How to monkey patch a class (aka static) method in Rails and a common bug

justin · July 12, 2021, 2:26am

I made this magic monkey patch to inform if the schema cache is really loaded.

However, loaded still does not mean it’s used. I contributed this rails/rails commit for that.

Here’s the code being patched:

rails/rails/blob/main/activerecord/lib/active_record/connection_adapters/schema_cache.rb#L8-L22


def self.load_from(filename)
  return unless File.file?(filename)

  read(filename) do |file|
    if filename.include?(".dump")
      Marshal.load(file)
    else
      if YAML.respond_to?(:unsafe_load)
        YAML.unsafe_load(file)
      else
        YAML.load(file)
      end
    end
  end
end

Do you see the subtle bug in the original, broken version?

Scroll down to see the explanation.

Original Broken Version

  module ConnectionAdapters
    class SchemaCache
      singleton_class.send(:alias_method, :old_load_from, :load_from)

      # Remove this once Rails is upgraded to 7.0 when this fix is available:
      # https://github.com/rails/rails/commit/9f9c1bf79eee9fd0d3cf059916e3a613d3cc4cbb
      def self.load_from(filename)
        old_load_from(filename)
        Rails.logger.info "Loaded schema cache file #{filename}"
      end
    end
  end
end

Fixed Version

  module ConnectionAdapters
    class SchemaCache
      singleton_class.send(:alias_method, :old_load_from, :load_from)

      # Remove this once Rails is upgraded to 7.0 when this fix is available:
      # https://github.com/rails/rails/commit/9f9c1bf79eee9fd0d3cf059916e3a613d3cc4cbb
      def self.load_from(filename)
        result = old_load_from(filename)
        Rails.logger.info "Loaded schema cache file #{filename}"
        result
      end
    end
  end
end

Summary

Yep, I added a logging statement to confirm the file was loaded, and I returned nil.

So the fix is to save the result in a local variable, then log, then return the result.