Rails: How does the respond_to block work?

Question

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.

Answer 1

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 conventions)
format.register('html')

// register json as well. the argument to .json is the second
// argument to method_missing ('json' is the first), which contains
// optional ways to configure the response. In this case, serialize as json.
format.register('json', renderOptions)

This part confused the heck out of me. I still find it unintuitive. Ruby seems to use this technique quite a bit. The entire class (responder) becomes the method implementation. In order to leverage method_missing, we need an instance of the class, so we're obliged to pass a callback into which they pass the method-like object. For someone who has coded in C-like languages for 20 some years, this is very backwards and unintuitive to me. Not that it's bad! But it's something a lot of people with that kind of background need to get their head around, and I think might be what the OP was after.

p.s. note that in RoR 4.2 respond_to was extracted into responders gem.