The cache store needs to be the actually store, not e.g. :memory_store

Status quo in test app:
In Rails
    ActionController::Base.cache_store = :memory_store
and then AMS railtie does:
  ActiveModelSerializers.config.cache_store = config.action_controller.cache_store
then, in the Railtie
1. ActiveSupport.on_load(:action_controller) fires
  - ActiveModelSerializers.config.cache_store #=> nil
  - ActionController::Base.cache_store        #=> #<ActiveSupport::Cache::FileStore:0x007fe319256760...]
2. After set_configs fires
  - ActiveModelSerializers.config.cache_store #+> #<ActiveSupport::Cache::FileStore:0x007fe319256760 ,
3. Tests pass, but notice that we're using the FileStore, not memory store

When we change the config to the test app:
  ActionController::Base.cache_store = :memory_store
  config = Rails.configuration
  config.action_controller.cache_store = :memory_store
then, in the Railtie:
1. ActiveSupport.on_load(:action_controller) fires
  - ActiveModelSerializers.config.cache_store #=> nil
  - ActionController::Base.cache_store        #=> #ActiveSupport::Cache::MemoryStore entries=0, size=0, options={}>]
2. After set_configs fires
  - ActiveModelSerializers.config.cache_store #=> :memory_store
3. And we get a lot of failures:
  NoMethodError: undefined method `fetch' for :memory_store:Symbol

So, we see that when we set the ActionController::Base.cache_store
directly in our test app, we could set
ActiveModelSerializers.config.cache_store in the :action_controller load
hook, but that would never use the Rails config.

To fix the Rails config, we change the config to the test app:
  config = Rails.configuration
  config.action_controller.cache_store = :memory_store
and then AMS railtie does:
  ActiveModelSerializers.config.cache_store = ActiveSupport::Cache.lookup_store(config.action_controller.cache_store
  ActiveSupport.on_load(:action_controller) do
    ::ActiveModelSerializers.config.cache_store = cache_store
  end
then
1. After set_configs fires
  - ActiveModelSerializers.config.cache_store #=> <#ActiveSupport::Cache::MemoryStore, object_id 70207113611740
2. ActiveSupport.on_load(:action_controller) fires
  - ActionController::Base.cache_store        #=> <#ActiveSupport::Cache::MemoryStore, object_id 70207106279660
  - ActiveModelSerializers.config.cache_store #=> <#ActiveSupport::Cache::MemoryStore, object_id 70207106279660
    (notice the object_id changed)
3. And we get a failure:

  1) Failure:
  ActiveModelSerializers::CacheTest#test_associations_cache_when_updated
  [active_model_serializers/test/cache_test.rb:141]:
  --- expected
  +++ actual
  @@ -1 +1 @@
  -{:id=>"post", :title=>"New Post", :body=>"Body"}
  +{:id=>"post", :title=>"New Post", :body=>"Body", :comments=>[{:id=>2, :body=>"ZOMG A NEW COMMENT"}], :blog=>{:id=>999, :name=>"Custom blog"}, :author=>{:id=>"author", :name=>"Joao M. D. Moura"}}

If we take out the on_load(:action_controller) hook, we get a ton of
failures.  So clearly, our code expects the controller cache to be the
same as the serializer cache.

So, we make sure we use an on_load(:action_controller) hook that runs
after set_configs

And look at the test and see it is filled with direct calls to ActionController::Base.cache_store

    assert_equal(new_comment_serializer.attributes, ActionController::Base.cache_store.fetch(new_comment.cache_key))
    assert_equal(@post_serializer.attributes, ActionController::Base.cache_store.fetch(@post.cache_key))

But that's not a problem in this case, since they're the same object.

For now, let's remove the :memory_store setting and use the default FileStore

lib/active_model_serializers/railtie.rb

@@ -25,8 +25,11 @@ class Railtie < Rails::Railtie
     # and also before eager_loading (if enabled).
     initializer 'active_model_serializers.set_configs', :after => 'action_controller.set_configs' do
       ActiveModelSerializers.logger = Rails.configuration.action_controller.logger
-      ActiveModelSerializers.config.cache_store     = Rails.configuration.action_controller.cache_store
       ActiveModelSerializers.config.perform_caching = Rails.configuration.action_controller.perform_caching
+      # We want this hook to run after the config has been set, even if ActionController has already loaded.
+      ActiveSupport.on_load(:action_controller) do
+        ActiveModelSerializers.config.cache_store = cache_store
+      end
     end
 
     generators do |app|

test/support/rails_app.rb

@@ -9,7 +9,6 @@ class ActiveModelSerializers::RailsApplication < Rails::Application
     config.logger = Logger.new(nil)
 
     config.action_controller.perform_caching = true
-    ActionController::Base.cache_store = :memory_store
 
     Rails.application.routes.default_url_options = { host: 'example.com' }
   end