Andrew Pashkin <> wrote:

> Working on this issue I encountered another problem.
> Most indices in the project has no names and because of that,
> developer must reverse-engineer them in every migration.
> Read about that also here [1].
> SQLAlchemy and Alembic provide feature for generation constraint
> names by pattern, specifically to resolve that kind of issues [1].
> I decided to introduce usage of this feature in Murano.
> I've implemented solution that preserves backward-compatibility
> for migration and allows to rename all constraints according
> to patterns safely [2]. With it user, that have already deployed Murano
> will be able to upgrade to new version of Murano without issues.
> There are downsides in this solution:
> - It assumes that all versions of Postgres and MySQL uses the
>  same patterns for constraints names generation.
> - It is hard to implement a test for this solution and it will be slow.
>  Because there is need to reproduce such situation when user has old
>  versions of migrations applied, and then tries to upgrade.

The patch seems to hardcode the conventions for MySQL and Postgresql.   The 
first thought I had was that in order to remove the dependence on them here, 
you’d need to instead simply turn off the “naming_convention” in the MetaData 
if you detect that you’re on one of those two databases.   That would be a 
safer idea than trying to hardcode these conventions (and would also work for 
other kinds of backends).

However, I’m not actually sure that you even need special behavior for these 
two backends.  If an operator runs these migrations on a clean database, then 
the constraints are generated with the consistent names on all backends.   if a 
target database already has these schema constructs present, then these 
migrations are never run; it doesn’t matter that they have the right or wrong 
names already.

I suppose then that the fear is that some PG/MySQL databases will have 
constraints that are named in one convention, and others will have constraints 
using the native conventions.    However, the case now is that all deployments 
are using native conventions, and being able to DROP these constraints is 
already not very feasible unless you again were willing to hardcode those 
naming conventions up forward.    The constraints in these initial migrations, 
assuming you don’t regenerate them, might just need to be left alone, and the 
project proceeds in the future with a consistent convention.

However, it’s probably worthwhile to introduce a migration that does in fact 
rename existing constraints on MySQL and Postgresql.  This would be a migration 
script that emits DROP CONSTRAINT and CREATE CONSTRAINT for all the above 
constraints that have an old name and a new name.  The script would need to 
check the backend, as you’re doing now, in order to run, and yes it would 
hardcode the names of those conventions, but at least it would just be a 
one-time run against only currently deployed databases.   Since your migrations 
are run “live”, the script can make itself a “conditional” run by checking for 
the “old” names and skipping those that don’t exist.      

> Another possible solution is to drop all current migrations and
> introduce new one with correct names.

you definitely shouldn’t need to do that.

> This brings us to new problem - migrations and models are out of sync
> right now in multiple places - there are different field types in
> migrations and models, migrations introduces indices that is absent
> in models, etc.
> And this solution has great downside - it is not backward-compatible,
> so all old users will lost their data.
> We (Murano team) should decide, what solution we want to use.
> [1]
> [2]
> -- 
> With kind regards, Andrew Pashkin.
> cell phone - +7 (985) 898 57 59
> Skype - waves_in_fluids
> e-mail -
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