From a951d1077052aec8446f1ddf40ec021044f14d11 Mon Sep 17 00:00:00 2001
From: Bastian Bartmann <bastian.bartmann@shopify.com>
Date: Wed, 4 Dec 2024 16:55:11 +0100
Subject: [PATCH] Changing column null does not change default function

---
 activerecord/CHANGELOG.md                            |  4 ++++
 .../connection_adapters/abstract_mysql_adapter.rb    |  6 +++++-
 activerecord/test/cases/migration/columns_test.rb    | 12 ++++++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 9abfc5799e566..c5f937d4a9283 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -102,4 +102,8 @@
 
     *Kazuma Watanabe*
 
+*   Fix MySQL default functions getting dropped when changing a column's nullability.
+
+    *Bastian Bartmann*
+
 Please check [8-0-stable](https://github.com/rails/rails/blob/8-0-stable/activerecord/CHANGELOG.md) for previous changes.
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index 4808d137cf2ab..4ec13c85b35de 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -403,7 +403,11 @@ def build_change_column_definition(table_name, column_name, type, **options) # :
         type ||= column.sql_type
 
         unless options.key?(:default)
-          options[:default] = column.default
+          options[:default] = if column.default_function
+            -> { column.default_function }
+          else
+            column.default
+          end
         end
 
         unless options.key?(:null)
diff --git a/activerecord/test/cases/migration/columns_test.rb b/activerecord/test/cases/migration/columns_test.rb
index 99a3c148b391d..51260e68a0679 100644
--- a/activerecord/test/cases/migration/columns_test.rb
+++ b/activerecord/test/cases/migration/columns_test.rb
@@ -346,6 +346,18 @@ def test_change_column_null_with_non_boolean_arguments_raises
         assert_equal "change_column_null expects a boolean value (true for NULL, false for NOT NULL). Got: #{{ from: true, to: false }}", e.message
       end
 
+      def test_change_column_null_does_not_change_default_functions
+        skip unless current_adapter?(:Mysql2Adapter, :TrilogyAdapter)
+
+        connection.change_column_default "test_models", "created_at", -> { "(NOW())" }
+        TestModel.reset_column_information
+        assert_equal "(now())", TestModel.columns_hash["created_at"].default_function
+
+        connection.change_column_null "test_models", "created_at", true
+        TestModel.reset_column_information
+        assert_equal "(now())", TestModel.columns_hash["created_at"].default_function
+      end
+
       def test_remove_column_no_second_parameter_raises_exception
         assert_raise(ArgumentError) { connection.remove_column("funny") }
       end