Add support for composite unique constraint in auto migration

[atkei]

Related to #172, #175 and based on #957.

Add UniqueConstraint to support composite unique constraint in auto migration.

from piccolo.columns import Text
from piccolo.constraint import UniqueConstraint
from piccolo.table import Table

class Band(Table):
    name = Text(required=True)
    label = Text(required=True)
    unique_name_label = UniqueConstraint(["name", "label"])

Some work remains such as adding doc and testing, but I would like to have feedback on my proposal first.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.85253% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 89.72%. Comparing base (363d683) to head (07f8b94). Report is 1 commits behind head on master.

Files	Patch %	Lines
piccolo/apps/migrations/auto/schema_differ.py	92.85%	3 Missing :warning:
piccolo/apps/migrations/auto/migration_manager.py	97.36%	2 Missing :warning:
piccolo/apps/migrations/auto/diffable_table.py	95.00%	1 Missing :warning:
piccolo/apps/migrations/auto/schema_snapshot.py	80.00%	1 Missing :warning:
piccolo/constraint.py	96.96%	1 Missing :warning:
piccolo/table.py	90.90%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #984 +/- ## ========================================== - Coverage 92.78% 89.72% -3.07% ========================================== Files 108 109 +1 Lines 8182 8399 +217 ========================================== - Hits 7592 7536 -56 - Misses 590 863 +273 ```

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[sinisaos]

@dantownsend I hope it's not a problem, but I have approved and run the workflow for this PR. I have already tried these changes in the @atkei local branch and in my case everything works great as I wrote in this comment. Can you try and review this PR. Thanks.

[dantownsend]

@sinisaos Awesome thanks - feel free to run the CI whenever you want.

@atkei Thanks a lot for this - looks great!

[AlexanderMakarov]

@sinisaos @dantownsend - could you make a hint - when it would be merged? Would it mean new version/release?

[dantownsend]

@AlexanderMakarov I'll try my best to review it properly this week.

[MoSheikh]

Currenty looking for this funtionality - would appreciate it someone on the team would be able to review!

[AlexanderMakarov]

@sinisaos @dantownsend - do you have some plans to deliver this functionality soon? Thank you in advance.

[sinisaos]

@AlexanderMakarov Sorry, but you have to wait for @dantownsend response.

[dantownsend]

Yes, it's definitely high on my radar. I need to try and get some improvements done to Piccolo Admin, but this is high on my list. Thanks for the patience.

[devsarvesh92]

@dantownsend Is this released?

[devsarvesh92]

@dantownsend is there any workaround to acheive this?

[sinisaos]

@devsarvesh92 For now, the workaround is to use raw sql (in migrations or script) to add a composite unique constraint, as described here.

[chilico]

@dantownsend Any advances on this getting merged & released?

[stronk7]

Hi,

great to see this coming! For sure support for composite unique indexes/constraints is a real use case!

I just wanted to introduce here a related question that is… why not make this to also support non-unique composite indexes? After all, the uniqueness is “only” an attribute of the index, isn’t it?

For sure, if in the future support for composite PKs/FKs is added, then the composite UK will need to be considered again (because FKs should be able to point to UKs too. But right now (after having read various issues here), that seems to be not trivial to happen.

So, back to the question, why not get the opportunity of this PR to, also, support non unique indexes?

Ciao :)

Disclaimer: I’m completely new to Piccolo, so maybe what I’m asking doesn’t make much sense or already can be done (apart from doing it in manual migrations), but from the research that I’m doing over various Python ORM libraries, where Piccolo is one of the “finalists”, this is pretty much, the only feature that I’m missing right now, so just sharing.