Ruby on Rails Session Variable not Showing on Front End on First Render

Question

I'm having a strange issue where the last value in a session array is not displaying on the front end when a partial is rendered for the first time. I'm using the add_to_box method to add a set of values to a session variable called 'session[:box]'. I then want to update the partial on the front end with the new list of values.

Here's the code... (sanitised slightly)

Controller:

def add_to_box(values_hash)

  session[:box] ||= []
  if params_valid?(values_hash)

    session[:box].push(values_hash)

    respond_to do |format|
      format.js
    end

  end

end

add_to_box.js.erb:

$("#box").html("<%= escape_javascript(render partial: 'box') %>");

Partial _box.html.erb:

<ul>
  <% session[:box].each do |m| %>
    <li>
      <%= "#{m['bottom_id']}, #{m['filling_id']}, #{m['top_id']}" %>
    </li>
  <% end %>
</ul>

Console output:

Started POST "/things/add_to_box" for ::1 at 2016-08-09 11:59:23 +0100
Processing by ThingController#add_to_box as JS
Rendered things/_box.html.erb (0.9ms)
Rendered things/add_to_box.js.erb (2.8ms)
Completed 200 OK in 17ms (Views: 8.2ms | ActiveRecord: 0.7ms)

The first time the method is called (values 1,1,1), the front end displays as:

∙ , ,

The second time the method is called (values 2,2,2), the front end displays as:

∙ 1, 1, 1

∙ , ,

The third time the method is called (values 3,3,3), the front end displays as:

∙ 1, 1, 1

∙ 2, 2, 2

∙ , ,

You get the picture... Essentially the latest value is never rendered.

BUT if I refresh the page, it shows correctly with all values:

∙ 1, 1, 1

∙ 2, 2, 2

∙ 3, 3, 3

I thought at first it could be because the values aren't saved in time and the render is happening first, but then how does it know there is a new set of values before they have been set?!

Need some help on this one

Answer 1

I have figured it out. As part of the params_valid? method I was doing the following:

  values_hash[:top_id] = params[:top_id].to_i
  values_hash[:filling_id] = params[:filling_id].to_i
  values_hash[:bottom_id] = params[:bottom_id].to_i

I added a logger.debug on session[:box] and got the following:

{"top_id"=>1, "filling_id"=>1, "bottom_id"=>1, "id"=>1}
{"top_id"=>1, "filling_id"=>1, "bottom_id"=>1, "id"=>2}
{"top_id"=>1, "filling_id"=>1, "bottom_id"=>1, "id"=>3}
{"top_id"=>1, "filling_id"=>1, "bottom_id"=>1, "id"=>4}
{:top_id=>1, :filling_id=>1, :bottom_id=>1, :id=>5}

You'll notice the difference in the last row. I changed the code above to be the following and it worked:

  values_hash["top_id"] = params[:top_id].to_i
  values_hash["filling_id"] = params[:filling_id].to_i
  values_hash["bottom_id"] = params[:bottom_id].to_i

The strange thing about this, and the reason I didn't catch it was that when refreshing the page it was calling the same session[:box] variable and displaying fine. Something must be happening to the session variable in the background to tidy it up.

Would be interested if anyone could share some insight on this