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, 6 insertions, 0 deletions

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 ed32997d25..3ca1c3c49a 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -662,7 +662,13 @@ module ActiveRecord
       #
       #   remove_reference(:products, :supplier, polymorphic: true)
       #
+      # ====== Remove the reference with a foreign key
+      #
+      #   remove_reference(:products, :user, index: true, foreign_key: true)
+      #
       def remove_reference(table_name, ref_name, options = {})
+        remove_foreign_key table_name, ref_name if options[:foreign_key]
+
         remove_column(table_name, "#{ref_name}_id")
         remove_column(table_name, "#{ref_name}_type") if options[:polymorphic]
       end