Merge pull request #45744 from fatkodima/mysql-change-column-collation

Preserve collation when changing column in MySQL

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -391,6 +391,10 @@ def build_change_column_definition(table_name, column_name, type, **options) # :
           options[:comment] = column.comment
         end
 
+        unless options.key?(:collation)
+          options[:collation] = column.collation
+        end
+
         unless options.key?(:auto_increment)
           options[:auto_increment] = column.auto_increment?
         end

activerecord/test/cases/adapters/mysql2/charset_collation_test.rb

@@ -48,6 +48,15 @@ class Mysql2CharsetCollationTest < ActiveRecord::Mysql2TestCase
     assert_equal "utf8mb4_general_ci", column.collation
   end
 
+  test "change column preserves collation" do
+    @connection.add_column :charset_collations, :description, :string, charset: "utf8mb4", collation: "utf8mb4_unicode_ci"
+    @connection.change_column :charset_collations, :description, :text
+
+    column = @connection.columns(:charset_collations).find { |c| c.name == "description" }
+    assert_equal :text, column.type
+    assert_equal "utf8mb4_unicode_ci", column.collation
+  end
+
   test "schema dump includes collation" do
     output = dump_table_schema("charset_collations")
     assert_match %r/create_table "charset_collations", id: { type: :string, collation: "utf8mb4_bin" }/, output