sql - ruby rails : Best way to Iterate a Relation or Associations

Question

我看到 .where 语句使用大量 CACHE User Load 消息发出大量请求而不是关联。这是真的还是假的？

在这种情况下，我得到一个ActiveRecord_Relation:

@dogs = Dog.where(user_id: current_user.id).order('created_at DESC')

在另一种情况下，我得到一个 ActiveRecord_Associations_CollectionProxy:

@dogs = current_user.dogs.order('created_at DESC')

当我在 View 中迭代时

<% @dogs.each do |dog| %>
<div><%= dog.name %></div>
<% end %>

我在控制台日志中收到不同的消息:

ActiveRecord_Relation:

User Load (0.3ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
  CACHE User Load (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]

ActiveRecord_Associations_CollectionProxy:

User Load (0.3ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 15], ["LIMIT", 1]]
Dog Load (0.6ms)  SELECT "dogs".* FROM "dogs" WHERE "dogs"."user_id" = ? ORDER BY "dogs"."created_at" DESC  [["user_id", 15]]

哪种方法更好？非常感谢。

Answer 1

似乎在这两种情况下，current_user 都应该只被调用一次。大概 current_user 方法有这样的实现:

def current_user
  # read some cookies or something
  User.find_by([...])
end

我不清楚为什么 User.find_by 会在“ActiveRecord_Relation”版本中被多次调用，但这似乎一定是正在发生的事情。 Rails 正在缓存此查询的结果，因此每次在您第一次访问热缓存后。

要排除这种情况，您可以 memoize #current_user。这应该可以防止 Active Record 在第一次调用 current_user 之后每次都看到查询。