From 79096618f04e08b9c1e9ba3100dd889ba8fd08e6 Mon Sep 17 00:00:00 2001
From: Iliana Hadzhiatanasova <me@iliana.dev>
Date: Mon, 1 Aug 2022 18:26:27 +0100
Subject: [PATCH] Optimize add_timestamps to use a single SQL statement

---
 activerecord/CHANGELOG.md                       | 14 ++++++++++++++
 .../abstract/schema_statements.rb               | 10 ++--------
 .../connection_adapters/sqlite3_adapter.rb      | 13 +++++++++++++
 .../test/cases/migration/columns_test.rb        | 17 +++++++++++++++++
 4 files changed, 46 insertions(+), 8 deletions(-)

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 501ef1bd32092..aae2d4850ecfb 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,17 @@
+*   Optimize `add_timestamps` to use a single SQL statement.
+
+    ```ruby
+    add_timestamps :my_table
+    ```
+
+    Now results in the following SQL:
+
+    ```sql
+    ALTER TABLE "my_table" ADD COLUMN "created_at" datetime(6) NOT NULL, ADD COLUMN "updated_at" datetime(6) NOT NULL
+    ```
+
+    *Iliana Hadzhiatanasova*
+
 *   Add `drop_enum` migration command for PostgreSQL
 
     This does the inverse of `create_enum`. Before dropping an enum, ensure you have
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index 8eb4dd9e061fb..bbfe017709b53 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -1355,14 +1355,8 @@ def distinct_relation_for_primary_key(relation) # :nodoc:
       #   add_timestamps(:suppliers, null: true)
       #
       def add_timestamps(table_name, **options)
-        options[:null] = false if options[:null].nil?
-
-        if !options.key?(:precision) && supports_datetime_with_precision?
-          options[:precision] = 6
-        end
-
-        add_column table_name, :created_at, :datetime, **options
-        add_column table_name, :updated_at, :datetime, **options
+        fragments = add_timestamps_for_alter(table_name, **options)
+        execute "ALTER TABLE #{quote_table_name(table_name)} #{fragments.join(', ')}"
       end
 
       # Removes the timestamp columns (+created_at+ and +updated_at+) from the table definition.
diff --git a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
index 6c0d7c9064de7..a5f1e8f3d6f78 100644
--- a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
@@ -318,6 +318,19 @@ def rename_column(table_name, column_name, new_column_name) # :nodoc:
         rename_column_indexes(table_name, column.name, new_column_name)
       end
 
+      def add_timestamps(table_name, **options)
+        options[:null] = false if options[:null].nil?
+
+        if !options.key?(:precision)
+          options[:precision] = 6
+        end
+
+        alter_table(table_name) do |definition|
+          definition.column :created_at, :datetime, **options
+          definition.column :updated_at, :datetime, **options
+        end
+      end
+
       def add_reference(table_name, ref_name, **options) # :nodoc:
         super(table_name, ref_name, type: :integer, **options)
       end
diff --git a/activerecord/test/cases/migration/columns_test.rb b/activerecord/test/cases/migration/columns_test.rb
index 73252b885ce47..496825c397671 100644
--- a/activerecord/test/cases/migration/columns_test.rb
+++ b/activerecord/test/cases/migration/columns_test.rb
@@ -368,6 +368,23 @@ def test_remove_columns_single_statement
       ensure
         connection.drop_table :my_table, if_exists: true
       end
+
+      def test_add_timestamps_single_statement
+        connection.create_table "my_table"
+
+        # SQLite3's ALTER TABLE statement has several limitations. To manage
+        # this, the adapter creates a temporary table, copies the data, drops
+        # the old table, creates the new table, then copies the data back.
+        expected_query_count = current_adapter?(:SQLite3Adapter) ? 12 : 1
+        assert_queries(expected_query_count) do
+          connection.add_timestamps("my_table")
+        end
+
+        columns = connection.columns("my_table").map(&:name)
+        assert_equal ["id", "created_at", "updated_at"], columns
+      ensure
+        connection.drop_table :my_table, if_exists: true
+      end
     end
   end
 end