foo.rb

class Foo < ActiveRecord::Base

  serialize :check_in, TimeOfDayAttribute # dump/load for valid, empty, nil values tested independently

  # Setter will handle both String & TimeOfDay input
  # I want to test the getter before defining (and depending on) this method
  def check_in=(param)
    # ...
  end
end

foo_spec.rb

describe Foo

  describe "#check_in"

    it "returns a TimeOfDay from a valid value" do
      # How to set this as though it came from the database without using setter method?
      subject[:check_in] = "16:30"

  #attributes_before_type_cast[:check_in]
  #<struct ActiveRecord::AttributeMethods::Serialization::Attribute
  # coder=TimeOfDayAttribute,
  # value="16:30",
  # state=:unserialized>

      subject.check_in.should == TimeOfDay.new(16,30)
    end

    it "returns nil from an invalid value" do
      # How to set this as though it came from the database without using setter method?
      subject.attributes[:check_in] = "doh"

      subject.check_in.should be nil
    end

  end

end