Fix/236 unique index before self fk

[WillG2001]

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[WillG2001]

Summary

• Fix migration generation so identity-based unique indexes on a table are created before self-referential foreign keys that depend on them (issue Migration script places unique index below relationships they're needed for #236).
• Add a regression test to ensure the unique index on template_phase.id is emitted before the follows foreign key to template_phase.id.

Background / Motivation

When a resource uses:

• an identity on :id (creating a unique index on id), and
• a self-referential belongs_to (e.g. follows pointing to id on the same table),

the migration generator previously produced migrations where the foreign key on follows could be emitted before (or otherwise out of order with respect to) the unique index on id. This could cause migration failures or brittle ordering.

Fixes issue: #236

Implementation

• In AshPostgres.MigrationGenerator.Operation.AddUniqueIndex, add an insert_after_attribute_order field to track where the identity unique index should sit relative to added attributes.
• In AshPostgres.MigrationGenerator:
  • Compute insert_after_attribute_order from the resource’s attributes when building %Operation.AddUniqueIndex{}.
  • In do_fetch_operations/4, split attribute operations into:
    • creates_and_adds (create/add operations), and
    • alter_and_rest (alter operations),
      and splice unique_indexes_to_add between these so identity unique indexes are generated after new columns but before alters that add FKs.
  • Extend after?/2 ordering rules so that:
    • CreateTable is always ordered before AddUniqueIndex for the same table.
    • AddUniqueIndex is placed after the last AddAttribute for the table but not after AlterAttribute that introduces a FK which depends on that index.
• In AshPostgres.MigrationGeneratorTest:
  • Add "unique index is created before dependent foreign key (issue #236)" which defines Template, Phase, and TemplatePhase with:
    • uuid_primary_key :id,
    • an identity on :id,
    • a self-referential belongs_to :template_phase, __MODULE__, source_attribute: :follows.
  • Generate migrations into a tmp dir and assert that:
    • create unique_index(:template_phase, [:id], ...) is present, and
    • it appears before the references(:template_phase, ...) line used for the follows foreign key.

How to test

From the ash_postgres repo root:

Run the focused regression test

mix test test/migration_generator_test.exs --only issue_236

Run the full test suite

mix test

Static analysis (as per project conventions)

mix credo
mix dialyzer

[zachdaniel]

Doesn't this just end up placing them at the end if its using the max order?

[zachdaniel]

If its being used to sort after a given attribute then it would make sense to track the name instead I think.