db_splitting_example.rb

class ImportBase
  include Mongoid::Document
  include Mongoid::Timestamps

  field :import_version, type: String, default: 'v1'  # legacy imports use 'v1' connection

  before_create do
    self.import_version = 'v2'  # new imports use 'v2' connection
  end

  protected

  def choose_connection
    if import_version == 'v2'
      Mongoid.session('imports')  # alternate database configuration
    else
      self.class.mongo_session
    end
  end

end

class FooImport < ImportBase
  
  def insert_data
    # use choose_connection to insert data...
  end

  def get_data
    # use choose_connection to retrieve data...
  end

end