ruby-on-rails – NoMethodError – []的未定义方法’find_by’

我的观点/users/show.html.erb看起来像这样：

.
.
.
<% if current_user.following?(@user) %>
    <%= render 'unfollow' %>
<% else %>
    <%= render 'follow' %>
<% end %>

用户模型’models / user.rb’：

class User < ActiveRecord::Base
devise :database_authenticatable,:registerable,:recoverable,:rememberable,:trackable,:validatable

has_many :authentications
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

    def following?(other_user)
        relationships.find_by(followed_id: other_user.id)
    end

    def follow!(other_user)
        relationships.create!(followed_id: other_user.id)
    end

    def unfollow!(other_user)
        relationships.find_by(followed_id: other_user.id).destroy
    end

end

关系模型’models / relationship.rb’：

class Relationship < ActiveRecord::Base

  attr_accessible :followed_id,:follower_id

  belongs_to :follower,class_name: "User"
  belongs_to :followed,class_name: "User"

  validates :follower_id,presence: true
  validates :followed_id,presence: true

end

Rails告诉我问题在于用户模型：“relationships.find_by(followed_id：other_user.id)”因为mthod没有定义,但我不明白为什么？