Weird behaviour with respond_with syntax for a Rails 3 app?

While building an api for Rails 3.0.3 app that serves json, some unexpected behaviour is happening.

Following is the controller. The question is about the respond_with. I already have respond_to :json in the app controller.

The create action just works and the data is also sent back after creation.

But the update action's respond_with doesn't send back any data.

The response body is blank.

def create
  line = get_line
  input_header = line.input_headers.create(params[:input_header])
  respond_with(input_header, :location => api_v1_line_input_header_url(line,input_header))
end

def show
 input_header = get_input_header
 respond_with(input_header.to_json)
end

def update
  input_header = get_input_header
  input_header.update_attributes(params[:input_header])

  respond_with(input_header, :location => api_v1_line_input_header_url(input_header.line,input_header))

  # render :json => input_header
end

When I use render :json => 开发者_StackOverflowinput_header instead of respond_with, it works. Why is this?

First, the show method is not too well, you do not need to call #to_json method, because respond_with is automatically calls the correspondent method when it detects the request needs json content or xml content.

Secondly, what happens exactly in get_input_header method? Can you replace it with standard rails InputHeader.find(params[:id]) solution for one shot?

respond_with returns body only for GET and POST requests, otherwise it returns only headers

look at this RoR's code

 # This is the common behavior for formats associated with APIs, such as :xml and :json.
def api_behavior(error)
  raise error unless resourceful?

  if get?
    display resource
  elsif post?
    display resource, :status => :created, :location => api_location
  else
    head :no_content
  end
end

and the josevalim's answer in this pull request's discution (Make respond_with return JSON/XML for PUT requests)

In my app I used monkeypatching for that :(

elsif put?
  display resource, status: :ok
...

Maybe you forget append to controller respond_to?

respond_to :json, :html, ...