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.

3 files changed, 47 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 490908417a..0492694dac 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   MySQL: Don't lose `auto_increment: true` in the `db/schema.rb`.
+
+    Fixes #30894.
+
+    *Ryuta Kamizono*
+
 *   Fix `COUNT(DISTINCT ...)` for `GROUP BY` with `ORDER BY` and `LIMIT`.
 
     Fixes #30886.
diff --git a/activerecord/lib/active_record/connection_adapters/mysql/schema_dumper.rb b/activerecord/lib/active_record/connection_adapters/mysql/schema_dumper.rb
index 95eb77aea4..d23178e43c 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/schema_dumper.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/schema_dumper.rb
@@ -8,6 +8,7 @@ module ActiveRecord
           def prepare_column_options(column)
             spec = super
             spec[:unsigned] = "true" if column.unsigned?
+            spec[:auto_increment] = "true" if column.auto_increment?
 
             if @connection.supports_virtual_columns? && column.virtual?
               spec[:as] = extract_expression_for_virtual_column(column)
@@ -18,6 +19,12 @@ module ActiveRecord
             spec
           end
 
+          def column_spec_for_primary_key(column)
+            spec = super
+            spec.delete(:auto_increment) if column.type == :integer && column.auto_increment?
+            spec
+          end
+
           def default_primary_key?(column)
             super && column.auto_increment? && !column.unsigned?
           end
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