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

@@            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.