Rails keep if false, render if true - ruby-on-rails

Question

I have a div tag that I want to change only if a function returns true, otherwise I want it to keep the same content. I used render: nothing => true but that removes everything.
<%= javascript_include_tag :defaults %>
<div id="x">Hi</div>
<% form_remote_tag :url => {:action => 'get'},
:update => 'x' do %>
<%= text_field_tag 'url' %>
<%= submit_tag "Submit" %>
<% end %>
def getImage
#img = params[:url]
if !verify #img
render :nothing => true
end
end
private
def verify url
return true if url =~ /https?:\/\/.*\.(jpg|png|gif|bmp)$/
return false
end
I want to make my rails code do this
var lastrequest = "";
function ajaxObj() { //Returns the ajax object based on browser
if ( window.XMLHttpRequest ) {
return new XMLHttpRequest(); //Firefox
} else if ( window.ActiveXObject ) {
return new ActiveXObject("Microsoft.XMLHTTP"); //IE
}
}
window.onload = function() {
document.getElementById("Submit").onclick = function() {
var request = ajaxObj();
request.onreadystatechange = function() {
if ( request.readyState == 4 ) {
document.getElementById("imgdisplay").innerHTML = request.responseText;
document.getElementById("imgurl").value = "";
}
}
if ( lastrequest != document.getElementById("imgurl").value ) { // Makes sure it doesn't request the same image twice
var parameters = "url=" + document.getElementById("imgurl").value;
request.open("POST", "imageget.php", true); // Sends the ajax request
request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
request.setRequestHeader("Content-length", parameters.length);
request.setRequestHeader("Connection", "close");
request.send(parameters);
lastrequest = document.getElementById("imgurl").value;
}
}
}

Answer 1

use status codes or a failure to control what happens.
http://api.rubyonrails.org/classes/ActionView/Helpers/PrototypeHelper.html#M001645

Answer 2

You can also remove the :update => 'x' part an use following code in your controller:
def getImage
#img = params[:url]
if !verify #img
render :nothing => true
else
render :update do |page|
page['x'].replace_html :partial => <some partial here>
end
end
end
Or, a bit briefer:
def getImage
#img = params[:url]
render :update do |page|
page['x'].replace_html(:partial => <some partial here>) if verify(#img)
end
end
Greets