respond_to

Without web-service support, an action which collects the data for displaying a list of people might look something like this:

  def index
    @people = Person.find(:all)
  end

Here’s the same action, with web-service support baked in:

  def index
    @people = Person.find(:all)

    respond_to do |format|
      format.html
      format.xml { render :xml => @people.to_xml }
    end
  end

What that says is, "if the client wants HTML in response to this action, just respond as we would have before, but if the client wants XML, return them the list of people in XML format." (Rails determines the desired response format from the HTTP Accept header submitted by the client.)

Supposing you have an action that adds a new person, optionally creating their company (by name) if it does not already exist, without web-services, it might look like this:

  def create
    @company = Company.find_or_create_by_name(params[:company][:name])
    @person  = @company.people.create(params[:person])

    redirect_to(person_list_url)
  end

Here’s the same action, with web-service support baked in:

  def create
    company  = params[:person].delete(:company)
    @company = Company.find_or_create_by_name(company[:name])
    @person  = @company.people.create(params[:person])

    respond_to do |format|
      format.html { redirect_to(person_list_url) }
      format.js
      format.xml  { render :xml => @person.to_xml(:include => @company) }
    end
  end

If the client wants HTML, we just redirect them back to the person list. If they want Javascript (format.js), then it is an RJS request and we render the RJS template associated with this action. Lastly, if the client wants XML, we render the created person as XML, but with a twist: we also include the person’s company in the rendered XML, so you get something like this:

  <person>
    <id>...</id>
    ...
    <company>
      <id>...</id>
      <name>...</name>
      ...
    </company>
  </person>

Note, however, the extra bit at the top of that action:

  company  = params[:person].delete(:company)
  @company = Company.find_or_create_by_name(company[:name])

This is because the incoming XML document (if a web-service request is in process) can only contain a single root-node. So, we have to rearrange things so that the request looks like this (url-encoded):

  person[name]=...&person[company][name]=...&...

And, like this (xml-encoded):

  <person>
    <name>...</name>
    <company>
      <name>...</name>
    </company>
  </person>

In other words, we make the request so that it operates on a single entity’s person. Then, in the action, we extract the company data from the request, find or create the company, and then create the new person with the remaining data.

Note that you can define your own XML parameter parser which would allow you to describe multiple entities in a single request (i.e., by wrapping them all in a single root node), but if you just go with the flow and accept Rails' defaults, life will be much easier.

If you need to use a MIME type which isn’t supported by default, you can register your own handlers in config/initializers/mime_types.rb as follows.

  Mime::Type.register "image/jpg", :jpg

Respond to also allows you to specify a common block for different formats by using any:

  def index
    @people = Person.find(:all)

    respond_to do |format|
      format.html
      format.any(:xml, :json) { render request.format.to_sym => @people }
    end
  end

In the example above, if the format is xml, it will render:

  render :xml => @people

Or if the format is json:

  render :json => @people

Since this is a common pattern, you can use the class method respond_to with the respond_with method to have the same results:

  class PeopleController < ApplicationController
    respond_to :html, :xml, :json

    def index
      @people = Person.find(:all)
      respond_with(@person)
    end
  end

Be sure to check respond_with and respond_to documentation for more examples.