Ruby on Rails：迭代关系或关联的最佳方式

Question

I see that .where statement make so much request with a lot of CACHE User Load message than association. Is this true or not?

In this case I get an ActiveRecord_Relation:

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

In this other case, I get a ActiveRecord_Associations_CollectionProxy:

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

When I iterate in the view

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

I get different message in my console log:

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]]

Wich is the better way? Thank you very much.

Answer 1

It seems like in both cases current_user should be invoked only once. Presumably the current_user method has an implementation like this:

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

It's not clear to me why User.find_by would be invoked many times in the "ActiveRecord_Relation" version, but that seems like it must be what's happening. Rails is caching the result of this query, so every time after the first time you hit a warm cache.

To rule this out, you can memoize #current_user . This should prevent Active Record from every seeing the query after the first invocation of current_user .