Phase 2: Auto-enable identity primary keys in Migration[8.2]+ (depends on Rails per-adapter-migration-compatibility) [DRAFT]

[yahonda]

Summary

Phase 2 of splitting #2579: build on Phase 1's explicit identity:
option (now merged) and flip the default for create_table to
identity: true on Oracle 12.1+, scoped to migrations declared at
Migration[8.2] or later. Migration[8.1] and earlier, Schema.define,
and direct connection.create_table keep the existing sequence + trigger
behavior, so applications see no change unless they explicitly adopt
Migration[8.2].

This PR is a draft because it depends on the unmerged Rails extension
point AbstractAdapter#migration_compatibility_module_for(migration_class)
plus the Migration::Compatibility::Versioned convention from
yahonda/rails per-adapter-migration-compatibility.
Once that lands on rails/rails#main:

1. Switch Gemfile back to rails/rails#main
2. Mark this PR ready for review

Behavior matrix

Caller	identity: arg	Oracle server	Result
Migration[8.2]+	omitted	≥ 12.1	Identity (V8_2 default)
Migration[8.2]+	omitted	< 12.1	Sequence (V8_2 auto-inject gated by supports_identity_columns?)
Migration[8.2]+	omitted, with sequence_name: / sequence_start_value:	any	Sequence (V8_2 treats sequence options as opt-out)
Migration[8.2]+	true	≥ 12.1	Identity (explicit)
Migration[8.2]+	true	< 12.1	ArgumentError (Phase 1)
Migration[8.2]+	false	any	Sequence (per-table opt-out)
Migration[8.1] or older	omitted	any	Sequence (forced by V8_1 module)
Migration[8.1] or older	true	≥ 12.1	Identity (explicit still honored)
Migration[8.1] or older	true	< 12.1	ArgumentError
Migration[8.1] or older	false	any	Sequence
Schema.define / direct connection.create_table	omitted	any	Sequence (no migration version context, default unchanged)
Schema.define / direct connection.create_table	true	≥ 12.1	Identity (explicit)
Schema.define / direct connection.create_table	false	any	Sequence

Implementation notes

• OracleEnhanced::MigrationCompatibility registers two compat modules
  via Rails' Migration::Compatibility::Versioned:
  • V8_2 forces options[:identity] = true when the option is omitted,
    no sequence_name: / sequence_start_value: is given,
    id: :primary_key is in effect (default), the PK is not composite,
    and the server supports identity columns. This is the new flip,
    scoped to migrations at Migration[8.2]+.
  • V8_1 forces options[:identity] = false for migrations at
    Migration[8.1] or earlier. Because Rails' versioned dispatch
    includes both modules for older migrations and V8_1 sets the
    explicit option first, V8_2 sees options.key?(:identity) and
    leaves it alone — older migrations keep sequence-backed behavior.
• OracleEnhancedSchemaStatements#create_table honors
  options[:identity] exactly as given and does no auto-injection
  of its own. Callers that bypass migration compat (Schema.define,
  direct connection.create_table) therefore keep the existing
  sequence-backed default unless they pass identity: true explicitly.
• OracleEnhancedSchemaStatements#rename_table and #drop_table are
  now identity-aware: they no longer rename or drop <table>_seq for
  tables whose primary key is an Oracle identity column. Previously,
  an unrelated application-owned sequence sharing that name would be
  silently mutated or removed. full_drop already had this protection
  via its ISEQ$$_ filter; this brings the table-level DDL paths in
  line.
• OracleEnhancedSchemaDumper emits identity: true / identity: false
  explicitly for every primary-key table when the connected server
  supports identity columns, so db/schema.rb roundtrips remain
  unambiguous regardless of which loader runs them.
• OracleEnhancedDatabaseStatements#empty_insert_statement_value is
  already in master from Implement empty_insert_statement_value for identity primary keys #2609 — this PR depends on that change so
  identity tables can satisfy Model.create! with no attributes.

Tests

• New migration_compatibility_spec.rb cases exercise the V8_2 / V8_1
  layering, the sequence-option opt-out, and the Schema.define /
  direct-call path that preserves sequence-backed PKs by default.
• New identity_primary_key_spec.rb cases cover identity-aware
  rename_table and drop_table against an unrelated app-owned
  <table>_seq, plus the identity counterparts to the existing
  sequence-prerequisite tests (pk_and_sequence_for, NEXTVAL-free
  insert, full_drop filter, long-name rename).

Test plan

• bundle exec rubocop — clean
• Full spec suite on Oracle 23ai (FREEPDB1) — 573 examples, 0 failures, 7 pending
• CI on full matrix (3.3 / 3.4 / 4.0 / jruby)
• Switch Gemfile back to rails/rails#main once per-adapter-migration-compatibility lands

Related

• Phase 1: Add explicit identity: option to create_table for Oracle 12.1+ #2579 (explicit identity: option) — merged
• Phase 2 prerequisite: Implement empty_insert_statement_value for identity primary keys #2609 (empty_insert_statement_value) — merged
• Rails dependency: yahonda/rails per-adapter-migration-compatibility

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