fix code typo in MysqlAdapter .Closes rails#12647.

Author: senny

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Fix code typo in `MysqlAdapter` when `Encoding.default_internal` is set.
+
+    Fixes #12647.
+
+    *Yves Senn*
+
 *   ActiveRecord::Base#attribute_for_inspect now truncates long arrays (more than 10 elements)
 
     *Jan Bernacki*

activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

@@ -559,7 +559,7 @@ def version
       def set_field_encoding field_name
         field_name.force_encoding(client_encoding)
         if internal_enc = Encoding.default_internal
-          field_name = field_name.encoding(internal_enc)
+          field_name = field_name.encode!(internal_enc)
         end
         field_name
       end

activerecord/test/cases/base_test.rb

@@ -604,10 +604,25 @@ def test_readonly_attributes
   end
 
   def test_unicode_column_name
+    Weird.reset_column_information
     weird = Weird.create(:なまえ => 'たこ焼き仮面')
     assert_equal 'たこ焼き仮面', weird.なまえ
   end
 
+  def test_respect_internal_encoding
+    if current_adapter?(:PostgreSQLAdapter)
+      skip 'pg does not respect internal encoding and always returns utf8'
+    end
+    old_default_internal = Encoding.default_internal
+    silence_warnings { Encoding.default_internal = "EUC-JP" }
+
+    Weird.reset_column_information
+
+    assert_equal ["EUC-JP"], Weird.columns.map {|c| c.name.encoding.name }.uniq
+  ensure
+    silence_warnings { Encoding.default_internal = old_default_internal }
+  end
+
   def test_non_valid_identifier_column_name
     weird = Weird.create('a$b' => 'value')
     weird.reload