Rails: How to Unit test for validations of nested attributes?

I have a the following has_many model associations in my app:

User < Company < Deed < Subtransaction,

where Deed accepts_nested_attributes_for :subtransactions. I wish to test my Model validations using Minitest and fixtures.

I have trouble, however, testing the nested attributes for validity. For example , If I clear all nested attributes using

@deed.subtransactions.clear I correctly get a test response that the model is not valid.

However

@deed.subtransactions.first.num_shares = " " does not seem to work.

How do I properly test these nested attributes for validity?

My test:

  class DeedTest < ActiveSupport::TestCase

        def setup
            @user = users(:dagobert)
            @newco = companies(:newco)
            params = { :deed => 
                  {
                   :date => deeds(:inc_new).date,
                   :subtransactions_attributes =>
                        { '1' => 
                             {
                              :shareholder_name => "Shareholder 1",
                              :num_shares => subtransactions(:new_subt1).num_shares
                              },
                          '2' => 
                              {
                              :shareholder_name => "Shareholder 2",
                              :num_shares => subtransactions(:new_subt2).num_shares
                              } 
                        }
                  }
              }
           @deed = @newco.deeds.new(params[:deed]) 

        end

        # test: does not raise any error messages
           test "num_shares should be present for a subtransaction" do
              @deed.subtransactions.first.num_shares = nil
              assert_not @deed.valid?
           end            

        # test passing: The params are submitted correctly and pass validation 
        test "fixture values should be valid" do
            assert @deed.valid?
        end

        # test passing: I can test the validity of deed attributes
        test "date should be present" do
            @deed.date = " "
            assert_not @deed.valid?
        end    

        # test passing: when I clear @deed.subtransactions the object is no longer valid 
        test "a subtransaction should be present" do
             @deed.subtransactions.clear
            assert_not @deed.valid?
        end

end