How to fail Ajax request in Rails?

Question

When user clicks a specific item, I use jQuery's post method to update something in the database:

$.post("/posts/" + post_id + "/update_something", 
       { some_param: some_value }, 
       success_handler);

where update_something looks like this:

def update_something
  post = Post.find(params[:id])
  post.update_attributes(:some_field => params[:some_param])
  render :nothing => true
end

The problem is if update_attributes fails, the request still succeeds and success_handler is executed.

How could I cause the request to fail when update_attributes fails such that success_handler won't be executed?

Answer 1

Well, you have to add an error handler, and give it an error to handle. So, in your JavaScript:

$.post( "/posts/" + post_id + "/update_something",
        { some_param : some_value }
      )
  .done( successHandler )
  .fail( errorHandler )      // define errorHandler somewhere, obviously
;

And in Rails:

def update_something
  post    = Post.find params[ :id ]

  success = post.update_attributes :some_field => params[ :some_param ]

  head success ? :ok : :internal_server_error
end

Note: 500 may or may not be the appropriate error code here—choose whichever among the 400s and 500s is appropriate.

Answer 2

You can either do render :status => 400 (or some other error code) in Rails, which will trigger the error callback of $.ajax(), or you can render some JSON with an error message:

render :json => { :success => false }

Then in your success_handler function you would:

function success_handler (response) {
    if (response.success) {
        // do stuff
    }
}

Edit:

Oh, and update_attributes returns false when it fails. So you can render your response based on that.

Edit 2 years later:

After a couple years and seeing that this has a few upvotes, I'd highly recommend using the status: 400 method instead of rendering 200. It's what the error handler in AJAX requests are for, and should be used that way.