How to refer to high-level model from nested one?

I'm trying to make conditional validation based on Project status in Payment model. For example status can be "Talks" or "Active". What's the best way to do it considering the structure below?

class Project < ActiveRecord::Base
  has_many :costs, :dependent => :destroy
  has_many :payments, :through => :costs, :dependent => :destroy
  accepts_nested_attributes_for :costs, :allow_destroy => true
end
class Cost < ActiveRecord::Base
  has_many :payments, :dependent => :destroy
  accepts_nested_attributes_for :payments, :allow_destroy => true
  belongs_to :project
end
class Payment < ActiveRecord::Base
  belongs_to :cost
  validates_presence_of :value1, :if => :new?
  validates_presence_of :value1, :if => :talks?
  validates_presence_of :va开发者_StackOverflow中文版lue2, :if => :active?

  def new?
    # if controller action is new
  end
  def talks?
    # if project status is "Talks" (edit action)
  end
  def active?
    # if project status is "Active" (edit action)
  end
end

---

class Payment < ActiveRecord::Base
  belongs_to :cost
  has_one :project, :through => :cost
  validates_presence_of :value1, :if => :new?
  validates_presence_of :value1, :if => :talks?
  validates_presence_of :value2, :if => :active?

  def new?
    self.new_record?
  end
  def talks?
    project.status == "talks"
  end
  def active?
    project.status == "active"
  end
end