group model 代码

class Group < ActiveRecord::Base
  has_many :posts
  belongs_to :owner, :class_name => "User", :foreign_key => :user_id

  has_many :group_users
  has_many :participated_groups, :through => :group_users, :source => :group

  validates :title, :presence => true

  def editable_by?(user)
    user && user == owner
  end
end

user model 代码

class User < ActiveRecord::Base


  has_many :groups
  has_many :posts

  extend OmniauthCallbacks

  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable

  def join!(group)
    participated_groups << group
  end

  def quit!(group)
    participated_groups.delete(group)
  end

  after_create :join_owner_to_group

  def is_member_of?(group)
    participated_groups.include?(group)
  end

  def join_owner_to_group
    members << owner
  end
end

group view 代码

<div class="span12">
  <div class="group">
    <%= link_to("New group", new_group_path , :class => "btn btn-mini btn-primary pull-right")%>
  </div>
  <table class="table">
    <thead> <tr>
      <td> # </td>
      <td> Title </td>
      <td> Descroption </td>
      <td> Owner </td>
    </tr>
    </thead>
    <tbody>
    <% @groups.each do |group| %>
        <tr>
          <td> # </td>
          <td> <%= link_to(group.title, group_path(group)) %> </td>
          <td> <%= group.description %> </td>
          <td> <%= group.owner.name %> </td>
          <td>
            <% if current_user && group.editable_by?(current_user) %>
                <%= link_to("Edit", edit_group_path(group), :class => "btn btn-mini")%>
                <%= link_to("Delete", group_path(group), :class => "btn btn-mini",:method => :delete, :confirm => "Are you sure?" ) %>
            <% end %>
          </td>
        </tr>
    <% end %>
    </tbody>
  </table>
</div>

然后运行报错

NoMethodError at /

undefined method `name' for nil:NilClass 错误代码指向这一行

<%= group.owner.name %> 请指教。