Move db and shard selector initalizer to Railties

In #45162 it was discovered that the multi_db generator that was created
for 7.0 doesn't work correctly because that file is loaded _after_ the
initializer in active record is run. I tried moving everything to an
after_initialize but the middleware stack is frozen at that point. I
also attempted to fix this in #45353 but it just didn't feel right to
have to deprecate this behavior that _should_ work.

I then realized that most of the middleware in Rails is installed in the
middleware stack file in railties. If I move the middleware installation
to this file, everything works as necessary.

The only caveat is we need to check if `config` responds to
`active_record` but I think that should be fine - we already do that in
the framework defaults configuration.

Fixes #45162

activerecord/lib/active_record/railtie.rb

@@ -94,22 +94,6 @@ module ActiveRecord
       end
     end
 
-    initializer "active_record.database_selector" do
-      if options = config.active_record.database_selector
-        resolver = config.active_record.database_resolver
-        operations = config.active_record.database_resolver_context
-        config.app_middleware.use ActiveRecord::Middleware::DatabaseSelector, resolver, operations, options
-      end
-    end
-
-    initializer "active_record.shard_selector" do
-      if resolver = config.active_record.shard_resolver
-        options = config.active_record.shard_selector || {}
-
-        config.app_middleware.use ActiveRecord::Middleware::ShardSelector, resolver, options
-      end
-    end
-
     initializer "Check for cache versioning support" do
       config.after_initialize do |app|
         ActiveSupport.on_load(:active_record) do

railties/lib/rails/application/default_middleware_stack.rb

@@ -81,6 +81,21 @@ module Rails
           middleware.use ::Rack::ETag, "no-cache"
 
           middleware.use ::Rack::TempfileReaper unless config.api_only
+
+          if config.respond_to?(:active_record)
+            if selector_options = config.active_record.database_selector
+              resolver = config.active_record.database_resolver
+              context = config.active_record.database_resolver_context
+
+              middleware.use ::ActiveRecord::Middleware::DatabaseSelector, resolver, context, selector_options
+            end
+
+            if shard_resolver = config.active_record.shard_resolver
+              options = config.active_record.shard_selector || {}
+
+              middleware.use ::ActiveRecord::Middleware::ShardSelector, shard_resolver, options
+            end
+          end
         end
       end
 

railties/test/application/middleware_test.rb

@@ -345,6 +345,28 @@ module ApplicationTests
       assert_equal "/foo/?something", env["ORIGINAL_FULLPATH"]
     end
 
+    test "database selector middleware is installed from application.rb" do
+      add_to_config "config.active_record.database_selector = { delay: 10 }"
+
+      boot!
+
+      assert_includes middleware, "ActiveRecord::Middleware::DatabaseSelector"
+    end
+
+    test "database selector middleware is installed from config/initializers" do
+      app_file "config/initializers/multi_db.rb", <<-RUBY
+        Rails.application.configure do
+          config.active_record.database_selector = { delay: 15.seconds }
+          config.active_record.database_resolver = ActiveRecord::Middleware::DatabaseSelector::Resolver
+          config.active_record.database_resolver_context = ActiveRecord::Middleware::DatabaseSelector::Resolver::Session
+        end
+      RUBY
+
+      boot!
+
+      assert_includes middleware, "ActiveRecord::Middleware::DatabaseSelector"
+    end
+
     test "shard selector middleware is installed by config option" do
       add_to_config "config.active_record.shard_resolver = ->(*) { }"
 
@@ -353,6 +375,19 @@ module ApplicationTests
       assert_includes middleware, "ActiveRecord::Middleware::ShardSelector"
     end
 
+    test "shard selector middleware is installed from config/initializers" do
+      app_file "config/initializers/multi_db.rb", <<-RUBY
+        Rails.application.configure do
+          config.active_record.shard_selector = { lock: true }
+          config.active_record.shard_resolver = ->(request) { Tenant.find_by!(host: request.host).shard }
+        end
+      RUBY
+
+      boot!
+
+      assert_includes middleware, "ActiveRecord::Middleware::ShardSelector"
+    end
+
     private
       def boot!
         require "#{app_path}/config/environment"