## spec that fails

  describe "#destroy_if_empty!" do
    before(:each) do
      @clause = AndClause.build_clause
    end

    it "should call #destroy if there are sub_expressions.empty?" do
      @or_clause = OrClause.new(:parent => Condition.new)
      @clause.parent = @or_clause
      @clause.save
      @clause.condition_predicates = []
      @clause.or_clauses = []
      @clause.should_receive(:destroy)
      require "rubygems"; require "ruby-debug"; debugger
      @clause.destroy_if_empty!

      @clause.should be_destroyed
      @or_clause.should be_destroyed
    end
 end

## class method for AndClause

class AndClause < ConditionClause
  ...
  def sub_expressions
    condition_predicates + or_clauses
  end

  def destroy_if_empty!
    if sub_expressions.empty?
      self.destroy
    end
  end
  ...
end