Api errors customization for Rails 3 like Github api v3 - ruby

Question

I am adding an API on a Rails3 app and its pretty going good.
But I saw the following Github api v3 at http://developer.github.com/v3/
HTTP/1.1 422 Unprocessable Entity
Content-Length: 149
{
"message": "Validation Failed",
"errors": [
{
"resource": "Issue",
"field": "title",
"code": "missing_field"
}
]
}
I liked the error messages structure. But couldn't get it to reproduce.
How can I make my apis to make the response like it?

Answer 1

You could quite easily achieve that error format by adding an ActionController::Responder for your JSON format. See http://api.rubyonrails.org/classes/ActionController/Responder.html for the (extremely vague) documentation on this class, but in a nutshell, you need to override the to_json method.
In the example below I'm calling a private method in an ActionController:Responder which will construct the json response, including the customised error response of your choice; all you have to do is fill in the gaps, really:
def to_json
json, status = response_data
render :json => json, :status => status
end
def response_data
status = options[:status] || 200
message = options[:notice] || ''
data = options[:data] || []
if data.blank? && !resource.blank?
if has_errors?
# Do whatever you need to your response to make this happen.
# You'll generally just want to munge resource.errors here into the format you want.
else
# Do something here for other types of responses.
end
end
hash_for_json = { :data => data, :message => message }
[hash_for_json, status]
end