Remove special handling for ActiveRecordStore

activerecord-session_store was removed in 0ffe190, and has been
displaying a special error message when missing since Rails 4.0.

Replace the specific error message so that third party stores get nicer
error handling as well

actionpack/lib/action_dispatch.rb

@@ -94,6 +94,19 @@ module ActionDispatch
     autoload :CookieStore,         "action_dispatch/middleware/session/cookie_store"
     autoload :MemCacheStore,       "action_dispatch/middleware/session/mem_cache_store"
     autoload :CacheStore,          "action_dispatch/middleware/session/cache_store"
+
+    def self.resolve_store(session_store) # :nodoc:
+      self.const_get(session_store.to_s.camelize)
+    rescue NameError
+      raise <<~ERROR
+        Unable to resolve session store #{session_store.inspect}.
+
+        #{session_store.inspect} resolves to ActionDispatch::Session::#{session_store.to_s.camelize},
+        but that class is undefined.
+
+        Is #{session_store.inspect} spelled correctly, and are any necessary gems installed?
+      ERROR
+    end
   end
 
   mattr_accessor :test_app

guides/source/action_controller_overview.md

@@ -378,8 +378,11 @@ Your application has a session for each user in which you can store small amount
 
 * [`ActionDispatch::Session::CookieStore`][] - Stores everything on the client.
 * [`ActionDispatch::Session::CacheStore`][] - Stores the data in the Rails cache.
-* `ActionDispatch::Session::ActiveRecordStore` - Stores the data in a database using Active Record (requires the `activerecord-session_store` gem).
 * [`ActionDispatch::Session::MemCacheStore`][] - Stores the data in a memcached cluster (this is a legacy implementation; consider using `CacheStore` instead).
+* [`ActionDispatch::Session::ActiveRecordStore`][activerecord-session_store] -
+  Stores the data in a database using Active Record (requires the
+  [`activerecord-session_store`][activerecord-session_store] gem)
+* A custom store or a store provided by a third party gem
 
 All session stores use a cookie to store a unique ID for each session (you must use a cookie, Rails will not allow you to pass the session ID in the URL as this is less secure).
 
@@ -394,12 +397,12 @@ Read more about session storage in the [Security Guide](security.html).
 If you need a different session storage mechanism, you can change it in an initializer:
 
 ```ruby
-# Use the database for sessions instead of the cookie-based default,
-# which shouldn't be used to store highly confidential information
-# (create the session table with "rails g active_record:session_migration")
-# Rails.application.config.session_store :active_record_store
+Rails.application.config.session_store :cache_store
 ```
 
+See [`config.session_store`](configuring.html#config-session-store) in the
+configuration guide for more information.
+
 Rails sets up a session key (the name of the cookie) when signing the session data. These can also be changed in an initializer:
 
 ```ruby
@@ -430,6 +433,8 @@ NOTE: Changing the secret_key_base when using the `CookieStore` will invalidate
 [`ActionDispatch::Session::CookieStore`]: https://api.rubyonrails.org/classes/ActionDispatch/Session/CookieStore.html
 [`ActionDispatch::Session::CacheStore`]: https://api.rubyonrails.org/classes/ActionDispatch/Session/CacheStore.html
 [`ActionDispatch::Session::MemCacheStore`]: https://api.rubyonrails.org/classes/ActionDispatch/Session/MemCacheStore.html
+[activerecord-session_store]: https://github.com/rails/activerecord-session_store
+
 
 ### Accessing the Session
 

railties/lib/rails/application/configuration.rb

@@ -428,7 +428,7 @@ module Rails
 
       # Specifies what class to use to store the session. Possible values
       # are +:cache_store+, +:cookie_store+, +:mem_cache_store+, a custom
-      # store, or +:disabled+. +:disabled+ tells Rails not to deal with
+      # store, or +:disabled+. +:disabled+ tells \Rails not to deal with
       # sessions.
       #
       # Additional options will be set as +session_options+:
@@ -443,25 +443,14 @@ module Rails
       #   config.session_store :my_custom_store
       def session_store(new_session_store = nil, **options)
         if new_session_store
-          if new_session_store == :active_record_store
-            begin
-              ActionDispatch::Session::ActiveRecordStore
-            rescue NameError
-              raise "`ActiveRecord::SessionStore` is extracted out of Rails into a gem. " \
-                "Please add `activerecord-session_store` to your Gemfile to use it."
-            end
-          end
-
           @session_store = new_session_store
           @session_options = options || {}
         else
           case @session_store
           when :disabled
             nil
-          when :active_record_store
-            ActionDispatch::Session::ActiveRecordStore
           when Symbol
-            ActionDispatch::Session.const_get(@session_store.to_s.camelize)
+            ActionDispatch::Session.resolve_store(@session_store)
           else
             @session_store
           end

railties/test/application/configuration_test.rb

@@ -1647,13 +1647,14 @@ module ApplicationTests
       ActionDispatch::Session.send :remove_const, :ActiveRecordStore
     end
 
-    test "config.session_store with :active_record_store without activerecord-session_store gem" do
+    test "config.session_store with unknown store raises helpful error" do
       e = assert_raise RuntimeError do
         make_basic_app do |application|
-          application.config.session_store :active_record_store
+          application.config.session_store :unknown_store
         end
       end
-      assert_match(/activerecord-session_store/, e.message)
+
+      assert_match(/Unable to resolve session store :unknown_store/, e.message)
     end
 
     test "default session store initializer does not overwrite the user defined session store even if it is disabled" do