Untitled

Custom validation in Rails
class Comment < ActiveRecord::Base
  validates :post_id, presence: true, if: :post_is_published

  ...
  def post_is_publised
    post && post.published
  end
end

class Comment < ActiveRecord::Base
  validates :post_id, :presence => true, :if => :post_is_published

  def post_is_publised
    post.try(:published)
  end
end

class Comment < ActiveRecord::Base
  validate :post_has_to_be_published

  def post_has_to_be_published
    unless post.try(:published)
      self.errors.add(:base, "you can add comments only to published posts")
    end
  end
end