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.

activerecord/CHANGELOG.md

+*   `remove_reference` with `foreign_key: true` removes the foreign key before
+    removing the column. This fixes a bug where it was not possible to remove
+    the column on MySQL.
+
+    Fixes #18664.
+
+    *Yves Senn*
+
 *   `find_in_batches` now accepts an `:end_at` parameter that complements the `:start`
      parameter to specify where to stop batch processing.
 

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -662,7 +662,13 @@ def add_reference(table_name, *args)
       #
       #   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

activerecord/test/cases/migration/references_foreign_key_test.rb

@@ -95,6 +95,16 @@ class ReferencesForeignKeyTest < ActiveRecord::TestCase
           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