On Aug 26, 2010, at 11:42 AM, Chris Withers wrote:
> Hi All,
>
> For the following model:
>
> class Header(Base):
> __tablename__ = 'header'
> id = Column(Integer, primary_key=True)
> message_id = Column(Integer,ForeignKey('message.id'))
> name = Column(String(50))
> value = Column(Text(255))
>
> sqlalchemy-migrate's SchemaDiff tool against the table freshly created in
> MySQL gives:
>
> Schema diffs:
> tables with differences: header
> header with different declaration of columns in database:
> [(Column('value', Text(length=255, convert_unicode=False,
> assert_unicode=None, unicode_error=None, _warn_on_bytestring=False),
> table=<header>), Column(u'value', TINYTEXT(), table=<header>), 'value
> TEXT(255)', 'value TINYTEXT')]
>
> The schema diff stuff basically does the following to get the database column
> declaration with SA 0.6+:
>
> from sqlalchemy.ext import compiler
> from sqlalchemy.schema import DDLElement
> class DefineColumn(DDLElement):
> def __init__(self, col):
> self.col = col
>
> @compiler.compiles(DefineColumn)
> def compile(elem, compiler, **kw):
> return compiler.get_column_specification(elem.col)
>
> def get_column_specification(col):
> return str(DefineColumn(col).compile(dialect=self.conn.dialect))
>
> How can it be changed so that the column reflected from the DB and the column
> calculated from the model end up being the same?
if schemas are being diffed, types can be compared "generically" using "type
affinity". This is described at:
http://www.sqlalchemy.org/trac/wiki/06Migration#ReflectionReturnsDialect-SpecificTypes
You could also take a reflected table and force all of its types down to the
"generic" ones using a hack like column.type.__class__ =
column.type._type_affinity to change the class of each type down to its "basic"
version. But if a diff is all that's needed they should be using that.
(FYI alembic will have no schema diff feature its not something I buy into much)
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