017 多对多关系下的多选框

HABTM Checkboxes

It is often asked: how do I create a list of checkboxes for managing a HABTM association? Ask no more because this episode will show you how to do exactly that.

经常出现这样一个问题：我如何创建一个多选框列表来实现has_and_belongs_to_many（多对多）的关系？这节就来具体解决这个问题。

首先，有两个实体：
Category和product，很显然它们是多对多的关系：
#category.rb
class Category < ActiveRecord::Base
has_and_belongs_to_many :products
end

#product.rb

class Product < ActiveRecord::Base
has_and_belongs_to_many :categories
end

现在在表单中，譬如对于product的表单：

<p>
Name<br/>
<%= text_field :product, :name%>
</p>

<p>
<% Category.find(:all).each do |category|%>
<div>
<%= check_box_tag "product[category_ids][]", category.id, @product.categories.include?(category)%>
<%= category.name%>
</div>
<% end %>
</p>

在日志中：

Parameters:{"commit"=>"Edit", "product"=>{"name"=>"Television Stand", "category_ids"=>["1","3"]},"action"=>"update", "id"=>"1","controller"=>"products" }

category_ids是product和category多对多关联的一个属性,在console下product.category_ids是一个数组。

但是对于has_many ,through关系来说，这个xxx_ids属性就得手动写了。

此时，如果是edit多选框，取消所有已选框，不会发生变化，因为提交的时候没有category_ids参数了。所以，在controller的update action中要加一个语句：
def update
params[:product][:category_ids] ||=[]
...
end
这样当没有这个参数提交的时候就将其设为空数组。
相关内容可参见我的另一篇文章<%check_box%> in rails :coditions=>{:has_many , :through}
----------------------下面是railsbrain中check_box方法的描述：

check_box(object_name, method, options = {}, checked_value = "1", unchecked_value = "0")

Returns a checkbox tag tailored for accessing a specified attribute (identified by method) . an object assigned to the template (identified by object). It’s intended that method returns an integer and if that integer is above zero, then the checkbox is checked. Additional options . the input tag can be passed as a hash with options. The checked_value defaults to 1 while the default unchecked_value is set to 0 which is convenient for boolean values. Since HTTP standards say that unchecked checkboxes don’t post anything, we add a hidden value with the same name as the checkbox as a work around.

Examples

# Let's say that @post.validated? is 1:
check_box("post", "validated")
# => <input type="checkbox" id="post_validated" name="post[validated]" value="1" />
#    <input name="post[validated]" type="hidden" value="0" />
# Let's say that @puppy.gooddog is "no":
check_box("puppy", "gooddog", {}, "yes", "no")
# => <input type="checkbox" id="puppy_gooddog" name="puppy[gooddog]" value="yes" />
#    <input name="puppy[gooddog]" type="hidden" value="no" />
check_box("eula", "accepted", { :class => 'eula_check' }, "yes", "no")
# => <input type="checkbox" class="eula_check" id="eula_accepted" name="eula[accepted]" value="yes" />
#    <input name="eula[accepted]" type="hidden" value="no" />

下面是对check_box_tag的描述：

check_box_tag(name, value = "1", checked = false, options = {})

Creates a check box form input tag.

Options

:disabled - If set to true, the user will not be able to use this input.
Any other key creates standard HTML options for the tag.

Examples

check_box_tag 'accept'
# => <input id="accept" name="accept" type="checkbox" value="1" />
check_box_tag 'rock', 'rock music'
# => <input id="rock" name="rock" type="checkbox" value="rock music" />
check_box_tag 'receive_email', 'yes', true
# => <input checked="checked" id="receive_email" name="receive_email" type="checkbox" value="yes" />
check_box_tag 'tos', 'yes', false, :class => 'accept_tos'
# => <input class="accept_tos" id="tos" name="tos" type="checkbox" value="yes" />
check_box_tag 'eula', 'accepted', false, :disabled => true
# => <input disabled="disabled" id="eula" name="eula" type="checkbox" value="accepted" />