1. ActiveRecord::Enum 介紹

在 Rails 的 ActiveRecord 中，有一個(gè) ActiveRecord::Enum 的 Module，即枚舉對(duì)象。

Declare an enum attribute where the values map to integers in the database, but can be queried by name.

給數(shù)據(jù)庫(kù)中的整型字段聲明一個(gè)一一對(duì)應(yīng)的枚舉屬性值，這個(gè)值可以以字面量用于查詢。

拿到具體的運(yùn)用場(chǎng)景中去考慮，Enum 的主要用于數(shù)據(jù)庫(kù)中類似于 狀態(tài)(status) 的字段，這類字段用不同的 整數(shù)(Integer) 來(lái)表示不用的狀態(tài)。如果不使用 Enum，那就意味著我們代碼中會(huì)出現(xiàn)很多表示狀態(tài)的數(shù)字，他們可能會(huì)出現(xiàn)在查詢條件里，也可能會(huì)出現(xiàn)在判斷條件里，除非你記得或者拿著數(shù)據(jù)字典去看，否則你很難理解這段代碼的含義。

代碼中，以數(shù)字方式去表示數(shù)據(jù)狀態(tài)，導(dǎo)致代碼可讀性被破壞，這樣的數(shù)字被稱為『魔鬼數(shù)字』。

Enum 就是 Rails 用來(lái)消滅魔鬼數(shù)字的工具。

2. ActiveRecord::Enum 的使用

常規(guī)用法：

# migration
create_table :conversations do |t|
  t.integer :status, default: 0
end

# model
class Conversation < ActiveRecord::Base
  # 相比起使用 Hash，數(shù)組相當(dāng)于隱式指定了數(shù)字鍵值，字面量的順序就很關(guān)鍵
  enum status: [ :active, :archived ]
  # or, recommended
  enum status: {active:0, archived:1}
end

conversation.active! # = conversation.update! status: 0
conversation.active? # => true
conversation.status  # => "active"

conversation.archived! # conversation.update! status: 1
conversation.archived? # => true
conversation.status    # => "archived"

conversation.status = "archived" # conversation.status = 1 

conversation.status = nil
conversation.status.nil? # => true
conversation.status      # Conversation.where(status: active) => nil

Conversation.statuses

提供默認(rèn)scope：

Conversation.active  # => Conversation.where(status: 0)
Conversation. archived # => Conversation.where(status: 1)

獲得一個(gè)名為 statuses 的 HashWithIndifferentAccess

Conversation.statuses  #=> { "active" => 0, "archived" => 1 }
Conversation.statuses[:active]    # => 0
Conversation.statuses["archived"]  # => 1

3. ActiveRecord::Enum 在 Rails 5.0+ 中的新特性

• where 查詢支持直接使用字面量做查詢條件

# 在 Rails 4.1+ 中
Conversation.where(status: %i[active waiting]).to_sql
# => "SELECT `conversations`.* FROM `conversations` WHERE `conversations`.`status` IN (NULL, NULL)"

# 驚不驚喜，意不意外，這也是 4.x 版本 Enum 比較雞肋的原因，正確的你應(yīng)這樣寫
Conversation.where(status: %i[active waiting].map { |s| Conversation.statuses[s] }).to_sql
# => "SELECT `conversations`.* FROM `conversations` WHERE `conversations`.`status` IN (0, 1)"

# 好在，Rails 5.0 之后就可以這么用了
Conversation.where(status: %i[active waiting]).to_sql
# => "SELECT \"conversations\".* FROM \"conversations\" WHERE \"conversations\".\"status\" IN (0, 1)"

# BUT，當(dāng)你選擇手寫 SQL 查詢條件的時(shí)候，仍是需要自己轉(zhuǎn)義的
Conversation.where("status <> ?", Conversation.statuses[:archived])

• conversation[:status] 與 conversation.status 返回值一致，都是字面量

conversation.status # => "active"
conversation[:status] # => "active"
Conversation.pluck(:status) # => ["active", "archived",  ...]

• 增加了兩個(gè)可選參數(shù) _prefix 和 _suffix

在 Rails 4.1+ 的版本中，即使是不同的 enum 字段也不能有同名的值

# user.rb
  enum status: [:temporary, :active, :deleted]
  enum admin_status: [:active, :super_admin]

# rails console
irb(main):001:0> u = User.new
ArgumentError: You tried to define an enum named "admin_status" on the model "User", but this will generate a instance method "active?", which is already defined by another enum.
...
于是在 Rails 5 中，引入了 _prefix 和 _suffix 兩個(gè)選項(xiàng)來(lái)解決這個(gè)問(wèn)題，它會(huì)給對(duì)應(yīng)的 !、? 以及 scope 方法加上前/后綴以示區(qū)分

# user.rb
  enum status: [:temporary, :active, :deleted], _suffix: true
  enum admin_status: [:active, :super_admin]

# rails console
  user = User.active_status.first
  user.active_status?
  user.deleted_status!

# user.rb
  enum status: [:temporary, :active, :deleted], _suffix: :stat
  enum admin_status: [:active, :super_admin]

# rails console
  user = User.active_stat.first
  user.active_stat?
  user.deleted_stat!

4.實(shí)際使用中的一些經(jīng)驗(yàn)總結(jié)

• 不要使用數(shù)據(jù)庫(kù)的 enum ?。。?/strong>
• 盡量升級(jí)到 Rails 5 以上的版本
• 對(duì) enum 字段賦值時(shí)，已經(jīng)隱含了數(shù)據(jù)驗(yàn)證
• 盡量不要使用數(shù)組來(lái)定義 enum
• 手動(dòng)設(shè)置了 table_name 時(shí)，需要警惕關(guān)聯(lián)查詢的陷阱
  注：這條是 Rails 5.x 的專屬煩惱

# post.rb
class Post < ActiveRecord::Base
  self.table_name = :articles

  has_many :comments

  enum category: { it: 0, law: 1, medical: 2 }
end

# comment.rb
class Comment < ActiveRecord::Base
    belongs_to :post, foreign_key: :article_id
end

# 正常查詢是 OK 的
Post.law.to_sql 
#=> "SELECT \"articles\".* FROM \"articles\" WHERE \"articles\".\"category\" = 1"

# 作為關(guān)聯(lián)表的查詢條件，enum 字段就無(wú)法轉(zhuǎn)義了，查詢會(huì)報(bào)錯(cuò)
Comment.joins(:post).where(articles: { category: :law }).to_sql
# => "SELECT \"comments\".* FROM \"comments\" INNER JOIN \"articles\" ON \"articles\".\"id\" = \"comments\".\"article_id\" WHERE \"articles\".\"category\" = 'law'"

# 故意寫個(gè)錯(cuò)的查詢，看看錯(cuò)出在哪兒
Comment.joins(:post).where(post: { category: :law }).to_sql
# => "SELECT \"comments\".* FROM \"comments\" INNER JOIN \"articles\" ON \"articles\".\"id\" = \"comments\".\"article_id\" WHERE \"post\".\"category\" = 1"

Comment.joins(:post).where(articles: { category: [:law] })

Comment.joins(:post).where(articles: { category: [:law] }).to_sql
# => "SELECT \"comments\".* FROM \"comments\" INNER JOIN \"articles\" ON \"articles\".\"id\" = \"comments\".\"article_id\" WHERE \"articles\".\"category\" = 0"

create_table :conversations do |t|
  t.integer :status, limit: 2, default: 0, null: false
end

class AddDeletedStatusToConversations < ActiveRecord::Migration[5.1]
  def up
    raise "The value of deleted status has already been taken." if Conversation.deleted.count.positive?
    change_column :conversations, :status, :integer, default: 0, null: false, comment: "0 - active, 1 - waiting, 2 - archived, 3 - deleted"
  end
end

# zh-CN.yml
# zh-CN:
#   activerecord:
#     attributes:
#       conversation/status:
#         active: 當(dāng)前激活
#         waiting: 等待中
#         archived: 已歸檔

conversation.status # => "active"
Conversation.human_attribute_name("status.#{conversation.status}") # => "當(dāng)前激活"