Default to identity primary keys in Migration[8.2]+

[yahonda]

Adopt the standalone ActiveRecord::Migration::CompatibilityBehavior extension point (rails/rails PR-pending, currently tracked on yahonda/rails branch per-adapter-migration-compatibility-v7) to default new-enough migrations to Oracle identity primary keys. Supersedes #2813 and #2596.

Behavior

A migration declaring 8.2 now creates an identity primary key on Oracle Database 12.1 or higher, with no change to the migration code:

class CreatePosts < ActiveRecord::Migration[8.2]
  def change
    create_table :posts do |t|   # id: NUMBER GENERATED BY DEFAULT AS IDENTITY
      t.string :title
    end
  end
end

No posts_seq sequence is created, and inserts no longer query posts_seq.NEXTVAL — the database assigns the id.

The same table under Migration[8.1] or earlier keeps the sequence-backed primary key, so every existing migration replays unchanged:

class CreatePosts < ActiveRecord::Migration[8.1]
  def change
    create_table :posts do |t|   # id: NUMBER + posts_seq sequence, as today
      t.string :title
    end
  end
end

An explicit identity: always wins over the version default, in both directions:

class CreatePosts < ActiveRecord::Migration[8.1]
  def change
    create_table :posts, identity: true do |t|   # identity, even on 8.1
      t.string :title
    end
  end
end

class CreateAudits < ActiveRecord::Migration[8.2]
  def change
    create_table :audits, identity: false do |t| # sequence, even on 8.2
      t.string :action
    end
  end
end

Resulting primary key by caller and option (Oracle Database 12.1+)

caller	(no identity:)	identity: true	identity: false
Migration[8.2] and later	identity (the only cell this PR changes)	identity	sequence
Migration[8.1] and earlier	sequence	identity	sequence
Schema.define / ActiveRecord::Schema[x.y].define / direct connection.create_table	sequence	identity	sequence

On Oracle Database releases before 12.1 every cell resolves to a sequence-backed key, except identity: true, which raises ArgumentError — both as today. A Migration[8.2]+ create_table passing a sequence-backed option (sequence_name:, sequence_start_value:, primary_key_trigger:) behaves like the middle row: the option opts it out of the identity default instead of raising about an identity: true the author never wrote.

Schema dumps written by older releases reload unchanged (schema loads keep the sequence default), and identity-backed tables are dumped with identity: true, so dump -> load round-trips preserve both kinds.

Implementation

• Fills in the V8_2/V8_1 behaviors that Add CompatibilityBehavior plumbing for per-adapter migration compatibility #2823 introduces as empty skeletons:
  • V8_2#create_table forces options[:identity] = true when no :identity is given, no sequence options are set, the connection supports identity columns, and the caller is not a schema load (checked via ActiveRecord::Schema::Definition, which both plain Schema.define and versioned ActiveRecord::Schema[x.y].define include), so schema dumps keep the sequence default.
  • V8_1#create_table forces options[:identity] = false to keep the pre-8.2 sequence default on older migrations.
• rename_table and drop_table become identity-aware: they skip the paired <table>_seq rename/drop when the table's primary key is an Oracle identity column, so unrelated application-owned sequences with matching names are left alone.
• The schema dumper emits identity: true for identity-backed primary keys. Sequence-backed primary keys carry no annotation: identity: false is the adapter's baseline default (Schema.define, direct connection.create_table, and Migration[8.1] and earlier all keep the sequence default), so emitting it on every sequence-backed table would be redundant for reload and add schema.rb churn.

Tests

identity_primary_key_spec.rb (round-trip + identity-aware rename/drop; the previously always-skipped “identity counterparts” block now runs) and migration_compatibility_spec.rb covering the version matrix: Migration[8.2] / Migration[8.1] / Migration[7.0], plain Schema.define and versioned ActiveRecord::Schema[8.2].define loads, explicit identity: overrides, and the sequence-option / primary_key_trigger: opt-outs. CI runs the suite against Oracle.

Depends on

• Add CompatibilityBehavior plumbing for per-adapter migration compatibility #2823: Add CompatibilityBehavior plumbing for per-adapter migration compatibility — merge that first; its commit is the first of the two commits in this PR's diff.
• Rails framework PR introducing compatibility_behavior_for and Migration::CompatibilityBehavior (currently yahonda/rails branch per-adapter-migration-compatibility-v7). Add CompatibilityBehavior plumbing for per-adapter migration compatibility #2823's commit points the Gemfile at that branch.

Independent of #2817: after #2823, either PR can merge first; whichever merges second needs a trivial rebase (both touch V8_1#create_table and append to migration_compatibility_spec.rb).

Review follow-up (2026-07-02)

A deeper review pass added the following fixes, since folded into the single commit:

• Versioned schema loads kept the sequence default only for the unversioned form. rake db:schema:dump writes ActiveRecord::Schema[8.2].define, whose anonymous class is not a subclass of ActiveRecord::Schema, so the is_a?(ActiveRecord::Schema) guard missed it and db:schema:load would have recreated every sequence-backed table as identity (breaking dump -> load -> dump idempotence). The guard now checks ActiveRecord::Schema::Definition, with a spec loading through ActiveRecord::Schema[8.2].define.
• primary_key_trigger: true now opts out of the identity default. Previously a Migration[8.2] create_table :t, primary_key_trigger: true raised about combining it with an identity: true the author never wrote.
• The "identity counterparts" spec block was always skipped. Its guard read @oracle12c_or_higher, which is never assigned in that file; it now uses the database_version comparison. The unskipped NEXTVAL assertion was tightened to /\.NEXTVAL/ so it does not match the has_primary_key_trigger? lookup SQL, which contains the literal NEXTVAL INTO :NEW.
• Spec/comment cleanups: non-deprecated database_version comparison in a before(:all) hook, removal of an unused table cleanup.

[yahonda]

Builds on #2823 (its commit is included here), which in turn depends on rails/rails#58162 — the Active Record change that adds the compatibility_behavior_for extension point. CI here runs against that rails branch until #58162 merges.