`to_a`避免重新运行查询？

Question

我在控制台中注意到，to_a方法在ActiveRecord_Relation中有一个不寻常的用户。我将在下面复制它：

从表中选择变量a

a = Crypto::ExchangeQuotation.all
[DEBUG]   Crypto::ExchangeQuotation Load (121.1ms)  SELECT  "crypto_exchange_quotations".* FROM "crypto_exchange_quotations" LIMIT $1  [["LIMIT", 11]]

调用a

2.4.1 :003 > a
[DEBUG]   Crypto::ExchangeQuotation Load (0.8ms)  SELECT  "crypto_exchange_quotations".* FROM "crypto_exchange_quotations" LIMIT $1  [["LIMIT", 11]]

a数组：

2.4.1 :004 > a.to_a
[DEBUG]   Crypto::ExchangeQuotation Load (0.9ms)  SELECT "crypto_exchange_quotations".* FROM "crypto_exchange_quotations"

再次调用a：

2.4.1 :005 > a
 => #<ActiveRecord::Relation [#<Crypto::ExchangeQuotation id: 1, exchange_code: "ARN", base_currency_code: "BRL", currency_code: "BTC", origin: "bitvalor", created_at: "2018-03-15 23:24:06", updated_at: "2018-03-15 23:24:06", buy: 17810.0, sell: 17810.0>,

在变量中调用to_a，然后再调用它是否真的避免了再次运行查询？如果是，为什么？因为在a中返回的仍然是ActiveRecord_Relation。

Answer 1

调用to_a并不会改变基本对象，但它会复制这个对象并将其作为数组返回。下面看一下to_a的广告代码：

def to_a
  records.dup
end

若要真正将其更改为数组，必须重写变量a

a = a.to_a