Here is an example:
$('#your_model_user_id').on('change', function() {
$.ajax({url: "/your_model/getname/" + $(this).val(),type: 'get'});
});
Then create an action in your_model:
def getname
@yourmodel = YourModel.find(params[:id])
end
Create a route:
get '/your_model/getname/:id' => 'your_models#getname'
create the file app/views/your_models/getname.js.erb:
$('#your_model_username').val('<%= @yourmodel.name %>');
In a nutshell