Annotate PostgreSQL unique and exclusion constraints

[kamipo]

Summary

Adds annotation support for two PostgreSQL constraint types that annotaterb has been overlooking: unique constraints and exclusion constraints. Also fixes a related leak where the indexes backing those constraints were showing up in the Indexes section.

Motivation

In our application we lean on DEFERRABLE INITIALLY DEFERRED unique and exclusion constraints so we can swap values across rows within a transaction (e.g. reordering position on a list without hitting a mid-transaction uniqueness violation). This is a first-class PostgreSQL feature exposed by Rails 7.1+ as add_unique_constraint ..., deferrable: :deferred / add_exclusion_constraint ..., deferrable: :deferred, but none of it ever made it into annotaterb's output — the deferrable flag was dropped, and unique/exclusion constraints didn't have dedicated sections at all.

Worse, the underlying indexes were still enumerated: since PostgreSQL implements unique/exclusion constraints as indexes with the same name, connection.indexes(table) returns them, and annotaterb was rendering them as ordinary index rows. So a deferrable unique constraint appeared as a plain UNIQUE index with no hint of its actual semantics.

Changes

1. Two new annotation sections, opt-in via config or CLI flags:

   • show_unique_constraints / --show-unique-constraints
   • show_exclusion_constraints / --show-exclusion-constraints

   Both mirror the existing check-constraint annotation pattern (default, markdown, yard, rdoc formats). The output includes column list / expression, USING method, WHERE predicate, and DEFERRABLE INITIALLY IMMEDIATE|DEFERRED.

2. Filter constraint-backed indexes out of the Indexes section, matching ActiveRecord::SchemaDumper#indexes_in_create: indexes whose name matches a unique or exclusion constraint name are excluded. Without this the same object would show up twice once you enable the new sections.

Example output

#
# Indexes
#
#  index_reservations_on_room_id  (room_id)
#
# Unique Constraints
#
#  unique_position  (position) DEFERRABLE INITIALLY DEFERRED
#
# Exclusion Constraints
#
#  no_overlap       (room_id WITH =, during WITH &&) USING gist WHERE (canceled = false) DEFERRABLE INITIALLY DEFERRED

Compatibility

• PostgreSQL-only feature; the accessors (connection.unique_constraints, connection.exclusion_constraints, supports_*_constraints?) exist in Rails 7.1+.
• Guarded with respond_to?(:supports_*_constraints?), so on other adapters or older Rails versions the new sections silently do nothing and the index filter is a no-op — no risk of raising in projects that don't opt in.
• Both show_unique_constraints and show_exclusion_constraints default to false, so no existing annotations change unless the user opts in. The index-filtering change does apply unconditionally (matching Rails' schema dumper), so users of unique/exclusion constraints on Rails 7.1+ will see those rows disappear from Indexes — this is the desired behavior since it removes double reporting, and the info is preserved (and enriched) once the new sections are enabled.

[drwl]

Thanks for submitting this PR. It's a decent size changed so I will need to take time to sit down and review it. The write up is helpful in understanding the problem.

I will plan to do it tomorrow.

[drwl]

Looks good to me. I think we'll want to keep an eye out if any people report regressions and then we can direct them to this pr.