Rails: loading association at once vs multiple tiny queries - sql

Question

I've got a situation. I am rendering a paginated collection of resources(let's say Posts). I need to show whether the post is liked by the current user. So basically we have three tables: posts, users and likes(a join table o indicate if users like some posts).
I came up with three solutions:
class Post
# 1
def liked_by?(user)
likes.exists?(user: user) # usage: Post.limit(50).map { |post| [p.id, post.liked_by?(current_user)] }
end
# 2
def liked_by?(user)
likes.any? { |like| like.user_id == user.id } # usage: Post.limit(50).includes(:likes).map { |post| [p.id, post.liked_by?(current_user)] }
end
# 3
def liked_by?(user)
user.likes.exists?(post: self) # usage: Post.limit(50).includes(:likes).map { |post| [p.id, post.liked_by?(current_user)] }
end
# 4
def liked_by?(user)
user.likes.any? { |like| like.user_id == user.id } # usage: Post.limit(50).map { |post| [p.id, post.liked_by?(current_user)] }
end
end
I can imagine the advantages and disadvantages of each of them but would be happy to hear any thoughts from you guys.
Usually, we render 50 posts at a time and the current user has about a hundred likes, but some users have thousands of likes so it's hard to say that there is a general solution for this. Anyway, I'm open to hear your proposals. Probably there's way # 5...(like denormalizing tables, materialized views and etc).

Answer 1

None of the above as they will all create a N+1 query issue. Also interrogation methods (methods that end with ?) should by convention return a boolean in Ruby.
You can use EXISTS with a subquery:
SELECT "posts".* FROM "posts"
WHERE EXISTS (SELECT "likes".* FROM "likes" WHERE "likes"."user_id" = $1 AND "likes"."post_id" = "posts"."id")
class User < ApplicationRecord
has_many :likes
end
class Post < ApplicationRecord
has_many :likes
# returns posts liked by user
def self.liked_by_user(user)
where(
user.likes
.where(Like.arel_table[:post_id].eq(arel_table[:id]))
.arel.exists
)
end
end
If you want all the posts regardless if the have been liked or not and a boolean indicating if they have been liked by the user can put that same EXISTS into the SELECT clause:
class Post < ApplicationRecord
# ...
# selects posts and a liked_by_user boolean
def self.with_likes_by_user(user)
columns = self.respond_to?(:arel) ? arel.projections : arel_table[Arel.star]
select(
columns,
user.likes
.where(Like.arel_table[:post_id].eq(arel_table[:id]))
.arel.exists.as('liked_by_user')
)
end
end
SELECT
"posts".*,
EXISTS (
SELECT "likes".*
FROM "likes" WHERE "likes"."user_id" = $1
AND "likes"."post_id" = "posts"."id"
) AS liked_by_user
FROM "posts"