donation.rb

 def apply_current_grant!
    current_grant = project.current_grant
    current_grant.apply_to_donation(self)
  end

  def grant_can_be_applied?
    grants.current.present? && (total_amount <= grant_amount)
  end

  def apply_multiple_grants
    if grants.current.remaining_amount >= total_amount
      apply_current_grant!
    else
      while grant_can_be_applied?
        apply_current_grant!
      end
    end
  end

  def apply_grant!
    grant_count = project.grants.active.count
    if grant_count == 1
      apply_current_grant!
    elsif grant_count > 1
      apply_multiple_grants
    else
    end
  end

grant.rb

  def apply_to_donation(donation)
    if active?
      amount_to_apply = donation.project_amount
      donation.grant_amount ||= 0
      if remaining_amount >= amount_to_apply
        # Case: There is enough remaining to match the gift
        donation.grant_amount += amount_to_apply
        self.remaining_amount  -= amount_to_apply
      else
        # Case: There is NOT enough remaining to match the gift
        donation.grant_amount += amount_remaining
        self.remaining_amount  -= amount_remaining
      end
      donation.grants.push(self)
      donation.save!
      self.save!
    end
  end