forms - 当字段为空或用户已被邀请时，设计邀请会引发错误

Question

我有带有可邀请设计的 Rails 应用程序。如果我邀请尚未注册的用户，那么它工作正常。如果我输入“fsafsdda”之类的内容，它会告诉我使用有效的形式。但是，当我将该字段留空或邀请现有用户时，它会为 devise/invitations/new.html.erb 中的 <%= f.input field %> 行引发以下错误

undefined method `input' for <ActionView::Helpers::FormBuilder:0x007fd644108d68>

我错过了什么？

html.erb

<div class="invite">  
  <%= simple_form_for(User.new, url: user_invitation_path) do |f| %>
    <div class="form-group">
      <div class="input-group input-group">
        <%= f.input :email, placeholder: 'Invite via email', label: false, class: 'form-control' %>
        <span class="input-group-btn">
          <%= content_tag :button, type: :submit, class: 'btn btn-success' do %>
            <span class="glyphicon glyphicon-envelope"></span>
          <%end%>
        </span>
      </div>
    </div>
  <% end %> 
</div>

默认设计/邀请/new.html.erb

<h2><%= t "devise.invitations.new.header" %></h2>

    <%= form_for resource, as: resource_name, url: invitation_path(resource_name), html: { method: :post} do |f| %>
      <%= devise_error_messages! %>

      <div class="field form-group">
      <% resource.class.invite_key_fields.each do |field| -%>
        <%= f.input field %> ##ERROR THROWN HERE
      <% end -%>
      </div>

      <div class="actions">
        <%= f.button :submit, t("devise.invitations.new.submit_button"), class: "btn btn-primary" %>
      </div>
    <% end %>