ruby on rails counter_cache 计数

# 运营商 model : Carrier

# sim卡 model : SimCard

使用counter_cache 用来查询每个运营商下的sim卡的数量

1.增加字段（如果运营商已经有sim卡记录，则需要更新现有数量，两种方法）

class AddSimCardsCountToCarriers < ActiveRecord::Migration[5.2]
  def change
    add_column :carriers, :sim_cards_count, :integer, default: 0
    #### 方法1直接用sql更新（sim_cards_count只读的）
    execute 'update carriers set sim_cards_count =
      (select count(*) from "sim_cards" where "sim_cards"."deleted_at" is null and "sim_cards"."carrier_id" = carriers.id)'
    #### 方法2使用 reset_counters 
    Carrier.reset_column_information
    Carrier.find_each do |carrier|
      Carrier.reset_counters carrier.id, :sim_cards
    end
  end
end

2.关联

class SimCard < ApplicationRecord
  belongs_to :carrier, counter_cache: true
end

class Carrier < ApplicationRecord
  has_many :sim_cards, dependent: :destroy
end