Active Record with Delegate and conditions

No, you can’t, but you can pass the :allow_nil => true option to return nil if the master is nil.

class User < ActiveRecord::Base
  delegate :company, :to => :master, :allow_nil => true

  # ...
end

user.master = nil
user.company 
# => nil

user.master = <#User ...>
user.company 
# => ...

Otherwise, you need to write your own custom method instead using the delegate macro for more complex options.

class User < ActiveRecord::Base
  # ...

  def company
    master.company if has_master?
  end

end