MySQL: Don't lose `auto_increment: true` in the `db/schema.rb`

Currently `AUTO_INCREMENT` is implicitly used in the default primary key
definition. But `AUTO_INCREMENT` is not only used for single column
primary key, but also for composite primary key. In that case,
`auto_increment: true` should be dumped explicitly in the
`db/schema.rb`.

Fixes #30894.

1 files changed, 34 insertions, 0 deletions

diff --git a/activerecord/test/cases/adapters/mysql2/auto_increment_test.rb b/activerecord/test/cases/adapters/mysql2/auto_increment_test.rb
new file mode 100644
index 0000000000..4c67633946
--- /dev/null
+++ b/activerecord/test/cases/adapters/mysql2/auto_increment_test.rb
@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "support/schema_dumping_helper"
+
+class Mysql2AutoIncrementTest < ActiveRecord::Mysql2TestCase
+  include SchemaDumpingHelper
+
+  def setup
+    @connection = ActiveRecord::Base.connection
+  end
+
+  def teardown
+    @connection.drop_table :auto_increments, if_exists: true
+  end
+
+  def test_auto_increment_without_primary_key
+    @connection.create_table :auto_increments, id: false, force: true do |t|
+      t.integer :id, null: false, auto_increment: true
+      t.index :id
+    end
+    output = dump_table_schema("auto_increments")
+    assert_match(/t\.integer\s+"id",\s+null: false,\s+auto_increment: true$/, output)
+  end
+
+  def test_auto_increment_with_composite_primary_key
+    @connection.create_table :auto_increments, primary_key: [:id, :created_at], force: true do |t|
+      t.integer :id, null: false, auto_increment: true
+      t.datetime :created_at, null: false
+    end
+    output = dump_table_schema("auto_increments")
+    assert_match(/t\.integer\s+"id",\s+null: false,\s+auto_increment: true$/, output)
+  end
+end