sqlalchemy.exc.CompileError with MYSQL->MYSQL migrate

[buctwang]

Hi,

My source mysql database 'test' has one table and it has two columns:

    id int(11)
    context   longtext

when I migrate it to another database, The type of 'longtext' cause one exception:
sqlalchemy.exc.CompileError: (in table 'new_table', column 'context'): VARCHAR requires a length on dialect mysql

I debug the sqlalchemy source and find the function called is visit_VARCHAR:

def visit_LONGTEXT(self, type_, **kw):
    return self._extend_string(type_, {}, "LONGTEXT")

def visit_VARCHAR(self, type_, **kw):
   if type_.length:
       return self._extend_string(
           type_, {}, "VARCHAR(%d)" % type_.length)
   else:
       raise exc.CompileError(
           "VARCHAR requires a length on dialect %s" %
           self.dialect.name)

I don't understand all _StringType type call this funtion visit_VARCHAR, not vistit_LONGTEXT? And how to deal with longtext type? Thanks.