Tracking issue: cache issues on deployments

[ArtOfCode-]

We have a repeating error when we update versions or deploy, which we've seen in local dev environments, on the dev server, and now in prod. The traces seem to point back to something in Rails itself, which seems unlikely, but it's also quite hard to reproduce predictably. The error is of the form:

[meta] [ff794e9a-2385-4ea9-a617-a1e436a4284c] ActionView::Template::Error (undefined method `each_with_index' for nil:NilClass

          list.each_with_index do |x, i|
              ^^^^^^^^^^^^^^^^):
[meta] [ff794e9a-2385-4ea9-a617-a1e436a4284c]
Causes:
[meta] [ff794e9a-2385-4ea9-a617-a1e436a4284c] NoMethodError (undefined method `each_with_index' for nil:NilClass

          list.each_with_index do |x, i|
              ^^^^^^^^^^^^^^^^)
[meta] [ff794e9a-2385-4ea9-a617-a1e436a4284c]     75:             <% end %>
[meta] [ff794e9a-2385-4ea9-a617-a1e436a4284c]     76:           </div>
[meta] [ff794e9a-2385-4ea9-a617-a1e436a4284c]     77:         <% end %>
[meta] [ff794e9a-2385-4ea9-a617-a1e436a4284c]     78:         <% if user_signed_in? && post.post_type.has_reactions && post.post_type.reactions.any? %>
[meta] [ff794e9a-2385-4ea9-a617-a1e436a4284c]     79:           <div class="post--react has-text-align-center">
[meta] [ff794e9a-2385-4ea9-a617-a1e436a4284c]     80:             <% unless post.locked? %>
[meta] [ff794e9a-2385-4ea9-a617-a1e436a4284c]     81:               <button class="react-button button is-muted is-icon-only-button h-fw-bold"

The consistent part is that each_with_index is being called on a nil by the framework. Not sure why. Initial thoughts and tasks to come in comments.

[Oaphi]

Just so as it doesn't get lost in our chat discussions - this seems to be closely tied to us caching arrays of ActiveRecord objects as the error is actually an Arel error:

def inject_join(list, collector, join_str)
  list.each_with_index do |x, i|
    collector << join_str unless i == 0
    collector = visit(x, collector)
  end
  collector
end