ActiveRecord::Middleware

• activerecord/lib/active_record.rb
• activerecord/lib/active_record/middleware/database_selector.rb
• activerecord/lib/active_record/middleware/database_selector/resolver.rb
• activerecord/lib/active_record/middleware/database_selector/resolver/session.rb
• activerecord/lib/active_record/middleware/shard_selector.rb
• railties/lib/rails/application/default_middleware_stack.rb