增加、修改作业时提交项目增加空值选择，即不选定任何关联项目

app/helpers/homework_attach_helper.rb

@@ -59,7 +59,11 @@ module HomeworkAttachHelper
     cond = Project.visible_condition(User.current) + " AND projects.project_type <> 1"
     memberships = User.current.memberships.all(:conditions => cond)
     projects = memberships.map(&:project)
+    not_have_project = []
+    not_have_project << "<<NULL>>"
+    not_have_project << 0
     type = []
+    type << not_have_project
     projects.each do |project|
       if project != nil
         option = []

app/views/homework_attach/new.html.erb

@@ -28,7 +28,7 @@
       </p>
       <p style=" padding: 3px 0 3px 0;padding-left: 50px;  clear:left;">
         <strong>提交项目：</strong>
-        <%= f.select :project_id, options_for_select(user_projects_option),:name => "project_id", :required => true%>
+        <%= f.select :project_id, options_for_select(user_projects_option),:name => "project_id", :required => true, :style => "width:490px;"%>
         <%= link_to '创建项目', new_project_path(course: 0, project_type: 0), :target => '_blank' %>
         <p class="font_lighter" style="padding-left:120px;clear:left;">提交项目可以为空</p>
       </p>