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The new REST web-service support in Rails 1.1 makes it so easy to add web-services to your app, you might as well do it earlier, rather than later.

Consider: if you have a page in your app that displays a list of people, it might look something like this, without web-service support:

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

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

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

    respond_to do |wants|
      wants.html
      wants.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.)

Now, let’s suppose 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 add
    @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 add
    company = params[:person].delete(:company)
    @company = Company.find_or_create_by_name(company[:name])
    @person  = @company.people.create(params[:person])

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

It was simple enough that I also added RJS support here. If the client wants HTML, we just redirect them back to the person list. If they want Javascript (wants.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—a 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 note), but if you just go with the flow and accept Rails’ defaults, life will be much easier.