Building an API with Rails and I wanted to respond back with a certain error message just using a hash. Easypeasy with JSON, however with XML it wasn't as straight forward.

Using this error_hash as our example response:
error_hash = {'LOGINFAILBOAT' => 'Yo Your Login/Password Failed'}

With JSON, you can pass that to your response block and works just fine; try to respond with XML and no bueno. However Nokogiri let's you make it work, by passing the key of your hash as the xml element name.

When I hit my conditions to throw an error, I have it call the following method with the type and message, which is mostly a generic respond_to; with the exception of the XML response.

def return_error_response(error_type, message)
  error_response = { error_type => message }

  respond_to do |format|
    format.json { render json: error_response }
    format.xml  { render xml: build_xml_error_message(error_response) }

And if it's an XML it gets sent to this method.

def build_xml_error_message(error_hash)
  error_attr = error_hash.keys.first
  error_valu = error_hash[error_attr]

  xml_error_message = do |xml|
    xml.send(error_attr.to_sym, error_valu)

  return xml_error_message


Pretty simple, but pretty simple was all I needed.