<%= f.select :order_status,
options_for_select(collection, value),
{}, class: 'form-control', multiple: true %>
这里无论怎么选择，都会提交成这种["", ........] ，烦了我好久。
就困于我，遂google之：stackoverflow，有一条Answers，告诉我需要看：https://api.rubyonrails.org/classes/ActionView/Helpers/FormOptionsHelper.html#method-i-select。
终于，在gotcha中找到了答案：
When no selection is made for a collection of checkboxes most web browsers will not send any value. If no category_ids are selected then we can safely assume this field will not be updated. This is possible thanks to a hidden field generated by the helper method for every collection of checkboxes. This hidden field is given the same field name as the checkboxes with a blank value. In the rare case you don’t want this hidden field, you can pass the
include_hidden: falseoption to the helper method.