Postamt

class UserController < ApplicationController
  use_db_connection :slave, for: ['User'], only: [:search]

  def search
    # SELECTs here are sent to slave
    # User#save and User.create would be sent to master anyways.
    # Everything in a transaction block too.
    @users = User.where(...) # sent to slave
    @something_else = SomethingElse.first # sent to master
  end

  def create
    @user = User.new(params[:user])
    @user.save! # sent to master
  end

  def invoice
    transaction do
      @user = User.where(...) # sent to master
      @invoices = Invoice.create(...) # sent to master
    end
  end
end

class ArchivedItem < ActiveRecord::Base
  # default_connection can be overwritten with
  # * Postamt.on(...) { ... },
  # * ActiveRecord::Base.transaction { ... }, and
  # * use_db_connection :other_connection, for: ['ArchivedItem'] in a controller.
  self.default_connection = :slave
end

User.where(...) # sent to master
item = ArchivedItem.where(...) # sent to slave
item.title = "changed title"
item.save! # sent to master
item.reload # sent to slave, beware of replication lag here!

ActiveRecord::Base.transaction do
  ArchivedItem.where(...) # sent to master, since we're in a transaction
  User.where(...) # sent to master
end

Postamt.on(:master) do
  ArchivedItem.where(...) # sent to master
  User.where(...) # sent to master
end

# If you don't want to test with a slave DB put this in config/environments/test.rb
Postamt.force_connection = :master