gistfile1.js

<script type="text/javascript">
   $(document).ready(function () {
       $("#project_postnumber").blur(function(){
        $.ajax({url: '/project/postplace', type: 'POST'})
        .done(function(response){
        $("#project_postplace").val(response);
        });
      });
   });
</script>

gistfile2.rb

def postplace
    @find_postplace = Postnumberregister.find_by_postnumber(params[:postnumber])
    render :text => @find_postplace.postplace
end