Which version do you prefere?

test.rb

require 'test/unit'

class KeyTest < Test::Unit::TestCase
  def test_key
    object = Object.new
    assert_equal(:submit, object_key(object))

    def object.persisted?; true end 
    assert_equal(:update, object_key(object))

    def object.persisted?; false end 
    assert_equal(:create, object_key(object))
  end 
end

v01.rb

def object_key
  if @object && @object.respond_to?(:persisted?)
    @object.persisted? ? :update : :create
  else
    :submit
  end
end

v02.rb

def object_key
  return :submit unless @object && @object.respond_to?(:persisted?)
  return @object.persisted? ? :update : :create
end

v03.rb

def object_key
  # assume that nil does not respond to :persisted? and if it does, then respect that
  return :submit unless @object.respond_to?(:persisted?)
  return @object.persisted? ? :update : :create
end

v04_yashke.rb

def object_key
  if @object && @object.respond_to?(:persisted?) && @object.persisted?
    :update
  elsif @object && @object.respond_to?(:persisted?)
    :create
  else
    :submit
  end
end

v05_andrzej.rb

def object_key
  return :submit if ! persistable?(@object)
  return :update if ! @object.persisted?
  return :create if   @object.persisted?
end

def persistable?(object)
  object && object.respond_to?(:persisted?)
end

v06.rb

def object_key
  return :submit unless @object.respond_to?(:persisted?)
  return :update if     @object.persisted?
  return :create
end

v07.rb

persisted = @object.respond_to?(:persisted?) ? @object.persisted? : nil
case persisted
  when true  then :update
  when false then :create
  else :submit
end

v08rescue.rb

def object_key(object)
  object.persisted? ? :update : :create
  rescue; :submit
end

v09alter_rescue.rb

def object_key(object)
  object.persisted? ? :update : :create rescue NoMethodError :submit
end