新增：父子关系判断以及可设置为父工作项的列表

app/services/api/v1/issues/concerns/checkable.rb

@@ -12,6 +12,10 @@ module Api::V1::Issues::Concerns::Checkable
     raise ApplicationService::Error, "Milestone不存在！" unless Version.find_by_id(milestone_id).present?
   end
 
+  def check_root_issue(issue, root_id) 
+    raise ApplicationService::Error, "父工作项与当前工作项已存在父子关系！" if Issue.full_children_issues(issue).map{|i| i.id}.include?(root_id)
+  end
+
   def check_issue_tags(issue_tag_ids)
     raise ApplicationService::Error, "请输入正确的标记ID数组！" unless issue_tag_ids.is_a?(Array)
     raise ApplicationService::Error, "最多可选择3个标记" if issue_tag_ids.size > 3