ActiveRecord Nested Atrributes 关联记录，对嵌套属性进行CURD

设置了Nested attributes后，你可以通过父记录来更新／新建／删除关联记录。

使用： #accepts_nested_attributes_for class method。

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例如：

class Member < ActiveRecord::Base
  has_one :author
  has_many :posts

  accepts_nested_attributes_for :author, :posts
end

于是就增加了2个方法：XXX_attributes=(attributes)

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分为2种情况：

• one to one
• ont to many

一对一的情况，生成的参数就是嵌套hash.

params = { member: { name: 'Jack', author_attributes: { icon: 'smiling' } } }

一对多的情况，参数包含了key :posts_attributes和它的属性，一个数组的hashes作为value。

params = { member: {
  name: 'joe',
  posts_attributes: [
    { title: 'Kari, the awesome Ruby documentation browser!' },
    { title: 'The egalitarian assumption of the modern citizen' },
    { title: '', _destroy: '1' } # this will be ignored
  ]
}}

member = Member.create(params[:member])
member.posts.length # => 2
member.posts.first.title # => 'Kari, the awesome Ruby documentation browser!'
member.posts.second.title # => 'The egalitarian assumption of the modern citizen'

在 controller中的参数验证方法需要加上：
params.require(:member).permit(:name, posts_attributes: [:title, :_destroy])

_destroy：'1'是当加上参数all_destroy: true后,用于删除所有关联的选项。

一对多，也可以使用嵌套hash代替：

params = { member: {
  name: 'joe',
  posts_attributes: {
    first: { title: 'Kari, the awesome Ruby documentation browser!' },
    second: { title: 'The egalitarian assumption of the modern citizen' },
    third:  { title: '', _destroy: '1' } # this will be ignored
  }
}}

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options

具体用法见api