No errors, but no success in jquery Ajax - php

Question

I'm doing an ajax call to my controller, but the alert in my success isn't displaying and I'm not getting errors in console. I don't know how to proceed now.
Controller - rate
function graph($userid, $courseid){
$i_am_admin = $this->logged_in->logged_as_admin();
if($this->session->userdata('id') == $userid || $i_am_admin ){
$this->load->model('rate_model');
$graph_data = array();
if($query = $this->rate_model->graphRate($userid, $courseid)){
$data['rate'] = $query;
}
$data['graph_json'] = json_encode($query);
$data['content'] = 'rate_graph_view';
$this->load->view('templates/template', $data);
return json_encode($query);
}
}
Script.js
$('.profileimg').click(function(){
$.ajax({
url: url, // url = http://localhost/herexamen/project/rate/graph/6/4
type:'POST',
dataType: 'json',
success: function(output_string){
alert(output_string);
alert("yes");
} // End of success function of ajax form
}); // End of ajax call
});

Answer 1

You aren't checking if the ajax call failed
The appended code isn't the solution, by any means, but can shed light on your problems. Notice I added the error section to the ajax request.
What is the result of this call?
$('.profileimg').click(function(){
$.ajax({
url: url,
type:'POST',
dataType: 'json',
success: function(output_string){
alert(output_string);
alert("yes");
},
error: function(jqXHR, textStatus) {
alert( "Request failed: " + textStatus );
}
}); // End of ajax call
});

Answer 2

While making ajax calls you are supposed to echo the content or response to send and you are using return try this
echo json_encode($query);
Why view is loaded when you are making json and only
function graph($userid, $courseid){
$i_am_admin = $this->logged_in->logged_as_admin();
if($this->session->userdata('id') == $userid || $i_am_admin ){
$this->load->model('rate_model');
$graph_data = array();
if($query = $this->rate_model->graphRate($userid, $courseid)){
$data['rate'] = $query;
}
$data['graph_json'] = json_encode($query);
$data['content'] = 'rate_graph_view';
$data['viewloaded']= $this->load->view('templates/template', $data,true);
echo json_encode($data);
die();
}
}
There is a third optional parameter lets you change the behavior of
the function so that it returns data as a string rather than sending
it to your browser. This can be useful if you want to process the data
in some way. If you set the parameter to true (boolean) it will return
data. The default behavior is false, which sends it to your browser.
Remember to assign it to a variable if you want the data returned
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