验证失败：后续不能为空

Question

我一直在尝试为我网站上有FB的朋友的每个用户实施自动关注功能。当朋友列表中的某人注册到我的网站时，新用户应该自动关注我网站上的人和他/她的朋友列表。

当我尝试在网站上注册第一个用户时，标题中出现错误。我该如何解决这个问题？请检查我的自动跟踪代码是否良好。

我也在使用G +进行身份验证。

这就是我现在在user.rb中的内容：

  has_many :relationships, foreign_key: "follower_id", dependent: :destroy
  has_many :followed_users, through: :relationships, source: :followed
  has_many :reverse_relationships, foreign_key: "followed_id",
                                   class_name:  "Relationship",
                                   dependent:   :destroy
  has_many :followers, through: :reverse_relationships, source: :follower

   after_create :follow_fbfriends!

  def self.from_omniauth(auth)
      where(auth.slice(:provider, :uid)).first_or_initialize.tap do |user|
        user.provider = auth.provider
        user.uid = auth.uid
        user.name = auth.info.name
        user.first_name = auth.info.first_name
        user.last_name = auth.info.last_name
        user.email = auth.info.email
        user.image = auth.info.image
        user.oauth_token = auth.credentials.token
        user.oauth_expires_at = Time.at(auth.credentials.expires_at)
        user.save!
       end
  end

   #gets uid of user's fb friends

   def fbfriends
     @graph = Koala::Facebook::API.new(oauth_token)
        begin
          @fbfriends = @graph.get_connections("me", "friends", fields: "id")
          @uids = @fbfriends.map{ |v| v.values }.flatten
        rescue Koala::Facebook::AuthenticationError => e
          redirect_to '/auth/facebook'
        end
          @friends = User.where(uid: @uids)
    end

  #User ID on my site(based on UID)
  def fb_user_id
    self.fbfriends.map{ |v| v.id }
  end

  #This should automatically follow user's fb friends if any of them are on my site
  def follow_fbfriends!
    relationships.create!(followed_id: fb_user_id)
  end

relationship.rb

class Relationship < ActiveRecord::Base
  belongs_to :follower, class_name: "User"
  belongs_to :followed, class_name: "User"
#  validates :follower_id, presence: true
#  validates :followed_id, presence: true
end