERE Product.id_tradeable = @id_tradeable_1'
> {'@id_tradeable_1': '1'}
>
> I am trying to put in place a workaround: If I can convert the sql statement
> to something like so
>
> 'SELECT SUBPRODUCT ... WHERE Product.id_tradeable = convert(bit,
> @id_tradeable_1)' {'@id_tradeable_1': '1'}
>
>
> I believe I can avoid the sybase SQL exception regardless of client driver
> version
>
> So I implemented this class
>
>
> import sqlalchemy.types as satypes
> from sqlalchemy import func
> from sqlalchemy.sql.expression import type_coerce
>
> class SybaseBit( satypes.TypeDecorator):
> impl = satypes.Boolean
>
> def bind_expression(self, bindvalue):
> # bindvalue = type_coerce(bindvalue, satypes.Integer)
> return func.convert(bit, bindvalue)
>
> It failed because 'bit' is not defined there. If I put in a string 'bit', the
> generated sql is very close to what I want.
>
> So my question is:
>
> What is the proper way to implement this?
you can get the string "bit" in there using sqlalchemy.literal_column('bit').
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