problems with validation on STI

I have the following STI table:

def self.up
    create_table :distributions do |t|
      t.string :type
      t.integer :simulation_id
      t.string :dist_name
      t.string :desc, :default=> 'fixed'
      t.float :param1, :default => 5.0
      t.float :param2, :param3
      t.timestamps

      #fields for RscDist
      t.integer :resource_id

      #fields for LoadDist
      t.integer :load_id
    end
  end

My model file looks like this:

class Distribution < ActiveRecord::Base

    validates_presence_of :desc, :param1
    validates_uniqueness_of :dist_name, :scope => "simulation_id"
     validates_inclusion_of :desc,
                        :in => %w{fixed exponential normal uniform
triangular},
                        :message => "should be 'fixed', 'exponential',
'normal', 'uniform', 'triangular'"
    validates_numericality_of :param1, :param2, :param3

    def to_s
        "desc = " + desc + " param1 = " + param1.to_s + " dist_name = "
+ dist_name + " type = " + type.to_s
    end

    protected
    def validate
        #desc can be fixed, exponential, normal, uniform, triangular
        errors.add("need 2nd parameter") if desc == 'normal' || desc ==
'uniform' || desc == 'triangular'
        errors.add("need 3rd parameter") if desc == 'triangular'
    end

end

class LoadDist < Distribution

    validates_uniqueness_of :dist_name, :scope => "load_id"

    belongs_to :load
end

class RscDist < Distribution
    validates_uniqueness_of :dist_name, :scope => "resource_id"

    belongs_to :resource
end

In my unit tests I can't seem to get a valid object. Here's my setup:

def setup
        @s1 = simulations(:base)
        @rsc1 = resources(:rsc1)
        @d1 = RscDist.new
        @d1.simulation_id = @s1.id
        @d1.resource_id = @rsc1.id
    end

and then the test:

def test_params_exp

    @d1.desc = 'exponential'

    assert !@d1.valid?

    @d1.param1 = 14
    @d1.dist_name = 'dist1'
    puts @d1.to_s
    assert @d1.valid?
  end

The final line "assert @d1.valid?" fails, even though I have a :desc
field that is properly spelled, a numerical :param1, and a unique
:dist_name. The message is:

  1) Failure:
test_params_exp(DistributionTest)
    [test/unit/distribution_test.rb:35:in `test_params_exp'
     c:/ruby/lib/ruby/gems/1.8/gems/activesupport-2.1.0/lib/active_support/testi
ng/setup_and_teardown.rb:33:in `__send__'
     c:/ruby/lib/ruby/gems/1.8/gems/activesupport-2.1.0/lib/active_support/testi
ng/setup_and_teardown.rb:33:in `run']:
<false> is not true.

What am I doing wrong?

I think I found the problem. I was expecting empty attributes for desc
and param1, but I set those to 'fixed' and 5 by default. *embarrassed
grin*

The final line "assert @d1.valid?" fails, even though I have a :desc
field that is properly spelled, a numerical :param1, and a unique
:dist_name. The message is:

I don't know what the problem is off the top of my head, but after calling @d1.valid?, calling
@d1.errors will show you the validation failures which should help you in tracking down the problem

Fred