Input formats and content types in Rails 1.2

One feature of recent releases of Rails I hadn’t spotted before is the ability to define your own parameter parsing based on content type. I’m working on an application that will employ a RESTful API and that I hope will take its input in either standard http parameters, microformatted HTML, XML or JSON.

I don’t really want to have to write custom code within the controllers to interpret the input based on content type, so I started looking for how rails parses XML input and came across the following in the actionpack changelog:

    # Assign a new param parser to a new content type
    ActionController::Base.param_parsers['application/atom+xml'] = do |data|
      node =
     { => node.root }

    # Assign the default XmlSimple to a new content type
    ActionController::Base.param_parsers['application/backpack+xml'] = :xml_simple

Looking at the actual source code it appears it’s actually being implemented slightly differently, with the Mime::Type object being used as the key, rather than the text content type. Since the json content type is already defined (with a reference to the object in Mime::JSON), JSON can (usually) be parsed as YAML, and the :yaml symbol is a shortcut to a YAML parser, handling it transparently is almost as simple as adding:

ActionController::Base.param_parsers[Mime::JSON] = :yaml

or if we wanted to be a bit more explicit:

ActionController::Base.param_parsers[Mime::JSON] = do |data|

to environment.rb. Building these APIs is even easier than I’d thought!

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