Fix ActiveRecord using two connection pools when no replica is defined (#27061)

app/helpers/database_helper.rb

@@ -1,11 +1,24 @@
 # frozen_string_literal: true
 
 module DatabaseHelper
+  def replica_enabled?
+    ENV['REPLICA_DB_NAME'] || ENV.fetch('REPLICA_DATABASE_URL', nil)
+  end
+  module_function :replica_enabled?
+
   def with_read_replica(&block)
-    ApplicationRecord.connected_to(role: :reading, prevent_writes: true, &block)
+    if replica_enabled?
+      ApplicationRecord.connected_to(role: :reading, prevent_writes: true, &block)
+    else
+      yield
+    end
   end
 
   def with_primary(&block)
-    ApplicationRecord.connected_to(role: :writing, &block)
+    if replica_enabled?
+      ApplicationRecord.connected_to(role: :writing, &block)
+    else
+      yield
+    end
   end
 end

app/models/application_record.rb

@@ -5,7 +5,7 @@ class ApplicationRecord < ActiveRecord::Base
 
   include Remotable
 
-  connects_to database: { writing: :primary, reading: ENV['REPLICA_DB_NAME'] || ENV['REPLICA_DATABASE_URL'] ? :replica : :primary }
+  connects_to database: { writing: :primary, reading: :replica } if DatabaseHelper.replica_enabled?
 
   class << self
     def update_index(_type_name, *_args, &_block)