caching_parts.html.erb

<%# Making sure a Rails partial's dependencies are represented in the cache keys is a common source %>
<%# of bugs that are hard to debug & often "bad" (revealing priviledged info). %>

<%# One strategy I've used is to cache outside a partial and try to pass the parts of the cache %>
<%# key to the partial as locals. This setup is beneficial because the proximity of the key to %>
<%# the locals makes it easier to realize/remember to update them at the same time. But they can %>
<%# still fall out of sync (also the partial can access `@post` or whatever behind your back). %>

<% cache [ @post, @user.can_edit?, @user.can_delete? ] do %>
  <%= render partial: 'post', locals: { post: @post, user_can_edit: @user.can_edit?, user_can_delete: user.can_delete? } %>
<% end %>

<%# One possible improvement would be to wrap the context needed by the partial in an object with %>
<%# a pair of methods for the cache key & the locals. In the object you would need to keep the %>
<%# key & data in sync but you should be able to do that automatically with some meta programming. %>

<% cacheable_post = CacheablePost.new( @post, @user ) %>
<% cache cacheable_post.cache_key do %>
  <%= render partial: 'post', locals: cacheable_post.locals_for_partial %>
<% end %>

<%# A solution like this (esp. w/ the meta programming) seems like it would work well as a gem so %>
<%# I'm wondering if this already exists and I just can't find it or if I'm going about this all "wrong". %>

<%# This still has the problem that the partial can technically access any controller variables %>
<%# (i.e. `@post`) that it wants to. AFAIK there's no way to restrict partials to just `locals`. %>