From 915c1202f873534431bbf1d0a6138cbe861853d0 Mon Sep 17 00:00:00 2001
From: Kevin McConnell <kevin@37signals.com>
Date: Tue, 11 Nov 2025 15:06:18 +0000
Subject: [PATCH] Initial minimal primary/replica switching

---
 app/models/application_record.rb             |  2 ++
 config/database.yml                          |  8 ++++++++
 config/environments/development.rb           |  3 +++
 config/initializers/database_role_logging.rb | 13 +++++++++++++
 config/initializers/multi_db.rb              |  5 +++++
 5 files changed, 31 insertions(+)
 create mode 100644 config/initializers/database_role_logging.rb
 create mode 100644 config/initializers/multi_db.rb

diff --git a/app/models/application_record.rb b/app/models/application_record.rb
index b63caeb8a..f0814baa6 100644
--- a/app/models/application_record.rb
+++ b/app/models/application_record.rb
@@ -1,3 +1,5 @@
 class ApplicationRecord < ActiveRecord::Base
   primary_abstract_class
+
+  connects_to database: { writing: :primary, reading: :replica }
 end
diff --git a/config/database.yml b/config/database.yml
index 2431e23ea..513794386 100644
--- a/config/database.yml
+++ b/config/database.yml
@@ -9,6 +9,10 @@ development:
   primary:
     <<: *default
     database: fizzy_development
+  replica:
+    <<: *default
+    database: fizzy_development
+    replica: true
   cable:
     <<: *default
     database: development_cable
@@ -29,6 +33,10 @@ test:
   primary:
     <<: *default
     database: fizzy_test
+  replica:
+    <<: *default
+    database: fizzy_test
+    replica: true
 
 production: &production
   primary:
diff --git a/config/environments/development.rb b/config/environments/development.rb
index a87ef7843..f4b10a3f0 100644
--- a/config/environments/development.rb
+++ b/config/environments/development.rb
@@ -73,6 +73,9 @@ Rails.application.configure do
   # Raise error when a before_action's only/except options reference missing actions
   config.action_controller.raise_on_missing_callback_actions = true
 
+  # Prepend all log lines with the following tags.
+  config.log_tags = [ :request_id ]
+
   if Rails.root.join("tmp/email-dev.txt").exist?
     config.action_mailer.delivery_method = :letter_opener
     config.action_mailer.perform_deliveries = true
diff --git a/config/initializers/database_role_logging.rb b/config/initializers/database_role_logging.rb
new file mode 100644
index 000000000..698d52080
--- /dev/null
+++ b/config/initializers/database_role_logging.rb
@@ -0,0 +1,13 @@
+class DatabaseRoleLogger
+  def initialize(app)
+    @app = app
+  end
+
+  def call(env)
+    Rails.logger.tagged(ActiveRecord::Base.current_role.to_s) do
+      @app.call(env)
+    end
+  end
+end
+
+Rails.application.config.middleware.insert_after ActiveRecord::Middleware::DatabaseSelector, DatabaseRoleLogger
diff --git a/config/initializers/multi_db.rb b/config/initializers/multi_db.rb
new file mode 100644
index 000000000..ae85d8527
--- /dev/null
+++ b/config/initializers/multi_db.rb
@@ -0,0 +1,5 @@
+Rails.application.configure do
+  config.active_record.database_selector = { delay: 1.second }
+  config.active_record.database_resolver = ActiveRecord::Middleware::DatabaseSelector::Resolver
+  config.active_record.database_resolver_context = ActiveRecord::Middleware::DatabaseSelector::Resolver::Session
+end