Open Stack

I've seen this come up a few times in reviews and was thinking we should 
put something in the general review checklist wiki for it [1].

Basically I have three things I'd like to have in the list for DB 
migrations:

1. Unique constraints should be named. Different DB engines and 
SQLAlchemy dialects automatically name the constraint their own way, 
which can be troublesome for universal migrations. We should avoid this 
by enforcing that UCs are named when they are created. This means not 
using the unique=True arg in UniqueConstraint if the name arg isn't 
provided.

2. Foreign keys should be named for the same reasons in #1.

3. Foreign keys shouldn't be created against nullable columns. Some DB 
engines don't allow unique constraints over nullable columns and if you 
can't create the unique constraint you can't create the foreign key, so 
we should avoid this. If you need the FK, then the pre-req is to make 
the target column non-nullable. Think of the instances.uuid column in 
nova for example.

Unless anyone has a strong objection to this, I'll update the review 
checklist wiki with these items.

[1] https://wiki.openstack.org/wiki/ReviewChecklist

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Thanks,

Matt Riedemann