fix `remove_reference` with `foreign_key: true` on MySQL. #18664.

MySQL rejects to remove an index which is used in a foreign key constraint:

```
ActiveRecord::StatementInvalid: Mysql2::Error: Cannot drop index 'index_copies_on_title_id': needed in a foreign key constraint: ALTER TABLE `copies` DROP `title_id`
```

Removing the constraint before removing the column (and the index) solves this problem.

1 files changed, 10 insertions, 0 deletions

diff --git a/activerecord/test/cases/migration/references_foreign_key_test.rb b/activerecord/test/cases/migration/references_foreign_key_test.rb
index 99de7db70c..cf1328e4d0 100644
--- a/activerecord/test/cases/migration/references_foreign_key_test.rb
+++ b/activerecord/test/cases/migration/references_foreign_key_test.rb
@@ -95,6 +95,16 @@ module ActiveRecord
           end
         end
       end
+
+      test "foreign key column can be removed" do
+        @connection.create_table :testings do |t|
+          t.references :testing_parent, index: true, foreign_key: true
+        end
+
+        assert_difference "@connection.foreign_keys('testings').size", -1 do
+          @connection.remove_reference :testings, :testing_parent, foreign_key: true
+        end
+      end
     end
   end
 end