知道关联是否存在的最佳方法是什么？

Question

i have to check user has photos or not, so i am doing

current_user.photos.blank?

it works good, but this is very ineffective since it loads all photos into memory (not sure, correct me if i'm wrong).

I don't want to do this because it will crash my server if

1. user can have thousands of photos
2. hundred of thousands of user visit page where current_user.photos.blank? type of conditions exists, so all this code demands the memory and I guess my server will crash.

I just want to check association ( photos ) is present or not effectively, that's all?

What is the best way to do it?

Answer 1

You can use ActiveRecord 's exists? or any? method to do what you want. This won't return a collection of objects and thus it won't load possible photos on memory. It will just let you know whether a user has any photos or not (true or false).

So for your example, you could have something like this:

current_user.photos.any?

At a lower level, exists? and any? are limiting their queries (via SQL's LIMIT ) to a single result so they can efficiently answer your question.

present? , on the other hand, will return all matching objects which might be an unnecessary load in your case but could serve better for other cases where the matching objects are supposed to be used in some way (eg iterating over them shortly after checking for their existence).