为什么我的 Rails 中的第一个元素总是使用嵌入式数组进行多重选择？

The hidden field is what is causing the issue. But it is there for a good reason: when all values are deselected, you still receive a subset_array parameter. From the Rails docs (you may have to scroll to the right to see all of this):

  # The HTML specification says when +multiple+ parameter passed to select and all options got deselected
# web browsers do not send any value to server. Unfortunately this introduces a gotcha:
# if an +User+ model has many +roles+ and have +role_ids+ accessor, and in the form that edits roles of the user
# the user deselects all roles from +role_ids+ multiple select box, no +role_ids+ parameter is sent. So,
# any mass-assignment idiom like
#
#   @user.update_attributes(params[:user])
#
# wouldn't update roles.
#
# To prevent this the helper generates an auxiliary hidden field before
# every multiple select. The hidden field has the same name as multiple select and blank value.
#
# This way, the client either sends only the hidden field (representing
# the deselected multiple select box), or both fields. Since the HTML specification
# says key/value pairs have to be sent in the same order they appear in the
# form, and parameters extraction gets the last occurrence of any repeated
# key in the query string, that works for ordinary forms.

EDIT: The last paragraph suggests that you shouldn't be seeing the empty one in the case when something is selected, but I think it is wrong. The person who made this commit to Rails (see https://github.com/rails/rails/commit/faba406fa15251cdc9588364d23c687a14ed6885) is trying to do the same trick that Rails uses for checkboxes (as mentioned here: https://github.com/rails/rails/pull/1552), but I don't think it can work for a multiple select box because the parameters sent over form an array in this case and so no value is ignored.

So my feeling is that this is a bug.

In Rails 4:

You will be able to pass :include_hidden option. https://github.com/rails/rails/pull/5414/files

As a quick fix for now: you can use right now in your model:

before_validation do |model|
model.subset_array.reject!(&:blank?) if model.subset_array
end

This will just delete all blank values at model level.

Another quick fix is to use this controller filter:

def clean_select_multiple_params hash = params
hash.each do |k, v|
case v
when Array then v.reject!(&:blank?)
when Hash then clean_select_multiple_params(v)
end
end
end

This way can be reused across controllers without touching the model layer.

In the controller:

arr = arr.delete_if { |x| x.empty? }

I make it work by writing this in the Javascript part of the page:

$("#model_subset_array").val( <%= @model.subset_array %> );

Mine looks more like following:

$("#modela_modelb_ids").val( <%= @modela.modelb_ids %> );

Not sure if this is going to get me headache in the future but now it works fine.

Use jQuery:

$('select option:empty').remove();

Option to remove blank options from drop down.

http://api.rubyonrails.org/classes/ActionView/Helpers/FormHelper.html#method-i-check_box

Gotcha

The HTML specification says unchecked check boxes or selects are not successful, and thus web browsers do not send them. Unfortunately this introduces a gotcha: if an Invoice model has a paid flag, and in the form that edits a paid invoice the user unchecks its check box, no paid parameter is sent. So, any mass-assignment idiom like

@invoice.update(params[:invoice]) wouldn't update the flag.

To prevent this the helper generates an auxiliary hidden field before the very check box. The hidden field has the same name and its attributes mimic an unchecked check box.

This way, the client either sends only the hidden field (representing the check box is unchecked), or both fields. Since the HTML specification says key/value pairs have to be sent in the same order they appear in the form, and parameters extraction gets the last occurrence of any repeated key in the query string, that works for ordinary forms.

To remove blank values:

  def myfield=(value)
value.reject!(&:blank?)
write_attribute(:myfield, value)
end

I fixed it using the params[:review][:staff_ids].delete("") in the controller before the update.

In my view:

= form_for @review do |f|
= f.collection_select :staff_ids, @business.staff, :id, :full_name, {}, {multiple:true}
= f.submit 'Submit Review'

In my controller:

class ReviewsController < ApplicationController
def create
....
params[:review][:staff_ids].delete("")
@review.update_attribute(:staff_ids, params[:review][:staff_ids].join(","))
....
end
end

In Rails 4+ set :include_hidden on select_tag to false

<%= form.grouped_collection_select :employee_id, Company.all, :employees, :name, :id, :name, { include_hidden: false }, { size: 6, multiple: true } %>