Alex Rivera | Logout

Rails: How does the respond_to block work?

Asked 2012-02-29T01:28:05.020
253

I'm going through the Getting Started with Rails guide and got confused with section 6.7. After generating a scaffold I find the following auto-generated block in my controller:

def index
  @posts = Post.all

  respond_to do |format|
    format.html  # index.html.erb
    format.json  { render :json => @posts }
  end
end

I'd like to understand how the respond_to block actually works. What type of variable is format? Are .html and .json methods of the format object? The documentation for

ActionController::MimeResponds::ClassMethods::respond_to

doesn't answer the question.

ruby-on-rails
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1 Answer

239

I am new to Ruby and got stuck at this same code. The parts that I got hung up on were a little more fundamental than some of the answers I found here. This may or may not help someone.

  • respond_to is a method on the superclass ActionController.
  • it takes a block, which is like a delegate. The block is from do until end, with |format| as an argument to the block.
  • respond_to executes your block, passing a Responder into the format argument.

http://api.rubyonrails.org/v4.1/classes/ActionController/Responder.html

  • The Responder does NOT contain a method for .html or .json, but we call these methods anyways! This part threw me for a loop.
  • Ruby has a feature called method_missing. If you call a method that doesn't exist (like json or html), Ruby calls the method_missing method instead.

http://ruby-metaprogramming.rubylearning.com/html/ruby_metaprogramming_2.html

  • The Responder class uses its method_missing as a kind of registration. When we call 'json', we are telling it to respond to requests with the .json extension by serializing to json. We need to call html with no arguments to tell it to handle .html requests in the default way (using conventions and views).

It could be written like this (using JS-like pseudocode):

// get an instance to a responder from the base class
var format = get_responder()

// register html to render in the default way
// (by way of the views and conventio
answered 2012-11-14T22:09:06.260

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