describe Campaign do
  before :each do
    @campaign = Campaign.new
  end

  it { should validate_presence_of :position }
end

class Campaign < ActiveRecord::Base
  attr_accessible :position

  validates :position, presence: true

  default_scope order(:position)

  before_validation :next_position

  # sets the position to the next id (1 if none exist) before validation
  def next_position
    if self.position.blank?
      self.position = Campaign.select("coalesce(max(position),0) + 1 as position").reorder(nil).first.position
    end
  end
end

Failures:

  1) Campaign should require position to be set
     Failure/Error: it { should validate_presence_of :position }
       Expected errors to include "can't be blank" when position is set to nil, got errors: ["name can't be blank (nil)"]
     # ./spec/models/campaign_spec.rb:9:in `block (2 levels) in <top (required)>'

it 'populates position if blank before validations' do
  subject.position = nil
  subject.valid?
  subject.position.should_not be_nil
end