ruby-on-rails - 外键约束确实会导致 N+1 查询但没有约束，预加载工作正常

Question

我在 markets 表中添加了 base_currency_id 和 base_currency_id2 分别有和没有外键约束，

def change
  create_table :currencies do |t|
    t.string :code
    t.timestamps
  end

  create_table :market2 do |t|
    t.string :code
    t.integer :base_currency_id
    t.integer :base_currency_id2
    t.foreign_key :currencies, column: :base_currency_id2
    t.integer :quote_currency_id
    t.timestamps
  end
end

ActiveRecord::Migration.change

Market 模型定义了以下关联，

class Market < ApplicationRecord
  belongs_to :base_currency, class_name: 'Currency', foreign_key: :base_currency_id
  belongs_to :base_currency2, class_name: 'Currency', foreign_key: :base_currency_id2
end

我不明白为什么以下关联之一会导致 N+1 查询，即使在此处进行预加载时也是如此，

Market.includes(:base_currency).each { |x| puts x.base_currency.code }
  # Market Load (0.6ms)  SELECT "markets".* FROM "markets"
  # Currency Load (0.3ms)  SELECT "currencies".* FROM "currencies" WHERE "currencies"."id" = $1  [["id", 1]]
#  INR
#  INR


Market.includes(:base_currency2).each { |x| puts x.base_currency.code }
  #  Market Load (0.5ms)  SELECT "markets".* FROM "markets"
  #  Currency Load (0.4ms)  SELECT  "currencies".* FROM "currencies" WHERE "currencies"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
# INR
  # Currency Load (0.4ms)  SELECT  "currencies".* FROM "currencies" WHERE "currencies"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
# INR

如果我在这里遗漏了什么，请解释。

Answer 1

将 block 内调用的关联更改为字母中的基础货币 2。

Market.includes(:base_currency2).each { |x| puts x.base_currency2.code }