On Thursday, June 7, 2012 4:16:12 PM UTC-4, Alex Ogier wrote:
> This isn't particularly robust. The SQL string generated by a
> particular backend isn't considered part of any API, and might change
> formatting or semantics on minor updates. Some SQL is quite
> complicated and dubiously related to the particular field, such as
> extra INDEX or UNIQUE constraint statements. Certain backends elide
> some arguments, performing constraint checking in the application (the
> sqlite backend for example just drops many constraints). In general,
> you want the migrations to be DB-agnostic, and checking SQL output
> loses this feature.
While the entire raw SQL generated by a field may be too unstable to
inspect, the underlying SQL shouldn't be completely ignored. At the very
least, the datatype needs to be inspected when freezing to detect if the
rug has been pulled out from under the ORM.
A real world use case. Django-mssql uses the datetime datatype for datetime
fields. Several years ago, Microsoft made the recommendation that all new
work should use datetime2. A future version of django-mssql will switch to
using datetime2. It would be great if the proposed migration framework
handled that sort of switch, or at the very least displayed a warning about
the need for manual user intervention.
The same type of problem could be encountered if a database backend becomes
unsupported and users are forced to switch to another backend where the
maintainer follows a different set of "best practices". There are many ways
of storing the same data that are all completely valid. The most likely
problem field will be IPAddressField due to varchar(15) vs. unsigned int.