allow library cover blank

app/models/library.rb

@@ -2,7 +2,7 @@ class Library < ActiveRecord::Base
   include AASM
 
   belongs_to :user
-  belongs_to :cover, class_name: 'Attachment', foreign_key: :cover_id
+  belongs_to :cover, class_name: 'Attachment', foreign_key: :cover_id, optional: true
 
   has_many :library_applies, dependent: :delete_all
   has_many :attachments, as: :container

app/services/libraries/save_service.rb

@@ -36,7 +36,7 @@ class Libraries::SaveService
 
   def validate_params!
     raise Error, '附件不能为空' if params[:attachment_ids].try(:compact).blank?
-    raise Error , '封面不能为空' if params[:cover_id].blank?
+    # raise Error , '封面不能为空' if params[:cover_id].blank?
   end
 
   def deal_library_tag!